first commit

core/modules/serialization/tests/modules/entity_serialization_test/entity_serialization_test.info.yml

@@ -0,0 +1,10 @@
+name: 'Entity serialization test support'
+type: module
+description: 'Provides test support for entity serialization tests.'
+package: Testing
+# version: VERSION
+
+# Information added by Drupal.org packaging script on 2024-07-04
+version: '10.3.1'
+project: 'drupal'
+datestamp: 1720094222

core/modules/serialization/tests/modules/entity_serialization_test/entity_serialization_test.module

@@ -0,0 +1,23 @@
+<?php
+
+/**
+ * @file
+ * Test support module for entity serialization tests.
+ */
+
+use Drupal\Core\Access\AccessResult;
+
+/**
+ * Implements hook_entity_field_access_alter().
+ *
+ * Overrides some default access control to support testing.
+ *
+ * @see Drupal\serialization\Tests\EntitySerializationTest::testUserNormalize()
+ */
+function entity_serialization_test_entity_field_access_alter(array &$grants, array $context) {
+  // Override default access control from UserAccessControlHandler to allow
+  // access to 'pass' field for the test user.
+  if ($context['field_definition']->getName() == 'pass' && $context['account']->getAccountName() == 'serialization_test_user') {
+    $grants[':default'] = AccessResult::allowed()->inheritCacheability($grants[':default'])->addCacheableDependency($context['items']->getEntity());
+  }
+}

core/modules/serialization/tests/modules/field_normalization_test/field_normalization_test.info.yml

@@ -0,0 +1,10 @@
+name: 'FieldItem normalization test support'
+type: module
+description: 'Provides test support for fieldItem normalization test support.'
+package: Testing
+# version: VERSION
+
+# Information added by Drupal.org packaging script on 2024-07-04
+version: '10.3.1'
+project: 'drupal'
+datestamp: 1720094222

core/modules/serialization/tests/modules/field_normalization_test/field_normalization_test.services.yml

@@ -0,0 +1,6 @@
+services:
+  serializer.normalizer.silly_field_item:
+    class: Drupal\field_normalization_test\Normalization\TextItemSillyNormalizer
+    tags:
+      # The priority must be higher than serialization.normalizer.field_item.
+      - { name: normalizer , priority: 9 }

core/modules/serialization/tests/modules/field_normalization_test/src/Normalization/TextItemSillyNormalizer.php

@@ -0,0 +1,38 @@
+<?php
+
+namespace Drupal\field_normalization_test\Normalization;
+
+use Drupal\serialization\Normalizer\FieldItemNormalizer;
+use Drupal\text\Plugin\Field\FieldType\TextItemBase;
+
+/**
+ * A test TextItem normalizer to test denormalization.
+ */
+class TextItemSillyNormalizer extends FieldItemNormalizer {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function normalize($object, $format = NULL, array $context = []): array|string|int|float|bool|\ArrayObject|NULL {
+    $data = parent::normalize($object, $format, $context);
+    $data['value'] .= '::silly_suffix';
+    return $data;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function constructValue($data, $context) {
+    $value = parent::constructValue($data, $context);
+    $value['value'] = str_replace('::silly_suffix', '', $value['value']);
+    return $value;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getSupportedTypes(?string $format): array {
+    return [TextItemBase::class => TRUE];
+  }
+
+}

core/modules/serialization/tests/modules/test_datatype_boolean_emoji_normalizer/src/Normalizer/BooleanNormalizer.php

@@ -0,0 +1,38 @@
+<?php
+
+namespace Drupal\test_datatype_boolean_emoji_normalizer\Normalizer;
+
+use Drupal\Core\TypedData\Plugin\DataType\BooleanData;
+use Drupal\serialization\Normalizer\NormalizerBase;
+use Symfony\Component\Serializer\Normalizer\DenormalizerInterface;
+
+/**
+ * Normalizes boolean data weirdly: renders them as 👍 (TRUE) or 👎 (FALSE).
+ */
+class BooleanNormalizer extends NormalizerBase implements DenormalizerInterface {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function normalize($object, $format = NULL, array $context = []): array|string|int|float|bool|\ArrayObject|NULL {
+    return $object->getValue() ? '👍' : '👎';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function denormalize($data, $class, $format = NULL, array $context = []): mixed {
+    if (!in_array($data, ['👍', '👎'], TRUE)) {
+      throw new \UnexpectedValueException('Only 👍 and 👎 are acceptable values.');
+    }
+    return $data === '👍';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getSupportedTypes(?string $format): array {
+    return [BooleanData::class => TRUE];
+  }
+
+}

core/modules/serialization/tests/modules/test_datatype_boolean_emoji_normalizer/test_datatype_boolean_emoji_normalizer.info.yml

@@ -0,0 +1,10 @@
+name: 'Test @DataType normalizer'
+type: module
+description: 'Provides test support for @DataType-level normalization.'
+package: Testing
+# version: VERSION
+
+# Information added by Drupal.org packaging script on 2024-07-04
+version: '10.3.1'
+project: 'drupal'
+datestamp: 1720094222

core/modules/serialization/tests/modules/test_datatype_boolean_emoji_normalizer/test_datatype_boolean_emoji_normalizer.services.yml

@@ -0,0 +1,6 @@
+services:
+  serializer.normalizer.boolean.datatype.emoji:
+    class: Drupal\test_datatype_boolean_emoji_normalizer\Normalizer\BooleanNormalizer
+    tags:
+      # The priority must be higher than serializer.normalizer.primitive_data.
+      - { name: normalizer , priority: 1000 }

core/modules/serialization/tests/modules/test_fieldtype_boolean_emoji_normalizer/src/Normalizer/BooleanItemNormalizer.php

@@ -0,0 +1,48 @@
+<?php
+
+namespace Drupal\test_fieldtype_boolean_emoji_normalizer\Normalizer;
+
+use Drupal\Core\Field\Plugin\Field\FieldType\BooleanItem;
+use Drupal\serialization\Normalizer\FieldItemNormalizer;
+use Symfony\Component\Serializer\Normalizer\DenormalizerInterface;
+
+/**
+ * Normalizes boolean fields weirdly: renders them as 👍 (TRUE) or 👎 (FALSE).
+ */
+class BooleanItemNormalizer extends FieldItemNormalizer implements DenormalizerInterface {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function normalize($object, $format = NULL, array $context = []): array|string|int|float|bool|\ArrayObject|NULL {
+    $data = parent::normalize($object, $format, $context);
+    $data['value'] = $data['value'] ? '👍' : '👎';
+    return $data;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function constructValue($data, $context) {
+    // Just like \Drupal\serialization\Normalizer\FieldItemNormalizer's logic
+    // for denormalization, which uses TypedDataInterface::setValue(), allow the
+    // keying by main property name ("value") to be implied.
+    if (!is_array($data)) {
+      $data = ['value' => $data];
+    }
+
+    if (!in_array($data['value'], ['👍', '👎'], TRUE)) {
+      throw new \UnexpectedValueException('Only 👍 and 👎 are acceptable values.');
+    }
+    $data['value'] = ($data['value'] === '👍');
+    return $data;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getSupportedTypes(?string $format): array {
+    return [BooleanItem::class => TRUE];
+  }
+
+}

core/modules/serialization/tests/modules/test_fieldtype_boolean_emoji_normalizer/test_fieldtype_boolean_emoji_normalizer.info.yml

@@ -0,0 +1,10 @@
+name: 'Test @FieldType normalizer'
+type: module
+description: 'Provides test support for @FieldType-level normalization.'
+package: Testing
+# version: VERSION
+
+# Information added by Drupal.org packaging script on 2024-07-04
+version: '10.3.1'
+project: 'drupal'
+datestamp: 1720094222

core/modules/serialization/tests/modules/test_fieldtype_boolean_emoji_normalizer/test_fieldtype_boolean_emoji_normalizer.services.yml

@@ -0,0 +1,6 @@
+services:
+  serializer.normalizer.boolean.fieldtype.emoji:
+    class: Drupal\test_fieldtype_boolean_emoji_normalizer\Normalizer\BooleanItemNormalizer
+    tags:
+      # The priority must be higher than serialization.normalizer.field_item.
+      - { name: normalizer , priority: 1000 }

core/modules/serialization/tests/serialization_test/serialization_test.info.yml

@@ -0,0 +1,10 @@
+name: Serialization test module
+type: module
+description: "Support module for serialization tests."
+package: Testing
+# version: VERSION
+
+# Information added by Drupal.org packaging script on 2024-07-04
+version: '10.3.1'
+project: 'drupal'
+datestamp: 1720094222

core/modules/serialization/tests/serialization_test/serialization_test.services.yml

@@ -0,0 +1,9 @@
+services:
+  serializer.normalizer.serialization_test:
+    class: Drupal\serialization_test\SerializationTestNormalizer
+    tags:
+      - { name: normalizer }
+  serializer.encoder.serialization_test:
+    class: Drupal\serialization_test\SerializationTestEncoder
+    tags:
+      - { name: encoder, format: serialization_test}

core/modules/serialization/tests/serialization_test/src/SerializationTestEncoder.php

@@ -0,0 +1,31 @@
+<?php
+
+namespace Drupal\serialization_test;
+
+use Symfony\Component\Serializer\Encoder\EncoderInterface;
+
+class SerializationTestEncoder implements EncoderInterface {
+
+  /**
+   * The format that this Encoder supports.
+   *
+   * @var string
+   */
+  protected static $format = 'serialization_test';
+
+  /**
+   * {@inheritdoc}
+   */
+  public function encode($data, $format, array $context = []): string {
+    // @see \Drupal\serialization_test\SerializationTestNormalizer::normalize().
+    return 'Normalized by ' . $data['normalized_by'] . ', Encoded by SerializationTestEncoder';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function supportsEncoding(string $format, array $context = []): bool {
+    return static::$format === $format;
+  }
+
+}

core/modules/serialization/tests/serialization_test/src/SerializationTestNormalizer.php

@@ -0,0 +1,43 @@
+<?php
+
+namespace Drupal\serialization_test;
+
+use Symfony\Component\Serializer\Normalizer\NormalizerInterface;
+
+class SerializationTestNormalizer implements NormalizerInterface {
+
+  /**
+   * The format that this Normalizer supports.
+   *
+   * @var string
+   */
+  protected static $format = 'serialization_test';
+
+  /**
+   * {@inheritdoc}
+   */
+  public function normalize($object, $format = NULL, array $context = []): array|string|int|float|bool|\ArrayObject|NULL {
+    $normalized = (array) $object;
+    // Add identifying value that can be used to verify that the expected
+    // normalizer was invoked.
+    $normalized['normalized_by'] = 'SerializationTestNormalizer';
+    return $normalized;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function supportsNormalization($data, ?string $format = NULL, array $context = []): bool {
+    return static::$format === $format;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getSupportedTypes(?string $format): array {
+    return [
+      \stdClass::class => TRUE,
+    ];
+  }
+
+}

core/modules/serialization/tests/src/Functional/GenericTest.php

@@ -0,0 +1,14 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\serialization\Functional;
+
+use Drupal\Tests\system\Functional\Module\GenericModuleTestBase;
+
+/**
+ * Generic module test for serialization.
+ *
+ * @group serialization
+ */
+class GenericTest extends GenericModuleTestBase {}

core/modules/serialization/tests/src/Kernel/EntitySerializationTest.php

@@ -0,0 +1,390 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\serialization\Kernel;
+
+use Drupal\Core\Cache\CacheableDependencyInterface;
+use Drupal\Core\Cache\CacheableMetadata;
+use Drupal\entity_test\Entity\EntityTestComputedField;
+use Drupal\Component\Serialization\Json;
+use Drupal\Component\Datetime\DateTimePlus;
+use Drupal\entity_test\Entity\EntitySerializedField;
+use Drupal\entity_test\Entity\EntityTestMulRev;
+use Drupal\filter\Entity\FilterFormat;
+use Drupal\serialization\Normalizer\CacheableNormalizerInterface;
+
+/**
+ * Tests that entities can be serialized to supported core formats.
+ *
+ * @group serialization
+ */
+class EntitySerializationTest extends NormalizerTestBase {
+
+  /**
+   * Modules to install.
+   *
+   * @var array
+   */
+  protected static $modules = [
+    'serialization',
+    'system',
+    'field',
+    'entity_test',
+    'text',
+    'filter',
+    'user',
+    'entity_serialization_test',
+  ];
+
+  /**
+   * The test values.
+   *
+   * @var array
+   */
+  protected $values;
+
+  /**
+   * The test entity.
+   *
+   * @var \Drupal\Core\Entity\ContentEntityInterface
+   */
+  protected $entity;
+
+  /**
+   * The test user.
+   *
+   * @var \Drupal\user\Entity\User
+   */
+  protected $user;
+
+  /**
+   * The serializer service.
+   *
+   * @var \Symfony\Component\Serializer\Serializer
+   */
+  protected $serializer;
+
+  /**
+   * The class name of the test class.
+   *
+   * @var string
+   */
+  protected $entityClass = 'Drupal\entity_test\Entity\EntityTest';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    FilterFormat::create([
+      'format' => 'my_text_format',
+      'name' => 'My Text Format',
+      'filters' => [
+        'filter_html' => [
+          'module' => 'filter',
+          'status' => TRUE,
+          'weight' => 10,
+          'settings' => [
+            'allowed_html' => '<p>',
+          ],
+        ],
+        'filter_autop' => [
+          'module' => 'filter',
+          'status' => TRUE,
+          'weight' => 10,
+          'settings' => [],
+        ],
+      ],
+    ])->save();
+
+    // Create a test user to use as the entity owner.
+    $this->user = \Drupal::entityTypeManager()->getStorage('user')->create([
+      'name' => 'serialization_test_user',
+      'mail' => 'foo@example.com',
+      'pass' => '123456',
+    ]);
+    $this->user->save();
+
+    // Create a test entity to serialize.
+    $test_text_value = $this->randomMachineName();
+    $this->values = [
+      'name' => $this->randomMachineName(),
+      'user_id' => $this->user->id(),
+      'field_test_text' => [
+        'value' => $test_text_value,
+        'format' => 'my_text_format',
+      ],
+    ];
+    $this->entity = EntityTestMulRev::create($this->values);
+    $this->entity->save();
+
+    $this->serializer = $this->container->get('serializer');
+
+    $this->installConfig(['field']);
+  }
+
+  /**
+   * Tests the normalize function.
+   */
+  public function testNormalize(): void {
+    $expected = [
+      'id' => [
+        ['value' => 1],
+      ],
+      'uuid' => [
+        ['value' => $this->entity->uuid()],
+      ],
+      'langcode' => [
+        ['value' => 'en'],
+      ],
+      'name' => [
+        ['value' => $this->values['name']],
+      ],
+      'type' => [
+        ['value' => 'entity_test_mulrev'],
+      ],
+      'created' => [
+        [
+          'value' => (new \DateTime())->setTimestamp((int) $this->entity->get('created')->value)->setTimezone(new \DateTimeZone('UTC'))->format(\DateTime::RFC3339),
+          'format' => \DateTime::RFC3339,
+        ],
+      ],
+      'user_id' => [
+        [
+          // id() will return the string value as it comes from the database.
+          'target_id' => (int) $this->user->id(),
+          'target_type' => $this->user->getEntityTypeId(),
+          'target_uuid' => $this->user->uuid(),
+          'url' => $this->user->toUrl()->toString(),
+        ],
+      ],
+      'revision_id' => [
+        ['value' => 1],
+      ],
+      'default_langcode' => [
+        ['value' => TRUE],
+      ],
+      'revision_translation_affected' => [
+        ['value' => TRUE],
+      ],
+      'non_rev_field' => [],
+      'non_mul_field' => [],
+      'field_test_text' => [
+        [
+          'value' => $this->values['field_test_text']['value'],
+          'format' => $this->values['field_test_text']['format'],
+          'processed' => "<p>{$this->values['field_test_text']['value']}</p>",
+        ],
+      ],
+    ];
+
+    $normalized = $this->serializer->normalize($this->entity);
+
+    foreach (array_keys($expected) as $fieldName) {
+      $this->assertSame($expected[$fieldName], $normalized[$fieldName], "Normalization produces expected array for $fieldName.");
+    }
+    $this->assertEquals([], array_diff_key($normalized, $expected), 'No unexpected data is added to the normalized array.');
+  }
+
+  /**
+   * Tests user normalization with some default access controls overridden.
+   *
+   * @see entity_serialization_test.module
+   */
+  public function testUserNormalize(): void {
+    // Test password isn't available.
+    $normalized = $this->serializer->normalize($this->user);
+
+    $this->assertArrayNotHasKey('pass', $normalized);
+    $this->assertArrayNotHasKey('mail', $normalized);
+
+    // Test again using our test user, so that our access control override will
+    // allow password viewing.
+    $normalized = $this->serializer->normalize($this->user, NULL, ['account' => $this->user]);
+
+    // The key 'pass' will now exist, but the password value should be
+    // normalized to NULL.
+    $this->assertSame([NULL], $normalized['pass'], '"pass" value is normalized to [NULL]');
+  }
+
+  /**
+   * Tests entity serialization for core's formats by a registered Serializer.
+   */
+  public function testSerialize(): void {
+    // Test that Serializer responds using the ComplexDataNormalizer and
+    // JsonEncoder. The output of ComplexDataNormalizer::normalize() is tested
+    // elsewhere, so we can just assume that it works properly here.
+    $normalized = $this->serializer->normalize($this->entity, 'json');
+    $expected = Json::encode($normalized);
+    // Test 'json'.
+    $actual = $this->serializer->serialize($this->entity, 'json');
+    $this->assertSame($expected, $actual, 'Entity serializes to JSON when "json" is requested.');
+    $actual = $this->serializer->serialize($normalized, 'json');
+    $this->assertSame($expected, $actual, 'A normalized array serializes to JSON when "json" is requested');
+    // Test 'ajax'.
+    $actual = $this->serializer->serialize($this->entity, 'ajax');
+    $this->assertSame($expected, $actual, 'Entity serializes to JSON when "ajax" is requested.');
+    $actual = $this->serializer->serialize($normalized, 'ajax');
+    $this->assertSame($expected, $actual, 'A normalized array serializes to JSON when "ajax" is requested');
+
+    // Generate the expected xml in a way that allows changes to entity property
+    // order.
+    $expected_created = [
+      'value' => DateTimePlus::createFromTimestamp($this->entity->created->value, 'UTC')->format(\DateTime::RFC3339),
+      'format' => \DateTime::RFC3339,
+    ];
+
+    $expected = [
+      'id' => '<id><value>' . $this->entity->id() . '</value></id>',
+      'uuid' => '<uuid><value>' . $this->entity->uuid() . '</value></uuid>',
+      'langcode' => '<langcode><value>en</value></langcode>',
+      'name' => '<name><value>' . $this->values['name'] . '</value></name>',
+      'type' => '<type><value>entity_test_mulrev</value></type>',
+      'created' => '<created><value>' . $expected_created['value'] . '</value><format>' . $expected_created['format'] . '</format></created>',
+      'user_id' => '<user_id><target_id>' . $this->user->id() . '</target_id><target_type>' . $this->user->getEntityTypeId() . '</target_type><target_uuid>' . $this->user->uuid() . '</target_uuid><url>' . $this->user->toUrl()->toString() . '</url></user_id>',
+      'revision_id' => '<revision_id><value>' . $this->entity->getRevisionId() . '</value></revision_id>',
+      'default_langcode' => '<default_langcode><value>1</value></default_langcode>',
+      'revision_translation_affected' => '<revision_translation_affected><value>1</value></revision_translation_affected>',
+      'non_mul_field' => '<non_mul_field/>',
+      'non_rev_field' => '<non_rev_field/>',
+      'field_test_text' => '<field_test_text><value>' . $this->values['field_test_text']['value'] . '</value><format>' . $this->values['field_test_text']['format'] . '</format><processed><![CDATA[<p>' . $this->values['field_test_text']['value'] . '</p>]]></processed></field_test_text>',
+    ];
+    // Sort it in the same order as normalized.
+    $expected = array_merge($normalized, $expected);
+    // Add header and footer.
+    array_unshift($expected, '<?xml version="1.0"?>' . PHP_EOL . '<response>');
+    $expected[] = '</response>' . PHP_EOL;
+    // Reduced the array to a string.
+    $expected = implode('', $expected);
+    // Test 'xml'. The output should match that of Symfony's XmlEncoder.
+    $actual = $this->serializer->serialize($this->entity, 'xml');
+    $this->assertSame($expected, $actual);
+    $actual = $this->serializer->serialize($normalized, 'xml');
+    $this->assertSame($expected, $actual);
+  }
+
+  /**
+   * Tests denormalization of an entity.
+   */
+  public function testDenormalize(): void {
+    $normalized = $this->serializer->normalize($this->entity);
+
+    foreach (['json', 'xml'] as $type) {
+      $denormalized = $this->serializer->denormalize($normalized, $this->entityClass, $type, ['entity_type' => 'entity_test_mulrev']);
+      $this->assertInstanceOf($this->entityClass, $denormalized);
+      $this->assertSame($this->entity->getEntityTypeId(), $denormalized->getEntityTypeId(), 'Expected entity type found.');
+      $this->assertSame($this->entity->bundle(), $denormalized->bundle(), 'Expected entity bundle found.');
+      $this->assertSame($this->entity->uuid(), $denormalized->uuid(), 'Expected entity UUID found.');
+    }
+  }
+
+  /**
+   * Tests denormalizing serialized columns.
+   */
+  public function testDenormalizeSerializedItem(): void {
+    $this->expectException(\LogicException::class);
+    $this->expectExceptionMessage('The generic FieldItemNormalizer cannot denormalize string values for "value" properties of the "serialized" field (field item class: Drupal\entity_test\Plugin\Field\FieldType\SerializedItem).');
+    $this->serializer->denormalize([
+      'serialized' => [
+        [
+          'value' => 'boo',
+        ],
+      ],
+      'type' => 'entity_test_serialized_field',
+    ], EntitySerializedField::class);
+  }
+
+  /**
+   * Tests normalizing/denormalizing custom serialized columns.
+   */
+  public function testDenormalizeCustomSerializedItem(): void {
+    $entity = EntitySerializedField::create(['serialized_text' => serialize(['Hello world!'])]);
+    $normalized = $this->serializer->normalize($entity);
+    $this->assertEquals(['Hello world!'], $normalized['serialized_text'][0]['value']);
+    $this->expectException(\LogicException::class);
+    $this->expectExceptionMessage('The generic FieldItemNormalizer cannot denormalize string values for "value" properties of the "serialized_text" field (field item class: Drupal\entity_test\Plugin\Field\FieldType\SerializedPropertyItem).');
+    $this->serializer->denormalize([
+      'serialized_text' => [
+        [
+          'value' => 'boo',
+        ],
+      ],
+      'type' => 'entity_test_serialized_field',
+    ], EntitySerializedField::class);
+  }
+
+  /**
+   * Tests normalizing/denormalizing invalid custom serialized fields.
+   */
+  public function testDenormalizeInvalidCustomSerializedField(): void {
+    $entity = EntitySerializedField::create(['serialized_long' => serialize(['Hello world!'])]);
+    $normalized = $this->serializer->normalize($entity);
+    $this->assertEquals(['Hello world!'], $normalized['serialized_long'][0]['value']);
+    $this->expectException(\LogicException::class);
+    $this->expectExceptionMessage('The generic FieldItemNormalizer cannot denormalize string values for "value" properties of the "serialized_long" field (field item class: Drupal\Core\Field\Plugin\Field\FieldType\StringLongItem).');
+    $this->serializer->denormalize([
+      'serialized_long' => [
+        [
+          'value' => 'boo',
+        ],
+      ],
+      'type' => 'entity_test_serialized_field',
+    ], EntitySerializedField::class);
+  }
+
+  /**
+   * Tests normalizing/denormalizing empty custom serialized fields.
+   */
+  public function testDenormalizeEmptyCustomSerializedField(): void {
+    $entity = EntitySerializedField::create(['serialized_long' => serialize([])]);
+    $normalized = $this->serializer->normalize($entity);
+    $this->assertEquals([], $normalized['serialized_long'][0]['value']);
+
+    $entity = $this->serializer->denormalize($normalized, EntitySerializedField::class);
+
+    $this->assertEquals(serialize([]), $entity->get('serialized_long')->value);
+  }
+
+  /**
+   * Tests normalizing/denormalizing valid custom serialized fields.
+   */
+  public function testDenormalizeValidCustomSerializedField(): void {
+    $entity = EntitySerializedField::create(['serialized_long' => serialize(['key' => 'value'])]);
+    $normalized = $this->serializer->normalize($entity);
+    $this->assertEquals(['key' => 'value'], $normalized['serialized_long'][0]['value']);
+
+    $entity = $this->serializer->denormalize($normalized, EntitySerializedField::class);
+
+    $this->assertEquals(serialize(['key' => 'value']), $entity->get('serialized_long')->value);
+  }
+
+  /**
+   * Tests normalizing/denormalizing using string values.
+   */
+  public function testDenormalizeStringValue(): void {
+    $this->expectException(\LogicException::class);
+    $this->expectExceptionMessage('The generic FieldItemNormalizer cannot denormalize string values for "value" properties of the "serialized_long" field (field item class: Drupal\Core\Field\Plugin\Field\FieldType\StringLongItem).');
+    $this->serializer->denormalize([
+      'serialized_long' => ['boo'],
+      'type' => 'entity_test_serialized_field',
+    ], EntitySerializedField::class);
+  }
+
+  /**
+   * Tests normalizing cacheable computed field.
+   */
+  public function testCacheableComputedField(): void {
+    $context[CacheableNormalizerInterface::SERIALIZATION_CONTEXT_CACHEABILITY] = new CacheableMetadata();
+    $entity = EntityTestComputedField::create();
+    $normalized = $this->serializer->normalize($entity, NULL, $context);
+    $this->assertEquals('computed test cacheable string field', $normalized['computed_test_cacheable_string_field'][0]['value']);
+    $this->assertInstanceOf(CacheableDependencyInterface::class, $context[CacheableNormalizerInterface::SERIALIZATION_CONTEXT_CACHEABILITY]);
+    // See \Drupal\entity_test\Plugin\Field\ComputedTestCacheableStringItemList::computeValue().
+    $this->assertEquals($context[CacheableNormalizerInterface::SERIALIZATION_CONTEXT_CACHEABILITY]->getCacheContexts(), ['url.query_args:computed_test_cacheable_string_field']);
+    $this->assertEquals($context[CacheableNormalizerInterface::SERIALIZATION_CONTEXT_CACHEABILITY]->getCacheTags(), ['field:computed_test_cacheable_string_field']);
+    $this->assertEquals($context[CacheableNormalizerInterface::SERIALIZATION_CONTEXT_CACHEABILITY]->getCacheMaxAge(), 800);
+  }
+
+}

core/modules/serialization/tests/src/Kernel/FieldItemSerializationTest.php

@@ -0,0 +1,243 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\serialization\Kernel;
+
+use Drupal\entity_test\Entity\EntityTestMulRev;
+use Drupal\field\Entity\FieldConfig;
+use Drupal\field\Entity\FieldStorageConfig;
+use Symfony\Component\Serializer\Exception\UnexpectedValueException;
+
+/**
+ * Test field level normalization process.
+ *
+ * @group serialization
+ */
+class FieldItemSerializationTest extends NormalizerTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = [
+    'serialization',
+    'system',
+    'field',
+    'entity_test',
+    'text',
+    'filter',
+    'user',
+    'field_normalization_test',
+  ];
+
+  /**
+   * The class name of the test class.
+   *
+   * @var string
+   */
+  protected $entityClass = 'Drupal\entity_test\Entity\EntityTestMulRev';
+
+  /**
+   * The test values.
+   *
+   * @var array
+   */
+  protected $values;
+
+  /**
+   * The test entity.
+   *
+   * @var \Drupal\Core\Entity\ContentEntityBase
+   */
+  protected $entity;
+
+  /**
+   * The serializer service.
+   *
+   * @var \Symfony\Component\Serializer\Serializer
+   */
+  protected $serializer;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    // Auto-create a field for testing default field values.
+    FieldStorageConfig::create([
+      'entity_type' => 'entity_test_mulrev',
+      'field_name' => 'field_test_text_default',
+      'type' => 'text',
+      'cardinality' => 1,
+      'translatable' => FALSE,
+    ])->save();
+    FieldConfig::create([
+      'entity_type' => 'entity_test_mulrev',
+      'field_name' => 'field_test_text_default',
+      'bundle' => 'entity_test_mulrev',
+      'label' => 'Test text-field with default',
+      'default_value' => [
+        [
+          'value' => 'This is the default',
+          'format' => 'full_html',
+        ],
+      ],
+      'widget' => [
+        'type' => 'text_textfield',
+        'weight' => 0,
+      ],
+    ])->save();
+    FieldStorageConfig::create([
+      'entity_type' => 'entity_test_mulrev',
+      'field_name' => 'field_test_boolean',
+      'type' => 'boolean',
+      'cardinality' => 1,
+      'translatable' => FALSE,
+    ])->save();
+    FieldConfig::create([
+      'entity_type' => 'entity_test_mulrev',
+      'field_name' => 'field_test_boolean',
+      'bundle' => 'entity_test_mulrev',
+      'label' => 'Test boolean',
+    ])->save();
+
+    // Create a test entity to serialize.
+    $this->values = [
+      'name' => $this->randomMachineName(),
+      'field_test_text' => [
+        'value' => $this->randomMachineName(),
+        'format' => 'full_html',
+      ],
+      'field_test_boolean' => [
+        'value' => FALSE,
+      ],
+    ];
+    $this->entity = EntityTestMulRev::create($this->values);
+    $this->entity->save();
+
+    $this->serializer = $this->container->get('serializer');
+
+    $this->installConfig(['field']);
+  }
+
+  /**
+   * Tests normalizing and denormalizing an entity with field item normalizer.
+   */
+  public function testFieldNormalizeDenormalize(): void {
+    $normalized = $this->serializer->normalize($this->entity, 'json');
+
+    $expected_field_value = $this->entity->field_test_text[0]->getValue()['value'] . '::silly_suffix';
+    $this->assertEquals($expected_field_value, $normalized['field_test_text'][0]['value'], 'Text field item normalized');
+    $denormalized = $this->serializer->denormalize($normalized, $this->entityClass, 'json');
+
+    $this->assertEquals($denormalized->field_test_text[0]->getValue(), $this->entity->field_test_text[0]->getValue(), 'Text field item denormalized.');
+    $this->assertEquals($denormalized->field_test_text_default[0]->getValue(), $this->entity->field_test_text_default[0]->getValue(), 'Text field item with default denormalized.');
+
+    // Unset the values for text field that has a default value.
+    unset($normalized['field_test_text_default']);
+    $denormalized_without_all_fields = $this->serializer->denormalize($normalized, $this->entityClass, 'json');
+    // Check that denormalized entity is still the same even if not all fields
+    // are not provided.
+    $this->assertEquals($denormalized_without_all_fields->field_test_text[0]->getValue(), $this->entity->field_test_text[0]->getValue(), 'Text field item denormalized.');
+    // Even though field_test_text_default value was unset before
+    // denormalization it should still have the default values for the field.
+    $this->assertEquals($denormalized_without_all_fields->field_test_text_default[0]->getValue(), $this->entity->field_test_text_default[0]->getValue(), 'Text field item with default denormalized.');
+  }
+
+  /**
+   * Tests denormalizing using a scalar field value.
+   */
+  public function testFieldDenormalizeWithScalarValue(): void {
+    $this->expectException(UnexpectedValueException::class);
+    $this->expectExceptionMessage('Field values for "uuid" must use an array structure');
+
+    $normalized = $this->serializer->normalize($this->entity, 'json');
+
+    // Change the UUID value to use the UUID directly. No array structure.
+    $normalized['uuid'] = $normalized['uuid'][0]['value'];
+
+    $this->serializer->denormalize($normalized, $this->entityClass, 'json');
+  }
+
+  /**
+   * Tests a format-agnostic normalizer.
+   *
+   * @param string[] $test_modules
+   *   The test modules to install.
+   * @param string $format
+   *   The format to test. (NULL results in the format-agnostic normalization.)
+   *
+   * @dataProvider providerTestCustomBooleanNormalization
+   */
+  public function testCustomBooleanNormalization(array $test_modules, $format): void {
+    // Asserts the entity contains the value we set.
+    $this->assertFalse($this->entity->field_test_boolean->value);
+
+    // Asserts normalizing the entity using core's 'serializer' service DOES
+    // yield the value we set.
+    $core_normalization = $this->container->get('serializer')->normalize($this->entity, $format);
+    $this->assertFalse($core_normalization['field_test_boolean'][0]['value']);
+
+    $assert_denormalization = function (array $normalization) use ($format) {
+      $denormalized_entity = $this->container->get('serializer')->denormalize($normalization, EntityTestMulRev::class, $format, []);
+      $this->assertInstanceOf(EntityTestMulRev::class, $denormalized_entity);
+      $this->assertTrue($denormalized_entity->field_test_boolean->value);
+    };
+
+    // Asserts denormalizing the entity DOES yield the value we set:
+    // - when using the detailed representation
+    $core_normalization['field_test_boolean'][0]['value'] = TRUE;
+    $assert_denormalization($core_normalization);
+    // - and when using the shorthand representation
+    $core_normalization['field_test_boolean'][0] = TRUE;
+    $assert_denormalization($core_normalization);
+
+    // Install test module that contains a high-priority alternative normalizer.
+    $this->enableModules($test_modules);
+
+    // Asserts normalizing the entity DOES NOT ANYMORE yield the value we set.
+    $core_normalization = $this->container->get('serializer')->normalize($this->entity, $format);
+    $this->assertSame('👎', $core_normalization['field_test_boolean'][0]['value']);
+
+    // Asserts denormalizing the entity DOES NOT ANYMORE yield the value we set:
+    // - when using the detailed representation
+    $core_normalization['field_test_boolean'][0]['value'] = '👍';
+    $assert_denormalization($core_normalization);
+    // - and when using the shorthand representation
+    $core_normalization['field_test_boolean'][0] = '👍';
+    $assert_denormalization($core_normalization);
+  }
+
+  /**
+   * Data provider.
+   *
+   * @return array
+   *   Test cases.
+   */
+  public static function providerTestCustomBooleanNormalization() {
+    return [
+      'Format-agnostic @FieldType-level normalizers SHOULD be able to affect the format-agnostic normalization' => [
+        ['test_fieldtype_boolean_emoji_normalizer'],
+        NULL,
+      ],
+      'Format-agnostic @DataType-level normalizers SHOULD be able to affect the format-agnostic normalization' => [
+        ['test_datatype_boolean_emoji_normalizer'],
+        NULL,
+      ],
+      'Format-agnostic @FieldType-level normalizers SHOULD be able to affect the JSON normalization' => [
+        ['test_fieldtype_boolean_emoji_normalizer'],
+        'json',
+      ],
+      'Format-agnostic @DataType-level normalizers SHOULD be able to affect the JSON normalization' => [
+        ['test_datatype_boolean_emoji_normalizer'],
+        'json',
+      ],
+      'Format-agnostic @FieldType-level normalizers SHOULD be able to affect the XML normalization' => [
+        ['test_fieldtype_boolean_emoji_normalizer'],
+        'xml',
+      ],
+    ];
+  }
+
+}

core/modules/serialization/tests/src/Kernel/MapDataNormalizerTest.php

@@ -0,0 +1,138 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\serialization\Kernel;
+
+use Drupal\Core\TypedData\DataDefinition;
+use Drupal\Core\TypedData\MapDataDefinition;
+use Drupal\KernelTests\KernelTestBase;
+
+/**
+ * @group TypedData
+ */
+class MapDataNormalizerTest extends KernelTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = ['system', 'serialization'];
+
+  /**
+   * The serializer service.
+   *
+   * @var \Symfony\Component\Serializer\Serializer
+   */
+  protected $serializer;
+
+  /**
+   * The typed data manager.
+   *
+   * @var \Drupal\Core\TypedData\TypedDataManagerInterface
+   */
+  protected $typedDataManager;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+    $this->serializer = \Drupal::service('serializer');
+    $this->typedDataManager = \Drupal::typedDataManager();
+  }
+
+  /**
+   * Tests whether map data can be normalized.
+   */
+  public function testMapNormalize(): void {
+    $typed_data = $this->buildExampleTypedData();
+    $data = $this->serializer->normalize($typed_data, 'json');
+    $expect_value = [
+      'key1' => 'value1',
+      'key2' => 'value2',
+      'key3' => 3,
+      'key4' => [
+        0 => TRUE,
+        1 => 'value6',
+        'key7' => 'value7',
+      ],
+    ];
+    $this->assertSame($expect_value, $data);
+  }
+
+  /**
+   * Tests whether map data with properties can be normalized.
+   */
+  public function testMapWithPropertiesNormalize(): void {
+    $typed_data = $this->buildExampleTypedDataWithProperties();
+    $data = $this->serializer->normalize($typed_data, 'json');
+    $expect_value = [
+      'key1' => 'value1',
+      'key2' => 'value2',
+      'key3' => 3,
+      'key4' => [
+        0 => TRUE,
+        1 => 'value6',
+        'key7' => 'value7',
+      ],
+    ];
+    $this->assertSame($expect_value, $data);
+  }
+
+  /**
+   * Builds some example typed data object with no properties.
+   */
+  protected function buildExampleTypedData() {
+    $tree = [
+      'key1' => 'value1',
+      'key2' => 'value2',
+      'key3' => 3,
+      'key4' => [
+        0 => TRUE,
+        1 => 'value6',
+        'key7' => 'value7',
+      ],
+    ];
+    $map_data_definition = MapDataDefinition::create();
+    $typed_data = $this->typedDataManager->create(
+      $map_data_definition,
+      $tree,
+      'test name'
+    );
+    return $typed_data;
+  }
+
+  /**
+   * Builds some example typed data object with properties.
+   */
+  protected function buildExampleTypedDataWithProperties() {
+    $tree = [
+      'key1' => 'value1',
+      'key2' => 'value2',
+      'key3' => 3,
+      'key4' => [
+        0 => TRUE,
+        1 => 'value6',
+        'key7' => 'value7',
+      ],
+    ];
+    $map_data_definition = MapDataDefinition::create()
+      ->setPropertyDefinition('key1', DataDefinition::create('string'))
+      ->setPropertyDefinition('key2', DataDefinition::create('string'))
+      ->setPropertyDefinition('key3', DataDefinition::create('integer'))
+      ->setPropertyDefinition('key4', MapDataDefinition::create()
+        ->setPropertyDefinition(0, DataDefinition::create('boolean'))
+        ->setPropertyDefinition(1, DataDefinition::create('string'))
+        ->setPropertyDefinition('key7', DataDefinition::create('string'))
+    );
+
+    $typed_data = $this->typedDataManager->create(
+      $map_data_definition,
+      $tree,
+      'test name'
+    );
+
+    return $typed_data;
+  }
+
+}

core/modules/serialization/tests/src/Kernel/NormalizerTestBase.php

@@ -0,0 +1,62 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\serialization\Kernel;
+
+use Drupal\KernelTests\KernelTestBase;
+use Drupal\field\Entity\FieldConfig;
+use Drupal\field\Entity\FieldStorageConfig;
+
+/**
+ * Helper base class to set up some test fields for serialization testing.
+ */
+abstract class NormalizerTestBase extends KernelTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = [
+    'serialization',
+    'system',
+    'field',
+    'entity_test',
+    'text',
+    'filter',
+    'user',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $this->installEntitySchema('entity_test_mulrev');
+    $this->installEntitySchema('user');
+    $this->installConfig(['field']);
+    \Drupal::moduleHandler()->invoke('rest', 'install');
+
+    // Auto-create a field for testing.
+    FieldStorageConfig::create([
+      'entity_type' => 'entity_test_mulrev',
+      'field_name' => 'field_test_text',
+      'type' => 'text',
+      'cardinality' => 1,
+      'translatable' => FALSE,
+    ])->save();
+    FieldConfig::create([
+      'entity_type' => 'entity_test_mulrev',
+      'field_name' => 'field_test_text',
+      'bundle' => 'entity_test_mulrev',
+      'label' => 'Test text-field',
+      'widget' => [
+        'type' => 'text_textfield',
+        'weight' => 0,
+      ],
+    ])->save();
+  }
+
+}

core/modules/serialization/tests/src/Kernel/SerializationTest.php

@@ -0,0 +1,60 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\serialization\Kernel;
+
+use Drupal\KernelTests\KernelTestBase;
+use Symfony\Component\Serializer\Exception\UnexpectedValueException;
+
+/**
+ * Functional tests for serialization system.
+ *
+ * @group serialization
+ */
+class SerializationTest extends KernelTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = ['serialization', 'serialization_test'];
+
+  /**
+   * The serializer service to test.
+   *
+   * @var \Symfony\Component\Serializer\SerializerInterface
+   */
+  protected $serializer;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+    $this->serializer = $this->container->get('serializer');
+  }
+
+  /**
+   * Confirms that modules can register normalizers and encoders.
+   */
+  public function testSerializerComponentRegistration(): void {
+    $object = new \stdClass();
+    $format = 'serialization_test';
+    $expected = 'Normalized by SerializationTestNormalizer, Encoded by SerializationTestEncoder';
+
+    // Ensure the serialization invokes the expected normalizer and encoder.
+    $this->assertSame($expected, $this->serializer->serialize($object, $format));
+
+    // Ensure the serialization fails for an unsupported format.
+    try {
+      $this->serializer->serialize($object, 'unsupported_format');
+      $this->fail('The serializer was expected to throw an exception for an unsupported format, but did not.');
+    }
+    catch (UnexpectedValueException $e) {
+      // Expected exception; just continue testing.
+    }
+  }
+
+}

core/modules/serialization/tests/src/Unit/CompilerPass/RegisterSerializationClassesCompilerPassTest.php

@@ -0,0 +1,60 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\serialization\Unit\CompilerPass;
+
+use Drupal\Core\DependencyInjection\ContainerBuilder;
+use Drupal\serialization\RegisterSerializationClassesCompilerPass;
+use Drupal\Tests\UnitTestCase;
+use Symfony\Component\DependencyInjection\Definition;
+use Symfony\Component\Serializer\Serializer;
+
+/**
+ * @coversDefaultClass \Drupal\serialization\RegisterSerializationClassesCompilerPass
+ * @group serialization
+ */
+class RegisterSerializationClassesCompilerPassTest extends UnitTestCase {
+
+  /**
+   * @covers ::process
+   */
+  public function testEncoders(): void {
+    $container = new ContainerBuilder();
+
+    $serializer_definition = new Definition(Serializer::class, [[], []]);
+    $serializer_definition->setPublic(TRUE);
+    $container->setDefinition('serializer', $serializer_definition);
+
+    $encoder_1_definition = new Definition('TestClass');
+    $encoder_1_definition->addTag('encoder', ['format' => 'xml']);
+    $encoder_1_definition->addTag('_provider', ['provider' => 'test_provider_a']);
+    $encoder_1_definition->setPublic(TRUE);
+    $container->setDefinition('encoder_1', $encoder_1_definition);
+
+    $encoder_2_definition = new Definition('TestClass');
+    $encoder_2_definition->addTag('encoder', ['format' => 'json']);
+    $encoder_2_definition->addTag('_provider', ['provider' => 'test_provider_a']);
+    $encoder_2_definition->setPublic(TRUE);
+    $container->setDefinition('encoder_2', $encoder_2_definition);
+
+    $normalizer_1_definition = new Definition('TestClass');
+    $normalizer_1_definition->addTag('normalizer');
+    $normalizer_1_definition->setPublic(TRUE);
+    $container->setDefinition('normalizer_1', $normalizer_1_definition);
+
+    $compiler_pass = new RegisterSerializationClassesCompilerPass();
+    $compiler_pass->process($container);
+
+    // Check registration of formats and providers.
+    $this->assertEquals(['xml', 'json'], $container->getParameter('serializer.formats'));
+    $this->assertEquals(['xml' => 'test_provider_a', 'json' => 'test_provider_a'], $container->getParameter('serializer.format_providers'));
+
+    // Check all encoder and normalizer service definitions are marked private.
+    $this->assertFalse($encoder_1_definition->isPublic());
+    $this->assertFalse($encoder_2_definition->isPublic());
+
+    $this->assertFalse($normalizer_1_definition->isPublic());
+  }
+
+}

core/modules/serialization/tests/src/Unit/Encoder/JsonEncoderTest.php

@@ -0,0 +1,27 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\serialization\Unit\Encoder;
+
+use Drupal\serialization\Encoder\JsonEncoder;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * @coversDefaultClass \Drupal\serialization\Encoder\JsonEncoder
+ * @group serialization
+ */
+class JsonEncoderTest extends UnitTestCase {
+
+  /**
+   * Tests the supportsEncoding() method.
+   */
+  public function testSupportsEncoding(): void {
+    $encoder = new JsonEncoder();
+
+    $this->assertTrue($encoder->supportsEncoding('json'));
+    $this->assertTrue($encoder->supportsEncoding('ajax'));
+    $this->assertFalse($encoder->supportsEncoding('xml'));
+  }
+
+}

core/modules/serialization/tests/src/Unit/Encoder/XmlEncoderTest.php

@@ -0,0 +1,112 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\serialization\Unit\Encoder;
+
+use Drupal\serialization\Encoder\XmlEncoder;
+use Symfony\Component\Serializer\Encoder\XmlEncoder as BaseXmlEncoder;
+use Symfony\Component\Serializer\Normalizer\GetSetMethodNormalizer;
+use Symfony\Component\Serializer\Serializer;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * @coversDefaultClass \Drupal\serialization\Encoder\XmlEncoder
+ * @group serialization
+ */
+class XmlEncoderTest extends UnitTestCase {
+
+  /**
+   * The XmlEncoder instance.
+   *
+   * @var \Drupal\serialization\Encoder\XmlEncoder
+   */
+  protected $encoder;
+
+  /**
+   * @var \Symfony\Component\Serializer\Encoder\XmlEncoder|\PHPUnit\Framework\MockObject\MockObject
+   */
+  protected $baseEncoder;
+
+  /**
+   * An array of test data.
+   *
+   * @var array
+   */
+  protected $testArray = ['test' => 'test'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $this->baseEncoder = $this->createMock(BaseXmlEncoder::class);
+    $this->encoder = new XmlEncoder();
+    $this->encoder->setBaseEncoder($this->baseEncoder);
+  }
+
+  /**
+   * Tests the supportsEncoding() method.
+   */
+  public function testSupportsEncoding(): void {
+    $this->assertTrue($this->encoder->supportsEncoding('xml'));
+    $this->assertFalse($this->encoder->supportsEncoding('json'));
+  }
+
+  /**
+   * Tests the supportsDecoding() method.
+   */
+  public function testSupportsDecoding(): void {
+    $this->assertTrue($this->encoder->supportsDecoding('xml'));
+    $this->assertFalse($this->encoder->supportsDecoding('json'));
+  }
+
+  /**
+   * Tests the encode() method.
+   */
+  public function testEncode(): void {
+    $this->baseEncoder->expects($this->once())
+      ->method('encode')
+      ->with($this->testArray, 'test', [])
+      ->willReturn('test');
+
+    $this->assertEquals('test', $this->encoder->encode($this->testArray, 'test'));
+  }
+
+  /**
+   * Tests the decode() method.
+   */
+  public function testDecode(): void {
+    $this->baseEncoder->expects($this->once())
+      ->method('decode')
+      ->with('test', 'test', [])
+      ->willReturn($this->testArray);
+
+    $this->assertEquals($this->testArray, $this->encoder->decode('test', 'test'));
+  }
+
+  /**
+   * @covers ::getBaseEncoder
+   */
+  public function testDefaultEncoderHasSerializer(): void {
+    // The serializer should be set on the Drupal encoder, which should then
+    // set it on our default encoder.
+    $encoder = new XmlEncoder();
+    $serializer = new Serializer([new GetSetMethodNormalizer()]);
+    $encoder->setSerializer($serializer);
+    $base_encoder = $encoder->getBaseEncoder();
+    $this->assertInstanceOf(BaseXmlEncoder::class, $base_encoder);
+    // Test the encoder.
+    $base_encoder->encode(['a' => new TestObject()], 'xml');
+  }
+
+}
+
+class TestObject {
+
+  public function getA() {
+    return 'A';
+  }
+
+}

core/modules/serialization/tests/src/Unit/EntityResolver/ChainEntityResolverTest.php

@@ -0,0 +1,157 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\serialization\Unit\EntityResolver;
+
+use Drupal\Tests\UnitTestCase;
+use Drupal\serialization\EntityResolver\ChainEntityResolver;
+
+/**
+ * @coversDefaultClass \Drupal\serialization\EntityResolver\ChainEntityResolver
+ * @group serialization
+ */
+class ChainEntityResolverTest extends UnitTestCase {
+
+  /**
+   * A mocked normalizer.
+   *
+   * @var \Symfony\Component\Serializer\Normalizer\NormalizerInterface|\PHPUnit\Framework\MockObject\MockObject
+   */
+  protected $testNormalizer;
+
+  /**
+   * Test data passed to the resolve method.
+   *
+   * @var object
+   */
+  protected $testData;
+
+  /**
+   * A test entity type.
+   *
+   * @var string
+   */
+  protected $testEntityType = 'test_type';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $this->testNormalizer = $this->createMock('Symfony\Component\Serializer\Normalizer\NormalizerInterface');
+    $this->testData = new \stdClass();
+  }
+
+  /**
+   * Tests the resolve method with no matching resolvers.
+   *
+   * @covers ::__construct
+   * @covers ::resolve
+   */
+  public function testResolverWithNoneResolved(): void {
+    $resolvers = [
+      $this->createEntityResolverMock(),
+      $this->createEntityResolverMock(),
+    ];
+
+    $resolver = new ChainEntityResolver($resolvers);
+
+    $this->assertNull($resolver->resolve($this->testNormalizer, $this->testData, $this->testEntityType));
+  }
+
+  /**
+   * Tests the resolve method with no matching resolvers, using addResolver.
+   *
+   * @covers ::addResolver
+   * @covers ::resolve
+   */
+  public function testResolverWithNoneResolvedUsingAddResolver(): void {
+    $resolver = new ChainEntityResolver();
+    $resolver->addResolver($this->createEntityResolverMock());
+    $resolver->addResolver($this->createEntityResolverMock());
+
+    $this->assertNull($resolver->resolve($this->testNormalizer, $this->testData, $this->testEntityType));
+  }
+
+  /**
+   * Tests the resolve method with a matching resolver first.
+   *
+   * @covers ::__construct
+   * @covers ::resolve
+   */
+  public function testResolverWithFirstResolved(): void {
+    $resolvers = [
+      $this->createEntityResolverMock(10),
+      $this->createEntityResolverMock(NULL, FALSE),
+    ];
+
+    $resolver = new ChainEntityResolver($resolvers);
+
+    $this->assertSame(10, $resolver->resolve($this->testNormalizer, $this->testData, $this->testEntityType));
+  }
+
+  /**
+   * Tests the resolve method with a matching resolver last.
+   *
+   * @covers ::__construct
+   * @covers ::resolve
+   */
+  public function testResolverWithLastResolved(): void {
+    $resolvers = [
+      $this->createEntityResolverMock(),
+      $this->createEntityResolverMock(10),
+    ];
+
+    $resolver = new ChainEntityResolver($resolvers);
+
+    $this->assertSame(10, $resolver->resolve($this->testNormalizer, $this->testData, $this->testEntityType));
+  }
+
+  /**
+   * Tests the resolve method where one resolver returns 0.
+   *
+   * @covers ::__construct
+   * @covers ::resolve
+   */
+  public function testResolverWithResolvedToZero(): void {
+    $resolvers = [
+      $this->createEntityResolverMock(0),
+      $this->createEntityResolverMock(NULL, FALSE),
+    ];
+
+    $resolver = new ChainEntityResolver($resolvers);
+
+    $this->assertSame(0, $resolver->resolve($this->testNormalizer, $this->testData, $this->testEntityType));
+  }
+
+  /**
+   * Creates a mock entity resolver.
+   *
+   * @param null|int $return
+   *   Whether the mocked resolve method should return TRUE or FALSE.
+   * @param bool $called
+   *   Whether or not the resolve method is expected to be called.
+   *
+   * @return \Drupal\serialization\EntityResolver\EntityResolverInterface|\PHPUnit\Framework\MockObject\MockObject
+   *   The mocked entity resolver.
+   */
+  protected function createEntityResolverMock($return = NULL, $called = TRUE) {
+    $mock = $this->createMock('Drupal\serialization\EntityResolver\EntityResolverInterface');
+
+    if ($called) {
+      $mock->expects($this->once())
+        ->method('resolve')
+        ->with($this->testNormalizer, $this->testData, $this->testEntityType)
+        ->willReturn($return);
+    }
+    else {
+      $mock->expects($this->never())
+        ->method('resolve');
+    }
+
+    return $mock;
+  }
+
+}

core/modules/serialization/tests/src/Unit/EntityResolver/UuidResolverTest.php

@@ -0,0 +1,112 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\serialization\Unit\EntityResolver;
+
+use Drupal\Core\Entity\EntityRepositoryInterface;
+use Drupal\Tests\UnitTestCase;
+use Drupal\serialization\EntityResolver\UuidResolver;
+
+/**
+ * @coversDefaultClass \Drupal\serialization\EntityResolver\UuidResolver
+ * @group serialization
+ */
+class UuidResolverTest extends UnitTestCase {
+
+  /**
+   * The UuidResolver instance.
+   *
+   * @var \Drupal\serialization\EntityResolver\UuidResolver
+   */
+  protected $resolver;
+
+  /**
+   * The mock entity repository service.
+   *
+   * @var \Drupal\Core\Entity\EntityRepositoryInterface|\PHPUnit\Framework\MockObject\MockObject
+   */
+  protected $entityRepository;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $this->entityRepository = $this->createMock(EntityRepositoryInterface::class);
+
+    $this->resolver = new UuidResolver($this->entityRepository);
+  }
+
+  /**
+   * Tests resolve() with a class using the incorrect interface.
+   */
+  public function testResolveNotInInterface(): void {
+    $this->entityRepository->expects($this->never())
+      ->method('loadEntityByUuid');
+
+    $normalizer = $this->createMock('Symfony\Component\Serializer\Normalizer\NormalizerInterface');
+    $this->assertNull($this->resolver->resolve($normalizer, [], 'test_type'));
+  }
+
+  /**
+   * Tests resolve() with a class using the correct interface but no UUID.
+   */
+  public function testResolveNoUuid(): void {
+    $this->entityRepository->expects($this->never())
+      ->method('loadEntityByUuid');
+
+    $normalizer = $this->createMock('Drupal\serialization\EntityResolver\UuidReferenceInterface');
+    $normalizer->expects($this->once())
+      ->method('getUuid')
+      ->with([])
+      ->willReturn(NULL);
+    $this->assertNull($this->resolver->resolve($normalizer, [], 'test_type'));
+  }
+
+  /**
+   * Tests resolve() with correct interface but no matching entity for the UUID.
+   */
+  public function testResolveNoEntity(): void {
+    $uuid = '392eab92-35c2-4625-872d-a9dab4da008e';
+
+    $this->entityRepository->expects($this->once())
+      ->method('loadEntityByUuid')
+      ->with('test_type')
+      ->willReturn(NULL);
+
+    $normalizer = $this->createMock('Drupal\serialization\EntityResolver\UuidReferenceInterface');
+    $normalizer->expects($this->once())
+      ->method('getUuid')
+      ->with([])
+      ->willReturn($uuid);
+
+    $this->assertNull($this->resolver->resolve($normalizer, [], 'test_type'));
+  }
+
+  /**
+   * Tests resolve() when a UUID corresponds to an entity.
+   */
+  public function testResolveWithEntity(): void {
+    $uuid = '392eab92-35c2-4625-872d-a9dab4da008e';
+
+    $entity = $this->createMock('Drupal\Core\Entity\EntityInterface');
+    $entity->expects($this->once())
+      ->method('id')
+      ->willReturn(1);
+
+    $this->entityRepository->expects($this->once())
+      ->method('loadEntityByUuid')
+      ->with('test_type', $uuid)
+      ->willReturn($entity);
+
+    $normalizer = $this->createMock('Drupal\serialization\EntityResolver\UuidReferenceInterface');
+    $normalizer->expects($this->once())
+      ->method('getUuid')
+      ->with([])
+      ->willReturn($uuid);
+    $this->assertSame(1, $this->resolver->resolve($normalizer, [], 'test_type'));
+  }
+
+}

core/modules/serialization/tests/src/Unit/EventSubscriber/DefaultExceptionSubscriberTest.php

@@ -0,0 +1,44 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\serialization\Unit\EventSubscriber;
+
+use Drupal\serialization\Encoder\JsonEncoder;
+use Drupal\serialization\EventSubscriber\DefaultExceptionSubscriber;
+use Drupal\Tests\UnitTestCase;
+use Symfony\Component\HttpFoundation\Request;
+use Symfony\Component\HttpFoundation\Response;
+use Symfony\Component\HttpKernel\Event\ExceptionEvent;
+use Symfony\Component\HttpKernel\Exception\MethodNotAllowedHttpException;
+use Symfony\Component\HttpKernel\HttpKernelInterface;
+use Symfony\Component\Serializer\Serializer;
+
+/**
+ * @coversDefaultClass \Drupal\serialization\EventSubscriber\DefaultExceptionSubscriber
+ * @group serialization
+ */
+class DefaultExceptionSubscriberTest extends UnitTestCase {
+
+  /**
+   * @covers ::on4xx
+   */
+  public function testOn4xx(): void {
+    $kernel = $this->prophesize(HttpKernelInterface::class);
+    $request = Request::create('/test');
+    $request->setRequestFormat('json');
+
+    $e = new MethodNotAllowedHttpException(['POST', 'PUT'], 'test message');
+    $event = new ExceptionEvent($kernel->reveal(), $request, HttpKernelInterface::MAIN_REQUEST, $e);
+    $subscriber = new DefaultExceptionSubscriber(new Serializer([], [new JsonEncoder()]), []);
+    $subscriber->on4xx($event);
+    $response = $event->getResponse();
+
+    $this->assertInstanceOf(Response::class, $response);
+    $this->assertEquals('{"message":"test message"}', $response->getContent());
+    $this->assertEquals(405, $response->getStatusCode());
+    $this->assertEquals('POST, PUT', $response->headers->get('Allow'));
+    $this->assertEquals('application/json', $response->headers->get('Content-Type'));
+  }
+
+}

core/modules/serialization/tests/src/Unit/Normalizer/ComplexDataNormalizerTest.php

@@ -0,0 +1,128 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\serialization\Unit\Normalizer;
+
+use Drupal\Core\TypedData\ComplexDataInterface;
+use Drupal\serialization\Normalizer\ComplexDataNormalizer;
+use Drupal\Tests\UnitTestCase;
+use Symfony\Component\Serializer\Serializer;
+
+/**
+ * @coversDefaultClass \Drupal\serialization\Normalizer\ComplexDataNormalizer
+ * @group serialization
+ */
+class ComplexDataNormalizerTest extends UnitTestCase {
+
+  use InternalTypedDataTestTrait;
+
+  /**
+   * Test format string.
+   *
+   * @var string
+   */
+  const TEST_FORMAT = 'test_format';
+
+  /**
+   * The Complex data normalizer under test.
+   *
+   * @var \Drupal\serialization\Normalizer\ComplexDataNormalizer
+   */
+  protected $normalizer;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $this->normalizer = new ComplexDataNormalizer();
+  }
+
+  /**
+   * @covers ::supportsNormalization
+   */
+  public function testSupportsNormalization(): void {
+    $complex_data = $this->prophesize(ComplexDataInterface::class)->reveal();
+    $this->assertTrue($this->normalizer->supportsNormalization($complex_data));
+    // Also test that an object not implementing ComplexDataInterface fails.
+    $this->assertFalse($this->normalizer->supportsNormalization(new \stdClass()));
+  }
+
+  /**
+   * Tests normalizing complex data.
+   *
+   * @covers ::normalize
+   */
+  public function testNormalizeComplexData(): void {
+    $serializer_prophecy = $this->prophesize(Serializer::class);
+
+    $non_internal_property = $this->getTypedDataProperty(FALSE);
+
+    $serializer_prophecy->normalize($non_internal_property, static::TEST_FORMAT, [])
+      ->willReturn('A-normalized')
+      ->shouldBeCalled();
+
+    $this->normalizer->setSerializer($serializer_prophecy->reveal());
+
+    $complex_data = $this->prophesize(ComplexDataInterface::class);
+    $complex_data->getProperties(TRUE)
+      ->willReturn([
+        'prop:a' => $non_internal_property,
+        'prop:internal' => $this->getTypedDataProperty(TRUE),
+      ])
+      ->shouldBeCalled();
+
+    $normalized = $this->normalizer->normalize($complex_data->reveal(), static::TEST_FORMAT);
+    $this->assertEquals(['prop:a' => 'A-normalized'], $normalized);
+  }
+
+  /**
+   * Tests normalize() where $object does not implement ComplexDataInterface.
+   *
+   * Normalizers extending ComplexDataNormalizer may have a different supported
+   * class.
+   *
+   * @covers ::normalize
+   */
+  public function testNormalizeNonComplex(): void {
+    $normalizer = new TestExtendedNormalizer();
+    $serialization_context = ['test' => 'test'];
+
+    $serializer_prophecy = $this->prophesize(Serializer::class);
+    $serializer_prophecy->normalize('A', static::TEST_FORMAT, $serialization_context)
+      ->willReturn('A-normalized')
+      ->shouldBeCalled();
+    $serializer_prophecy->normalize('B', static::TEST_FORMAT, $serialization_context)
+      ->willReturn('B-normalized')
+      ->shouldBeCalled();
+
+    $normalizer->setSerializer($serializer_prophecy->reveal());
+
+    $stdClass = new \stdClass();
+    $stdClass->a = 'A';
+    $stdClass->b = 'B';
+
+    $normalized = $normalizer->normalize($stdClass, static::TEST_FORMAT, $serialization_context);
+    $this->assertEquals(['a' => 'A-normalized', 'b' => 'B-normalized'], $normalized);
+
+  }
+
+}
+
+/**
+ * Test normalizer with a different supported class.
+ */
+class TestExtendedNormalizer extends ComplexDataNormalizer {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getSupportedTypes(?string $format): array {
+    return [
+      \stdClass::class => TRUE,
+    ];
+  }
+
+}

core/modules/serialization/tests/src/Unit/Normalizer/ConfigEntityNormalizerTest.php

@@ -0,0 +1,109 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\serialization\Unit\Normalizer;
+
+use Drupal\Core\Config\Entity\ConfigEntityTypeInterface;
+use Drupal\Core\Entity\EntityFieldManagerInterface;
+use Drupal\Core\Entity\EntityStorageInterface;
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Drupal\Core\Entity\EntityTypeRepositoryInterface;
+use Drupal\serialization\Normalizer\ConfigEntityNormalizer;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * @coversDefaultClass \Drupal\serialization\Normalizer\ConfigEntityNormalizer
+ * @group serialization
+ */
+class ConfigEntityNormalizerTest extends UnitTestCase {
+
+  /**
+   * Tests the normalize() method.
+   *
+   * @covers ::normalize
+   */
+  public function testNormalize(): void {
+    $test_export_properties = [
+      'test' => 'test',
+      '_core' => [
+        'default_config_hash' => $this->randomMachineName(),
+        $this->randomMachineName() => 'some random key',
+      ],
+    ];
+
+    $entity_field_manager = $this->createMock(EntityFieldManagerInterface::class);
+    $entity_type_manager = $this->createMock(EntityTypeManagerInterface::class);
+    $entity_type_repository = $this->createMock(EntityTypeRepositoryInterface::class);
+    $normalizer = new ConfigEntityNormalizer(
+      $entity_type_manager,
+      $entity_type_repository,
+      $entity_field_manager
+    );
+
+    $config_entity = $this->createMock('Drupal\Core\Config\Entity\ConfigEntityInterface');
+    $config_entity->expects($this->once())
+      ->method('toArray')
+      ->willReturn($test_export_properties);
+
+    $this->assertSame(['test' => 'test'], $normalizer->normalize($config_entity));
+  }
+
+  /**
+   * @covers ::denormalize
+   */
+  public function testDenormalize(): void {
+    $test_value = $this->randomMachineName();
+    $data = [
+      'test' => $test_value,
+      '_core' => [
+        'default_config_hash' => $this->randomMachineName(),
+        $this->randomMachineName() => 'some random key',
+      ],
+    ];
+
+    $expected_storage_data = [
+      'test' => $test_value,
+    ];
+
+    // Mock of the entity storage, to test our expectation that the '_core' key
+    // never makes it to that point, thanks to the denormalizer omitting it.
+    $entity_storage = $this->prophesize(EntityStorageInterface::class);
+    $entity_storage->create($expected_storage_data)
+      ->shouldBeCalled()
+      ->will(function ($args) {
+        $entity = new \stdClass();
+        $entity->received_data = $args[0];
+        return $entity;
+      });
+
+    // Stubs for the denormalizer going from entity type manager to entity
+    // storage.
+    $entity_type_id = $this->randomMachineName();
+    $entity_type_class = $this->randomMachineName();
+    $entity_type_manager = $this->prophesize(EntityTypeManagerInterface::class);
+    $entity_type_manager->getDefinition($entity_type_id, FALSE)
+      ->willReturn($this->prophesize(ConfigEntityTypeInterface::class)->reveal());
+    $entity_type_manager->getStorage($entity_type_id)
+      ->willReturn($entity_storage->reveal());
+    $entity_type_repository = $this->prophesize(EntityTypeRepositoryInterface::class);
+    $entity_type_repository->getEntityTypeFromClass($entity_type_class)
+      ->willReturn($entity_type_id);
+    $entity_field_manager = $this->prophesize(EntityFieldManagerInterface::class);
+    $normalizer = new ConfigEntityNormalizer($entity_type_manager->reveal(), $entity_type_repository->reveal(), $entity_field_manager->reveal());
+
+    // Verify the denormalizer still works correctly: the mock above creates an
+    // artificial entity object containing exactly the data it received. It also
+    // should still set _restSubmittedFields correctly.
+    $expected_denormalization = (object) [
+      '_restSubmittedFields' => [
+        'test',
+      ],
+      'received_data' => [
+        'test' => $test_value,
+      ],
+    ];
+    $this->assertEquals($expected_denormalization, $normalizer->denormalize($data, $entity_type_class, 'json'));
+  }
+
+}

core/modules/serialization/tests/src/Unit/Normalizer/ContentEntityNormalizerTest.php

@@ -0,0 +1,176 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\serialization\Unit\Normalizer;
+
+use Drupal\Core\Entity\EntityFieldManagerInterface;
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Drupal\Core\Entity\EntityTypeRepositoryInterface;
+use Drupal\Core\Field\FieldItemListInterface;
+use Drupal\Core\Session\AccountInterface;
+use Drupal\Core\TypedData\ComplexDataInterface;
+use Drupal\Core\TypedData\DataDefinitionInterface;
+use Drupal\serialization\Normalizer\ContentEntityNormalizer;
+use Drupal\Tests\Core\Entity\ContentEntityBaseMockableClass;
+use Drupal\Tests\UnitTestCase;
+use Prophecy\Argument;
+use Symfony\Component\Serializer\Serializer;
+
+/**
+ * @coversDefaultClass \Drupal\serialization\Normalizer\ContentEntityNormalizer
+ * @group serialization
+ */
+class ContentEntityNormalizerTest extends UnitTestCase {
+
+  /**
+   * The mock serializer.
+   *
+   * @var \Symfony\Component\Serializer\Serializer|\Prophecy\Prophecy\ObjectProphecy
+   */
+  protected $serializer;
+
+  /**
+   * The normalizer under test.
+   *
+   * @var \Drupal\serialization\Normalizer\ContentEntityNormalizer
+   */
+  protected $contentEntityNormalizer;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $entity_field_manager = $this->createMock(EntityFieldManagerInterface::class);
+    $entity_type_manager = $this->createMock(EntityTypeManagerInterface::class);
+    $entity_type_repository = $this->createMock(EntityTypeRepositoryInterface::class);
+
+    $this->contentEntityNormalizer = new ContentEntityNormalizer($entity_type_manager, $entity_type_repository, $entity_field_manager);
+
+    $this->serializer = $this->prophesize(Serializer::class);
+    $this->contentEntityNormalizer->setSerializer($this->serializer->reveal());
+  }
+
+  /**
+   * @covers ::supportsNormalization
+   */
+  public function testSupportsNormalization(): void {
+    $content_mock = $this->createMock('Drupal\Core\Entity\ContentEntityInterface');
+    $config_mock = $this->createMock('Drupal\Core\Config\Entity\ConfigEntityInterface');
+    $this->assertTrue($this->contentEntityNormalizer->supportsNormalization($content_mock));
+    $this->assertFalse($this->contentEntityNormalizer->supportsNormalization($config_mock));
+  }
+
+  /**
+   * Tests the normalize() method.
+   *
+   * @covers ::normalize
+   */
+  public function testNormalize(): void {
+    $this->serializer->normalize(Argument::type(FieldItemListInterface::class),
+      'test_format', ['account' => NULL])->willReturn('test');
+
+    $definitions = [
+      'field_accessible_external' => $this->createMockFieldListItem(TRUE, FALSE),
+      'field_non-accessible_external' => $this->createMockFieldListItem(FALSE, FALSE),
+      'field_accessible_internal' => $this->createMockFieldListItem(TRUE, TRUE),
+      'field_non-accessible_internal' => $this->createMockFieldListItem(FALSE, TRUE),
+    ];
+    $content_entity_mock = $this->createMockForContentEntity($definitions);
+
+    $normalized = $this->contentEntityNormalizer->normalize($content_entity_mock, 'test_format');
+
+    $this->assertArrayHasKey('field_accessible_external', $normalized);
+    $this->assertEquals('test', $normalized['field_accessible_external']);
+    $this->assertArrayNotHasKey('field_non-accessible_external', $normalized);
+    $this->assertArrayNotHasKey('field_accessible_internal', $normalized);
+    $this->assertArrayNotHasKey('field_non-accessible_internal', $normalized);
+  }
+
+  /**
+   * Tests the normalize() method with account context passed.
+   *
+   * @covers ::normalize
+   */
+  public function testNormalizeWithAccountContext(): void {
+    $mock_account = $this->createMock('Drupal\Core\Session\AccountInterface');
+
+    $context = [
+      'account' => $mock_account,
+    ];
+
+    $this->serializer->normalize(Argument::type(FieldItemListInterface::class),
+      'test_format', $context)->willReturn('test');
+
+    // The mock account should get passed directly into the access() method on
+    // field items from $context['account'].
+    $definitions = [
+      'field_1' => $this->createMockFieldListItem(TRUE, FALSE, $mock_account),
+      'field_2' => $this->createMockFieldListItem(FALSE, FALSE, $mock_account),
+    ];
+    $content_entity_mock = $this->createMockForContentEntity($definitions);
+
+    $normalized = $this->contentEntityNormalizer->normalize($content_entity_mock, 'test_format', $context);
+
+    $this->assertArrayHasKey('field_1', $normalized);
+    $this->assertEquals('test', $normalized['field_1']);
+    $this->assertArrayNotHasKey('field_2', $normalized);
+  }
+
+  /**
+   * Creates a mock content entity.
+   *
+   * @param $definitions
+   *   The properties the will be returned.
+   *
+   * @return \PHPUnit\Framework\MockObject\MockObject
+   */
+  public function createMockForContentEntity($definitions) {
+    $content_entity_mock = $this->getMockBuilder(ContentEntityBaseMockableClass::class)
+      ->disableOriginalConstructor()
+      ->onlyMethods(['getTypedData'])
+      ->getMock();
+    $typed_data = $this->prophesize(ComplexDataInterface::class);
+    $typed_data->getProperties(TRUE)
+      ->willReturn($definitions)
+      ->shouldBeCalled();
+    $content_entity_mock->expects($this->any())
+      ->method('getTypedData')
+      ->willReturn($typed_data->reveal());
+
+    return $content_entity_mock;
+  }
+
+  /**
+   * Creates a mock field list item.
+   *
+   * @param bool $access
+   *   The value that access() will return.
+   * @param bool $internal
+   *   The value that isInternal() will return.
+   * @param \Drupal\Core\Session\AccountInterface $user_context
+   *   The user context used for the access check.
+   *
+   * @return \Drupal\Core\Field\FieldItemListInterface|\PHPUnit\Framework\MockObject\MockObject
+   */
+  protected function createMockFieldListItem($access, $internal, ?AccountInterface $user_context = NULL) {
+    $data_definition = $this->prophesize(DataDefinitionInterface::class);
+    $mock = $this->createMock('Drupal\Core\Field\FieldItemListInterface');
+    $mock->expects($this->once())
+      ->method('getDataDefinition')
+      ->willReturn($data_definition->reveal());
+    $data_definition->isInternal()
+      ->willReturn($internal)
+      ->shouldBeCalled();
+    if (!$internal) {
+      $mock->expects($this->once())
+        ->method('access')
+        ->with('view', $user_context)
+        ->willReturn($access);
+    }
+    return $mock;
+  }
+
+}

core/modules/serialization/tests/src/Unit/Normalizer/DateTimeIso8601NormalizerTest.php

@@ -0,0 +1,275 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\serialization\Unit\Normalizer;
+
+use Drupal\Core\Config\ConfigFactoryInterface;
+use Drupal\Core\Config\ImmutableConfig;
+use Drupal\Core\Datetime\DrupalDateTime;
+use Drupal\Core\Field\FieldDefinitionInterface;
+use Drupal\Core\Field\FieldItemBase;
+use Drupal\Core\Field\FieldStorageDefinitionInterface;
+use Drupal\Core\TypedData\Plugin\DataType\DateTimeIso8601;
+use Drupal\Core\TypedData\Plugin\DataType\IntegerData;
+use Drupal\Core\TypedData\Type\DateTimeInterface;
+use Drupal\datetime\Plugin\Field\FieldType\DateTimeItem;
+use Drupal\serialization\Normalizer\DateTimeIso8601Normalizer;
+use Drupal\Tests\UnitTestCase;
+use Prophecy\Argument;
+use Symfony\Component\Serializer\Exception\InvalidArgumentException;
+use Symfony\Component\Serializer\Exception\UnexpectedValueException;
+
+/**
+ * Unit test coverage for the "datetime_iso8601" @DataType.
+ *
+ * @coversDefaultClass \Drupal\serialization\Normalizer\DateTimeIso8601Normalizer
+ * @group serialization
+ * @see \Drupal\Core\TypedData\Plugin\DataType\DateTimeIso8601
+ * @see \Drupal\datetime\Plugin\Field\FieldType\DateTimeItem::DATETIME_TYPE_DATE
+ */
+class DateTimeIso8601NormalizerTest extends UnitTestCase {
+
+  /**
+   * The tested data type's normalizer.
+   *
+   * @var \Drupal\serialization\Normalizer\DateTimeIso8601Normalizer
+   */
+  protected $normalizer;
+
+  /**
+   * The tested data type.
+   *
+   * @var \Drupal\Core\TypedData\Plugin\DataType\DateTimeIso8601
+   */
+  protected $data;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $system_date_config = $this->prophesize(ImmutableConfig::class);
+    $system_date_config->get('timezone.default')
+      ->willReturn('Australia/Sydney');
+    $config_factory = $this->prophesize(ConfigFactoryInterface::class);
+    $config_factory->get('system.date')
+      ->willReturn($system_date_config->reveal());
+
+    $this->normalizer = new DateTimeIso8601Normalizer($config_factory->reveal());
+    $this->data = $this->prophesize(DateTimeIso8601::class);
+  }
+
+  /**
+   * @covers ::supportsNormalization
+   */
+  public function testSupportsNormalization(): void {
+    $this->assertTrue($this->normalizer->supportsNormalization($this->data->reveal()));
+
+    $datetime = $this->prophesize(DateTimeInterface::class);
+    $this->assertFalse($this->normalizer->supportsNormalization($datetime->reveal()));
+
+    $integer = $this->prophesize(IntegerData::class);
+    $this->assertFalse($this->normalizer->supportsNormalization($integer->reveal()));
+  }
+
+  /**
+   * @covers ::supportsDenormalization
+   */
+  public function testSupportsDenormalization(): void {
+    $this->assertTrue($this->normalizer->supportsDenormalization($this->data->reveal(), DateTimeIso8601::class));
+  }
+
+  /**
+   * @covers ::normalize
+   * @dataProvider providerTestNormalize
+   */
+  public function testNormalize($parent_field_item_class, $datetime_type, $expected_format): void {
+    $formatted_string = $this->randomMachineName();
+
+    $field_item = $this->prophesize($parent_field_item_class);
+    if ($parent_field_item_class === DateTimeItem::class) {
+      $field_storage_definition = $this->prophesize(FieldStorageDefinitionInterface::class);
+      $field_storage_definition->getSetting('datetime_type')
+        ->willReturn($datetime_type);
+      $field_definition = $this->prophesize(FieldDefinitionInterface::class);
+      $field_definition->getFieldStorageDefinition()
+        ->willReturn($field_storage_definition);
+      $field_item->getFieldDefinition()
+        ->willReturn($field_definition);
+    }
+    else {
+      $field_item->getFieldDefinition(Argument::any())
+        ->shouldNotBeCalled();
+    }
+    $this->data->getParent()
+      ->willReturn($field_item);
+
+    $drupal_date_time = $this->prophesize(DateTimeIso8601NormalizerTestDrupalDateTime::class);
+    $drupal_date_time->setTimezone(new \DateTimeZone('Australia/Sydney'))
+      ->willReturn($drupal_date_time->reveal());
+    $drupal_date_time->format($expected_format)
+      ->willReturn($formatted_string);
+    $this->data->getDateTime()
+      ->willReturn($drupal_date_time->reveal());
+
+    $normalized = $this->normalizer->normalize($this->data->reveal());
+    $this->assertSame($formatted_string, $normalized);
+  }
+
+  /**
+   * @covers ::normalize
+   * @dataProvider providerTestNormalize
+   */
+  public function testNormalizeWhenNull($parent_field_item_class, $datetime_type, $expected_format): void {
+    $field_item = $this->prophesize($parent_field_item_class);
+    if ($parent_field_item_class === DateTimeItem::class) {
+      $field_storage_definition = $this->prophesize(FieldStorageDefinitionInterface::class);
+      $field_storage_definition->getSetting('datetime_type')
+        ->willReturn($datetime_type);
+      $field_definition = $this->prophesize(FieldDefinitionInterface::class);
+      $field_definition->getFieldStorageDefinition()
+        ->willReturn($field_storage_definition);
+      $field_item->getFieldDefinition()
+        ->willReturn($field_definition);
+    }
+    else {
+      $field_item->getFieldDefinition(Argument::any())
+        ->shouldNotBeCalled();
+    }
+    $this->data->getParent()
+      ->willReturn($field_item);
+
+    $this->data->getDateTime()
+      ->willReturn(NULL);
+
+    $normalized = $this->normalizer->normalize($this->data->reveal());
+    $this->assertNull($normalized);
+  }
+
+  /**
+   * Data provider for testNormalize.
+   *
+   * @return array
+   */
+  public static function providerTestNormalize() {
+    return [
+      // @see \Drupal\datetime\Plugin\Field\FieldType\DateTimeItem::DATETIME_TYPE_DATE
+      'datetime field, configured to store only date: must be handled by DateTimeIso8601Normalizer' => [
+        DateTimeItem::class,
+        DateTimeItem::DATETIME_TYPE_DATE,
+        // This expected format call proves that normalization is handled by \Drupal\serialization\Normalizer\DateTimeIso8601Normalizer::normalize().
+        'Y-m-d',
+      ],
+      // @see \Drupal\datetime\Plugin\Field\FieldType\DateTimeItem::DATETIME_TYPE_DATETIME
+      'datetime field, configured to store date and time; must be handled by the parent normalizer' => [
+        DateTimeItem::class,
+        DateTimeItem::DATETIME_TYPE_DATETIME,
+        \DateTime::RFC3339,
+      ],
+      'non-datetime field; must be handled by the parent normalizer' => [
+        FieldItemBase::class,
+        NULL,
+        \DateTime::RFC3339,
+      ],
+
+    ];
+  }
+
+  /**
+   * Tests the denormalize function with good data.
+   *
+   * @covers ::denormalize
+   * @dataProvider providerTestDenormalizeValidFormats
+   */
+  public function testDenormalizeValidFormats($type, $normalized, $expected): void {
+    $field_definition = $this->prophesize(FieldDefinitionInterface::class);
+    $field_definition->getSetting('datetime_type')->willReturn($type === 'date-only' ? DateTimeItem::DATETIME_TYPE_DATE : DateTimeItem::DATETIME_TYPE_DATETIME);
+    $denormalized = $this->normalizer->denormalize($normalized, DateTimeIso8601::class, NULL, [
+      'field_definition' => $field_definition->reveal(),
+    ]);
+    $this->assertSame($expected, $denormalized);
+  }
+
+  /**
+   * Data provider for testDenormalizeValidFormats.
+   *
+   * @return array
+   */
+  public static function providerTestDenormalizeValidFormats() {
+    $data = [];
+    $data['just a date'] = ['date-only', '2016-11-06', '2016-11-06'];
+
+    $data['RFC3339'] = ['date+time', '2016-11-06T09:02:00+00:00', '2016-11-06T09:02:00'];
+    $data['RFC3339 +0100'] = ['date+time', '2016-11-06T09:02:00+01:00', '2016-11-06T08:02:00'];
+    $data['RFC3339 -0600'] = ['date+time', '2016-11-06T09:02:00-06:00', '2016-11-06T15:02:00'];
+
+    $data['ISO8601'] = ['date+time', '2016-11-06T09:02:00+0000', '2016-11-06T09:02:00'];
+    $data['ISO8601 +0100'] = ['date+time', '2016-11-06T09:02:00+0100', '2016-11-06T08:02:00'];
+    $data['ISO8601 -0600'] = ['date+time', '2016-11-06T09:02:00-0600', '2016-11-06T15:02:00'];
+
+    return $data;
+  }
+
+  /**
+   * Tests the denormalize function with bad data for the date-only case.
+   *
+   * @covers ::denormalize
+   */
+  public function testDenormalizeDateOnlyException(): void {
+    $this->expectException(UnexpectedValueException::class);
+    $this->expectExceptionMessage('The specified date "2016/11/06" is not in an accepted format: "Y-m-d" (date-only).');
+
+    $normalized = '2016/11/06';
+
+    $field_definition = $this->prophesize(FieldDefinitionInterface::class);
+    $field_definition->getSetting('datetime_type')->willReturn(DateTimeItem::DATETIME_TYPE_DATE);
+    $this->normalizer->denormalize($normalized, DateTimeIso8601::class, NULL, ['field_definition' => $field_definition->reveal()]);
+  }
+
+  /**
+   * Tests the denormalize function with bad data for the date+time case.
+   *
+   * @covers ::denormalize
+   */
+  public function testDenormalizeDateAndTimeException(): void {
+    $this->expectException(UnexpectedValueException::class);
+    $this->expectExceptionMessage('The specified date "on a rainy day" is not in an accepted format: "Y-m-d\TH:i:sP" (RFC 3339), "Y-m-d\TH:i:sO" (ISO 8601).');
+
+    $normalized = 'on a rainy day';
+
+    $field_definition = $this->prophesize(FieldDefinitionInterface::class);
+    $field_definition->getSetting('datetime_type')->willReturn(DateTimeItem::DATETIME_TYPE_DATETIME);
+    $this->normalizer->denormalize($normalized, DateTimeIso8601::class, NULL, ['field_definition' => $field_definition->reveal()]);
+  }
+
+  /**
+   * Tests the denormalize function with incomplete serialization context.
+   *
+   * @covers ::denormalize
+   */
+  public function testDenormalizeNoTargetInstanceOrFieldDefinitionException(): void {
+    $this->expectException(InvalidArgumentException::class);
+    $this->expectExceptionMessage('$context[\'target_instance\'] or $context[\'field_definition\'] must be set to denormalize with the DateTimeIso8601Normalizer');
+    $this->normalizer->denormalize('', DateTimeIso8601::class, NULL, []);
+  }
+
+}
+
+/**
+ * Provides a test class for testing DrupalDateTime.
+ *
+ * Note: Prophecy does not support magic methods. By subclassing and specifying
+ * an explicit method, Prophecy works.
+ * @see https://github.com/phpspec/prophecy/issues/338
+ * @see https://github.com/phpspec/prophecy/issues/34
+ * @see https://github.com/phpspec/prophecy/issues/80
+ */
+class DateTimeIso8601NormalizerTestDrupalDateTime extends DrupalDateTime {
+
+  public function setTimezone(\DateTimeZone $timezone) {
+    parent::setTimezone($timezone);
+  }
+
+}

core/modules/serialization/tests/src/Unit/Normalizer/DateTimeNormalizerTest.php

@@ -0,0 +1,197 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\serialization\Unit\Normalizer;
+
+use Drupal\Core\Config\ConfigFactoryInterface;
+use Drupal\Core\Config\ImmutableConfig;
+use Drupal\Core\Datetime\DrupalDateTime;
+use Drupal\Core\TypedData\Plugin\DataType\DateTimeIso8601;
+use Drupal\Core\TypedData\Plugin\DataType\IntegerData;
+use Drupal\Core\TypedData\Type\DateTimeInterface;
+use Drupal\serialization\Normalizer\DateTimeNormalizer;
+use Drupal\Tests\UnitTestCase;
+use Symfony\Component\Serializer\Exception\UnexpectedValueException;
+
+/**
+ * Unit test coverage for @DataTypes implementing DateTimeInterface.
+ *
+ * @group serialization
+ * @coversDefaultClass \Drupal\serialization\Normalizer\DateTimeNormalizer
+ * @see \Drupal\Core\TypedData\Type\DateTimeInterface
+ */
+class DateTimeNormalizerTest extends UnitTestCase {
+
+  /**
+   * The tested data type's normalizer.
+   *
+   * @var \Drupal\serialization\Normalizer\DateTimeNormalizer
+   */
+  protected $normalizer;
+
+  /**
+   * The tested data type.
+   *
+   * @var \Drupal\Core\TypedData\Type\DateTimeInterface
+   */
+  protected $data;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $system_date_config = $this->prophesize(ImmutableConfig::class);
+    $system_date_config->get('timezone.default')
+      ->willReturn('Australia/Sydney');
+    $config_factory = $this->prophesize(ConfigFactoryInterface::class);
+    $config_factory->get('system.date')
+      ->willReturn($system_date_config->reveal());
+
+    $this->normalizer = new DateTimeNormalizer($config_factory->reveal());
+    $this->data = $this->prophesize(DateTimeInterface::class);
+  }
+
+  /**
+   * @covers ::supportsNormalization
+   */
+  public function testSupportsNormalization(): void {
+    $this->assertTrue($this->normalizer->supportsNormalization($this->data->reveal()));
+
+    $datetimeiso8601 = $this->prophesize(DateTimeIso8601::class);
+    $this->assertTrue($this->normalizer->supportsNormalization($datetimeiso8601->reveal()));
+
+    $integer = $this->prophesize(IntegerData::class);
+    $this->assertFalse($this->normalizer->supportsNormalization($integer->reveal()));
+  }
+
+  /**
+   * @covers ::supportsDenormalization
+   */
+  public function testSupportsDenormalization(): void {
+    $this->assertTrue($this->normalizer->supportsDenormalization($this->data->reveal(), DateTimeInterface::class));
+  }
+
+  /**
+   * @covers ::normalize
+   */
+  public function testNormalize(): void {
+    $random_rfc_3339_string = $this->randomMachineName();
+
+    $drupal_date_time = $this->prophesize(DateTimeNormalizerTestDrupalDateTime::class);
+    $drupal_date_time->setTimezone(new \DateTimeZone('Australia/Sydney'))
+      ->willReturn($drupal_date_time->reveal());
+    $drupal_date_time->format(\DateTime::RFC3339)
+      ->willReturn($random_rfc_3339_string);
+
+    $this->data->getDateTime()
+      ->willReturn($drupal_date_time->reveal());
+
+    $normalized = $this->normalizer->normalize($this->data->reveal());
+    $this->assertSame($random_rfc_3339_string, $normalized);
+  }
+
+  /**
+   * @covers ::normalize
+   */
+  public function testNormalizeWhenNull(): void {
+    $this->data->getDateTime()
+      ->willReturn(NULL);
+
+    $normalized = $this->normalizer->normalize($this->data->reveal());
+    $this->assertNull($normalized);
+  }
+
+  /**
+   * Tests the denormalize function with good data.
+   *
+   * @covers ::denormalize
+   * @dataProvider providerTestDenormalizeValidFormats
+   */
+  public function testDenormalizeValidFormats($normalized, $expected): void {
+    $denormalized = $this->normalizer->denormalize($normalized, DateTimeInterface::class, NULL, []);
+    $this->assertSame(0, $denormalized->getTimestamp() - $expected->getTimestamp());
+    $this->assertEquals($expected, $denormalized);
+  }
+
+  /**
+   * Data provider for testDenormalizeValidFormats.
+   *
+   * @return array
+   */
+  public static function providerTestDenormalizeValidFormats() {
+    $data = [];
+
+    $data['RFC3339'] = ['2016-11-06T09:02:00+00:00', new \DateTimeImmutable('2016-11-06T09:02:00+00:00')];
+    $data['RFC3339 +0100'] = ['2016-11-06T09:02:00+01:00', new \DateTimeImmutable('2016-11-06T09:02:00+01:00')];
+    $data['RFC3339 -0600'] = ['2016-11-06T09:02:00-06:00', new \DateTimeImmutable('2016-11-06T09:02:00-06:00')];
+
+    $data['ISO8601'] = ['2016-11-06T09:02:00+0000', new \DateTimeImmutable('2016-11-06T09:02:00+00:00')];
+    $data['ISO8601 +0100'] = ['2016-11-06T09:02:00+0100', new \DateTimeImmutable('2016-11-06T09:02:00+01:00')];
+    $data['ISO8601 -0600'] = ['2016-11-06T09:02:00-0600', new \DateTimeImmutable('2016-11-06T09:02:00-06:00')];
+
+    return $data;
+  }
+
+  /**
+   * Tests the denormalize function with a user supplied format.
+   *
+   * @covers ::denormalize
+   * @dataProvider providerTestDenormalizeUserFormats
+   */
+  public function testDenormalizeUserFormats($normalized, $format, $expected): void {
+    $denormalized = $this->normalizer->denormalize($normalized, DateTimeInterface::class, NULL, ['datetime_allowed_formats' => [$format]]);
+    $this->assertSame(0, $denormalized->getTimestamp() - $expected->getTimestamp());
+    $this->assertEquals($expected, $denormalized);
+  }
+
+  /**
+   * Data provider for testDenormalizeUserFormats.
+   *
+   * @return array
+   */
+  public static function providerTestDenormalizeUserFormats() {
+    $data = [];
+
+    $data['Y/m/d H:i:s P'] = ['2016/11/06 09:02:00 +00:00', 'Y/m/d H:i:s P', new \DateTimeImmutable('2016-11-06T09:02:00+00:00')];
+    $data['H:i:s Y/m/d P'] = ['09:02:00 2016/11/06  +01:00', 'H:i:s Y/m/d P', new \DateTimeImmutable('2016-11-06T09:02:00+01:00')];
+    $data['Y/m/d H:i:s'] = ['09:02:00 2016/11/06', 'H:i:s Y/m/d', new \DateTimeImmutable('2016-11-06T09:02:00+11:00')];
+
+    return $data;
+  }
+
+  /**
+   * Tests the denormalize function with bad data.
+   *
+   * @covers ::denormalize
+   */
+  public function testDenormalizeException(): void {
+    $this->expectException(UnexpectedValueException::class);
+    $this->expectExceptionMessage('The specified date "2016/11/06 09:02am GMT" is not in an accepted format: "Y-m-d\TH:i:sP" (RFC 3339), "Y-m-d\TH:i:sO" (ISO 8601).');
+
+    $normalized = '2016/11/06 09:02am GMT';
+
+    $this->normalizer->denormalize($normalized, DateTimeInterface::class, NULL, []);
+  }
+
+}
+
+
+/**
+ * Provides a test class for testing DrupalDateTime.
+ *
+ * Note: Prophecy does not support magic methods. By subclassing and specifying
+ * an explicit method, Prophecy works.
+ * @see https://github.com/phpspec/prophecy/issues/338
+ * @see https://github.com/phpspec/prophecy/issues/34
+ * @see https://github.com/phpspec/prophecy/issues/80
+ */
+class DateTimeNormalizerTestDrupalDateTime extends DrupalDateTime {
+
+  public function setTimezone(\DateTimeZone $timezone) {
+    parent::setTimezone($timezone);
+  }
+
+}

core/modules/serialization/tests/src/Unit/Normalizer/EntityNormalizerTest.php

@@ -0,0 +1,425 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\serialization\Unit\Normalizer;
+
+use Drupal\Core\Entity\EntityFieldManagerInterface;
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Drupal\Core\Entity\EntityTypeRepositoryInterface;
+use Drupal\Core\Entity\FieldableEntityInterface;
+use Drupal\Core\Field\FieldItemListInterface;
+use Drupal\serialization\Normalizer\EntityNormalizer;
+use Drupal\Tests\Core\Entity\ContentEntityBaseMockableClass;
+use Drupal\Tests\UnitTestCase;
+use Symfony\Component\Serializer\Exception\UnexpectedValueException;
+
+/**
+ * @coversDefaultClass \Drupal\serialization\Normalizer\EntityNormalizer
+ * @group serialization
+ */
+class EntityNormalizerTest extends UnitTestCase {
+
+  /**
+   * The mock entity field manager.
+   *
+   * @var \Drupal\Core\Entity\EntityFieldManagerInterface|\PHPUnit\Framework\MockObject\MockObject
+   */
+  protected $entityFieldManager;
+
+  /**
+   * The mock entity type manager.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeManagerInterface|\PHPUnit\Framework\MockObject\MockObject
+   */
+  protected $entityTypeManager;
+
+  /**
+   * The mock entity type repository.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeRepositoryInterface|\PHPUnit\Framework\MockObject\MockObject
+   */
+  protected $entityTypeRepository;
+
+  /**
+   * The mock serializer.
+   *
+   * @var \Symfony\Component\Serializer\SerializerInterface|\PHPUnit\Framework\MockObject\MockObject
+   */
+  protected $serializer;
+
+  /**
+   * The entity normalizer.
+   *
+   * @var \Drupal\serialization\Normalizer\EntityNormalizer
+   */
+  protected $entityNormalizer;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $this->entityFieldManager = $this->createMock(EntityFieldManagerInterface::class);
+    $this->entityTypeManager = $this->createMock(EntityTypeManagerInterface::class);
+    $this->entityTypeRepository = $this->createMock(EntityTypeRepositoryInterface::class);
+
+    $this->entityNormalizer = new EntityNormalizer(
+      $this->entityTypeManager,
+      $this->entityTypeRepository,
+      $this->entityFieldManager
+    );
+  }
+
+  /**
+   * Tests the normalize() method.
+   *
+   * @covers ::normalize
+   */
+  public function testNormalize(): void {
+    $list_item_1 = $this->createMock('Drupal\Core\TypedData\TypedDataInterface');
+    $list_item_2 = $this->createMock('Drupal\Core\TypedData\TypedDataInterface');
+
+    $definitions = [
+      'field_1' => $list_item_1,
+      'field_2' => $list_item_2,
+    ];
+
+    $content_entity = $this->getMockBuilder(ContentEntityBaseMockableClass::class)
+      ->disableOriginalConstructor()
+      ->onlyMethods(['getFields'])
+      ->getMock();
+    $content_entity->expects($this->once())
+      ->method('getFields')
+      ->willReturn($definitions);
+
+    $serializer = $this->prophesize('Symfony\Component\Serializer\Serializer');
+    $serializer->normalize($list_item_1, 'test_format', [])->shouldBeCalled();
+    $serializer->normalize($list_item_2, 'test_format', [])->shouldBeCalled();
+
+    $this->entityNormalizer->setSerializer($serializer->reveal());
+
+    $this->entityNormalizer->normalize($content_entity, 'test_format');
+  }
+
+  /**
+   * Tests the denormalize() method with no entity type provided in context.
+   *
+   * @covers ::denormalize
+   */
+  public function testDenormalizeWithNoEntityType(): void {
+    $this->expectException(UnexpectedValueException::class);
+    $this->entityNormalizer->denormalize([], ContentEntityBaseMockableClass::class);
+  }
+
+  /**
+   * Tests the denormalize method with a bundle property.
+   *
+   * @covers ::denormalize
+   */
+  public function testDenormalizeWithValidBundle(): void {
+    $test_data = [
+      'key_1' => 'value_1',
+      'key_2' => 'value_2',
+      'test_type' => [
+        ['name' => 'test_bundle'],
+      ],
+    ];
+
+    $entity_type = $this->createMock('Drupal\Core\Entity\EntityTypeInterface');
+
+    $entity_type->expects($this->once())
+      ->method('id')
+      ->willReturn('test');
+    $entity_type->expects($this->once())
+      ->method('hasKey')
+      ->with('bundle')
+      ->willReturn(TRUE);
+    $entity_type->expects($this->once())
+      ->method('getKey')
+      ->with('bundle')
+      ->willReturn('test_type');
+    $entity_type->expects($this->once())
+      ->method('entityClassImplements')
+      ->with(FieldableEntityInterface::class)
+      ->willReturn(TRUE);
+
+    $entity_type->expects($this->once())
+      ->method('getBundleEntityType')
+      ->willReturn('test_bundle');
+
+    $entity_type_storage_definition = $this->createMock('Drupal\Core\Field\FieldStorageDefinitionInterface');
+    $entity_type_storage_definition->expects($this->once())
+      ->method('getMainPropertyName')
+      ->willReturn('name');
+
+    $entity_type_definition = $this->createMock('Drupal\Core\Field\FieldDefinitionInterface');
+    $entity_type_definition->expects($this->once())
+      ->method('getFieldStorageDefinition')
+      ->willReturn($entity_type_storage_definition);
+
+    $base_definitions = [
+      'test_type' => $entity_type_definition,
+    ];
+
+    $this->entityTypeManager->expects($this->once())
+      ->method('getDefinition')
+      ->with('test')
+      ->willReturn($entity_type);
+    $this->entityFieldManager->expects($this->once())
+      ->method('getBaseFieldDefinitions')
+      ->with('test')
+      ->willReturn($base_definitions);
+
+    $entity_query_mock = $this->createMock('Drupal\Core\Entity\Query\QueryInterface');
+    $entity_query_mock->expects($this->once())
+      ->method('accessCheck')
+      ->with(TRUE)
+      ->willReturn($entity_query_mock);
+    $entity_query_mock->expects($this->once())
+      ->method('execute')
+      ->willReturn(['test_bundle' => 'test_bundle']);
+
+    $entity_type_storage = $this->createMock('Drupal\Core\Entity\EntityStorageInterface');
+    $entity_type_storage->expects($this->once())
+      ->method('getQuery')
+      ->willReturn($entity_query_mock);
+
+    $key_1 = $this->createMock(FieldItemListInterface::class);
+    $key_2 = $this->createMock(FieldItemListInterface::class);
+
+    $entity = $this->createMock(ContentEntityBaseMockableClass::class);
+    $entity->expects($this->exactly(2))
+      ->method('get')
+      ->willReturnMap([
+        ['key_1', $key_1],
+        ['key_2', $key_2],
+      ]);
+
+    $storage = $this->createMock('Drupal\Core\Entity\EntityStorageInterface');
+    // Create should only be called with the bundle property at first.
+    $expected_test_data = [
+      'test_type' => 'test_bundle',
+    ];
+
+    $storage->expects($this->once())
+      ->method('create')
+      ->with($expected_test_data)
+      ->willReturn($entity);
+
+    $this->entityTypeManager->expects($this->exactly(2))
+      ->method('getStorage')
+      ->willReturnMap([
+        ['test_bundle', $entity_type_storage],
+        ['test', $storage],
+      ]);
+
+    // Setup expectations for the serializer. This will be called for each field
+    // item.
+    $serializer = $this->prophesize('Symfony\Component\Serializer\Serializer');
+    $serializer->denormalize('value_1', get_class($key_1), NULL, ['target_instance' => $key_1, 'entity_type' => 'test'])
+      ->willReturn(NULL)
+      ->shouldBeCalled();
+    $serializer->denormalize('value_2', get_class($key_2), NULL, ['target_instance' => $key_2, 'entity_type' => 'test'])
+      ->willReturn(NULL)
+      ->shouldBeCalled();
+
+    $this->entityNormalizer->setSerializer($serializer->reveal());
+
+    $this->assertNotNull($this->entityNormalizer->denormalize($test_data, ContentEntityBaseMockableClass::class, NULL, ['entity_type' => 'test']));
+  }
+
+  /**
+   * Tests the denormalize method with a bundle property.
+   *
+   * @covers ::denormalize
+   */
+  public function testDenormalizeWithInvalidBundle(): void {
+    $test_data = [
+      'key_1' => 'value_1',
+      'key_2' => 'value_2',
+      'test_type' => [
+        ['name' => 'test_bundle'],
+      ],
+    ];
+
+    $entity_type = $this->createMock('Drupal\Core\Entity\EntityTypeInterface');
+
+    $entity_type->expects($this->once())
+      ->method('id')
+      ->willReturn('test');
+    $entity_type->expects($this->once())
+      ->method('hasKey')
+      ->with('bundle')
+      ->willReturn(TRUE);
+    $entity_type->expects($this->once())
+      ->method('getKey')
+      ->with('bundle')
+      ->willReturn('test_type');
+    $entity_type->expects($this->once())
+      ->method('entityClassImplements')
+      ->with(FieldableEntityInterface::class)
+      ->willReturn(TRUE);
+
+    $entity_type->expects($this->once())
+      ->method('getBundleEntityType')
+      ->willReturn('test_bundle');
+
+    $entity_type_storage_definition = $this->createMock('Drupal\Core\Field\FieldStorageDefinitionInterface');
+    $entity_type_storage_definition->expects($this->once())
+      ->method('getMainPropertyName')
+      ->willReturn('name');
+
+    $entity_type_definition = $this->createMock('Drupal\Core\Field\FieldDefinitionInterface');
+    $entity_type_definition->expects($this->once())
+      ->method('getFieldStorageDefinition')
+      ->willReturn($entity_type_storage_definition);
+
+    $base_definitions = [
+      'test_type' => $entity_type_definition,
+    ];
+
+    $this->entityTypeManager->expects($this->once())
+      ->method('getDefinition')
+      ->with('test')
+      ->willReturn($entity_type);
+    $this->entityFieldManager->expects($this->once())
+      ->method('getBaseFieldDefinitions')
+      ->with('test')
+      ->willReturn($base_definitions);
+
+    $entity_query_mock = $this->createMock('Drupal\Core\Entity\Query\QueryInterface');
+    $entity_query_mock->expects($this->once())
+      ->method('accessCheck')
+      ->with(TRUE)
+      ->willReturn($entity_query_mock);
+    $entity_query_mock->expects($this->once())
+      ->method('execute')
+      ->willReturn(['test_bundle_other' => 'test_bundle_other']);
+
+    $entity_type_storage = $this->createMock('Drupal\Core\Entity\EntityStorageInterface');
+    $entity_type_storage->expects($this->once())
+      ->method('getQuery')
+      ->willReturn($entity_query_mock);
+
+    $this->entityTypeManager->expects($this->once())
+      ->method('getStorage')
+      ->with('test_bundle')
+      ->willReturn($entity_type_storage);
+
+    $this->expectException(UnexpectedValueException::class);
+    $this->entityNormalizer->denormalize($test_data, ContentEntityBaseMockableClass::class, NULL, ['entity_type' => 'test']);
+  }
+
+  /**
+   * Tests the denormalize method with no bundle defined.
+   *
+   * @covers ::denormalize
+   */
+  public function testDenormalizeWithNoBundle(): void {
+    $test_data = [
+      'key_1' => 'value_1',
+      'key_2' => 'value_2',
+    ];
+
+    $entity_type = $this->createMock('Drupal\Core\Entity\EntityTypeInterface');
+    $entity_type->expects($this->once())
+      ->method('entityClassImplements')
+      ->with(FieldableEntityInterface::class)
+      ->willReturn(TRUE);
+    $entity_type->expects($this->once())
+      ->method('hasKey')
+      ->with('bundle')
+      ->willReturn(FALSE);
+    $entity_type->expects($this->never())
+      ->method('getKey');
+
+    $this->entityTypeManager->expects($this->once())
+      ->method('getDefinition')
+      ->with('test')
+      ->willReturn($entity_type);
+
+    $key_1 = $this->createMock(FieldItemListInterface::class);
+    $key_2 = $this->createMock(FieldItemListInterface::class);
+
+    $entity = $this->createMock(ContentEntityBaseMockableClass::class);
+    $entity->expects($this->exactly(2))
+      ->method('get')
+      ->willReturnMap([
+        ['key_1', $key_1],
+        ['key_2', $key_2],
+      ]);
+
+    $storage = $this->createMock('Drupal\Core\Entity\EntityStorageInterface');
+    $storage->expects($this->once())
+      ->method('create')
+      ->with([])
+      ->willReturn($entity);
+
+    $this->entityTypeManager->expects($this->once())
+      ->method('getStorage')
+      ->with('test')
+      ->willReturn($storage);
+
+    $this->entityFieldManager->expects($this->never())
+      ->method('getBaseFieldDefinitions');
+
+    // Setup expectations for the serializer. This will be called for each field
+    // item.
+    $serializer = $this->prophesize('Symfony\Component\Serializer\Serializer');
+    $serializer->denormalize('value_1', get_class($key_1), NULL, ['target_instance' => $key_1, 'entity_type' => 'test'])
+      ->willReturn(NULL)
+      ->shouldBeCalled();
+    $serializer->denormalize('value_2', get_class($key_2), NULL, ['target_instance' => $key_2, 'entity_type' => 'test'])
+      ->willReturn(NULL)
+      ->shouldBeCalled();
+
+    $this->entityNormalizer->setSerializer($serializer->reveal());
+
+    $this->assertNotNull($this->entityNormalizer->denormalize($test_data, ContentEntityBaseMockableClass::class, NULL, ['entity_type' => 'test']));
+  }
+
+  /**
+   * Tests the denormalize method with no bundle defined.
+   *
+   * @covers ::denormalize
+   */
+  public function testDenormalizeWithNoFieldableEntityType(): void {
+    $test_data = [
+      'key_1' => 'value_1',
+      'key_2' => 'value_2',
+    ];
+
+    $entity_type = $this->createMock('Drupal\Core\Entity\EntityTypeInterface');
+    $entity_type->expects($this->once())
+      ->method('entityClassImplements')
+      ->with(FieldableEntityInterface::class)
+      ->willReturn(FALSE);
+
+    $entity_type->expects($this->never())
+      ->method('getKey');
+
+    $this->entityTypeManager->expects($this->once())
+      ->method('getDefinition')
+      ->with('test')
+      ->willReturn($entity_type);
+
+    $storage = $this->createMock('Drupal\Core\Entity\EntityStorageInterface');
+    $storage->expects($this->once())
+      ->method('create')
+      ->with($test_data)
+      ->willReturn($this->createMock(ContentEntityBaseMockableClass::class));
+
+    $this->entityTypeManager->expects($this->once())
+      ->method('getStorage')
+      ->with('test')
+      ->willReturn($storage);
+
+    $this->entityFieldManager->expects($this->never())
+      ->method('getBaseFieldDefinitions');
+
+    $this->assertNotNull($this->entityNormalizer->denormalize($test_data, ContentEntityBaseMockableClass::class, NULL, ['entity_type' => 'test']));
+  }
+
+}

core/modules/serialization/tests/src/Unit/Normalizer/EntityReferenceFieldItemNormalizerTest.php

@@ -0,0 +1,512 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\serialization\Unit\Normalizer;
+
+use Drupal\Core\Entity\EntityInterface;
+use Drupal\Core\Entity\EntityTypeInterface;
+use Drupal\Core\Field\FieldDefinitionInterface;
+use Drupal\Core\Field\TypedData\FieldItemDataDefinition;
+use Drupal\Core\GeneratedUrl;
+use Drupal\Core\TypedData\Type\IntegerInterface;
+use Drupal\Core\TypedData\TypedDataInterface;
+use Drupal\Core\Entity\EntityRepositoryInterface;
+use Drupal\Core\Entity\FieldableEntityInterface;
+use Drupal\Core\Field\FieldItemInterface;
+use Drupal\Core\Field\FieldItemListInterface;
+use Drupal\Core\Field\Plugin\Field\FieldType\EntityReferenceItem;
+use Drupal\Core\Url;
+use Drupal\locale\StringInterface;
+use Drupal\serialization\Normalizer\EntityReferenceFieldItemNormalizer;
+use Drupal\Tests\UnitTestCase;
+use Prophecy\Argument;
+use Symfony\Component\Serializer\Exception\InvalidArgumentException;
+use Symfony\Component\Serializer\Exception\UnexpectedValueException;
+use Symfony\Component\Serializer\Serializer;
+
+/**
+ * @coversDefaultClass \Drupal\serialization\Normalizer\EntityReferenceFieldItemNormalizer
+ * @group serialization
+ */
+class EntityReferenceFieldItemNormalizerTest extends UnitTestCase {
+
+  use InternalTypedDataTestTrait;
+
+  /**
+   * The mock serializer.
+   *
+   * @var \Symfony\Component\Serializer\SerializerInterface|\Prophecy\Prophecy\ObjectProphecy
+   */
+  protected $serializer;
+
+  /**
+   * The normalizer under test.
+   *
+   * @var \Drupal\serialization\Normalizer\EntityReferenceFieldItemNormalizer
+   */
+  protected $normalizer;
+
+  /**
+   * The mock field item.
+   *
+   * @var \Drupal\Core\Field\Plugin\Field\FieldType\EntityReferenceItem|\Prophecy\Prophecy\ObjectProphecy
+   */
+  protected $fieldItem;
+
+  /**
+   * The mock entity repository.
+   *
+   * @var \Drupal\Core\Entity\EntityRepositoryInterface|\Prophecy\Prophecy\ObjectProphecy
+   */
+  protected $entityRepository;
+
+  /**
+   * The mock field definition.
+   *
+   * @var \Drupal\Core\Field\FieldDefinitionInterface|\Prophecy\Prophecy\ObjectProphecy
+   */
+  protected $fieldDefinition;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $this->entityRepository = $this->prophesize(EntityRepositoryInterface::class);
+    $this->normalizer = new EntityReferenceFieldItemNormalizer($this->entityRepository->reveal());
+
+    $this->serializer = $this->prophesize(Serializer::class);
+    // Set up the serializer to return an entity property.
+    $this->serializer->normalize(Argument::cetera())
+      ->willReturn('test');
+
+    $this->normalizer->setSerializer($this->serializer->reveal());
+
+    $this->fieldItem = $this->prophesize(EntityReferenceItem::class);
+    $this->fieldItem->getIterator()
+      ->willReturn(new \ArrayIterator(['target_id' => []]));
+
+    $this->fieldDefinition = $this->prophesize(FieldDefinitionInterface::class);
+    $this->fieldDefinition->getItemDefinition()
+      ->willReturn($this->prophesize(FieldItemDataDefinition::class)->reveal());
+
+  }
+
+  /**
+   * @covers ::supportsNormalization
+   */
+  public function testSupportsNormalization(): void {
+    $this->assertTrue($this->normalizer->supportsNormalization($this->fieldItem->reveal()));
+    $this->assertFalse($this->normalizer->supportsNormalization(new \stdClass()));
+  }
+
+  /**
+   * @covers ::supportsDenormalization
+   */
+  public function testSupportsDenormalization(): void {
+    $this->assertTrue($this->normalizer->supportsDenormalization([], EntityReferenceItem::class));
+    $this->assertFalse($this->normalizer->supportsDenormalization([], FieldItemInterface::class));
+  }
+
+  /**
+   * @covers ::normalize
+   */
+  public function testNormalize(): void {
+    $test_url = '/test/100';
+
+    $generated_url = (new GeneratedUrl())->setGeneratedUrl($test_url);
+
+    $url = $this->prophesize(Url::class);
+    $url->toString(TRUE)
+      ->willReturn($generated_url);
+
+    $entity = $this->prophesize(EntityInterface::class);
+    $entity->hasLinkTemplate('canonical')
+      ->willReturn(TRUE);
+    $entity->isNew()
+      ->willReturn(FALSE)
+      ->shouldBeCalled();
+    $entity->toUrl('canonical')
+      ->willReturn($url)
+      ->shouldBeCalled();
+    $entity->uuid()
+      ->willReturn('080e3add-f9d5-41ac-9821-eea55b7b42fb')
+      ->shouldBeCalled();
+    $entity->getEntityTypeId()
+      ->willReturn('test_type')
+      ->shouldBeCalled();
+
+    $entity_reference = $this->prophesize(TypedDataInterface::class);
+    $entity_reference->getValue()
+      ->willReturn($entity->reveal())
+      ->shouldBeCalled();
+
+    $field_definition = $this->prophesize(FieldDefinitionInterface::class);
+    $field_definition->getSetting('target_type')
+      ->willReturn('test_type');
+
+    $this->fieldItem->getFieldDefinition()
+      ->willReturn($field_definition->reveal());
+
+    $this->fieldItem->get('entity')
+      ->willReturn($entity_reference)
+      ->shouldBeCalled();
+
+    $this->fieldItem->getProperties(TRUE)
+      ->willReturn(['target_id' => $this->getTypedDataProperty(FALSE)])
+      ->shouldBeCalled();
+
+    $normalized = $this->normalizer->normalize($this->fieldItem->reveal());
+
+    $expected = [
+      'target_id' => 'test',
+      'target_type' => 'test_type',
+      'target_uuid' => '080e3add-f9d5-41ac-9821-eea55b7b42fb',
+      'url' => $test_url,
+    ];
+    $this->assertSame($expected, $normalized);
+  }
+
+  public function testNormalizeWithNewEntityReference(): void {
+    $test_url = '/test/100';
+
+    $generated_url = (new GeneratedUrl())->setGeneratedUrl($test_url);
+
+    $url = $this->prophesize(Url::class);
+    $url->toString(TRUE)
+      ->willReturn($generated_url);
+
+    $entity = $this->prophesize(EntityInterface::class);
+    $entity->hasLinkTemplate('canonical')
+      ->willReturn(TRUE);
+    $entity->isNew()
+      ->willReturn(TRUE)
+      ->shouldBeCalled();
+    $entity->uuid()
+      ->willReturn('080e3add-f9d5-41ac-9821-eea55b7b42fb')
+      ->shouldBeCalled();
+    $entity->getEntityTypeId()
+      ->willReturn('test_type')
+      ->shouldBeCalled();
+    $entity->toUrl('canonical')
+      ->willReturn($url)
+      ->shouldNotBeCalled();
+
+    $entity_reference = $this->prophesize(TypedDataInterface::class);
+    $entity_reference->getValue()
+      ->willReturn($entity->reveal())
+      ->shouldBeCalled();
+
+    $field_definition = $this->prophesize(FieldDefinitionInterface::class);
+    $field_definition->getSetting('target_type')
+      ->willReturn('test_type');
+
+    $this->fieldItem->getFieldDefinition()
+      ->willReturn($field_definition->reveal());
+
+    $this->fieldItem->get('entity')
+      ->willReturn($entity_reference)
+      ->shouldBeCalled();
+
+    $this->fieldItem->getProperties(TRUE)
+      ->willReturn(['target_id' => $this->getTypedDataProperty(FALSE)])
+      ->shouldBeCalled();
+
+    $normalized = $this->normalizer->normalize($this->fieldItem->reveal());
+
+    $expected = [
+      'target_id' => 'test',
+      'target_type' => 'test_type',
+      'target_uuid' => '080e3add-f9d5-41ac-9821-eea55b7b42fb',
+    ];
+    $this->assertSame($expected, $normalized);
+  }
+
+  /**
+   * @covers ::normalize
+   */
+  public function testNormalizeWithEmptyTaxonomyTermReference(): void {
+    // Override the serializer prophecy from setUp() to return a zero value.
+    $this->serializer = $this->prophesize(Serializer::class);
+    // Set up the serializer to return an entity property.
+    $this->serializer->normalize(Argument::cetera())
+      ->willReturn(0);
+
+    $this->normalizer->setSerializer($this->serializer->reveal());
+
+    $entity_reference = $this->prophesize(TypedDataInterface::class);
+    $entity_reference->getValue()
+      ->willReturn(NULL)
+      ->shouldBeCalled();
+
+    $field_definition = $this->prophesize(FieldDefinitionInterface::class);
+    $field_definition->getSetting('target_type')
+      ->willReturn('taxonomy_term');
+
+    $this->fieldItem->getFieldDefinition()
+      ->willReturn($field_definition->reveal());
+
+    $this->fieldItem->get('entity')
+      ->willReturn($entity_reference)
+      ->shouldBeCalled();
+
+    $this->fieldItem->getProperties(TRUE)
+      ->willReturn(['target_id' => $this->getTypedDataProperty(FALSE)])
+      ->shouldBeCalled();
+
+    $normalized = $this->normalizer->normalize($this->fieldItem->reveal());
+
+    $expected = [
+      'target_id' => NULL,
+    ];
+    $this->assertSame($expected, $normalized);
+  }
+
+  /**
+   * @covers ::normalize
+   */
+  public function testNormalizeWithNoEntity(): void {
+    $entity_reference = $this->prophesize(TypedDataInterface::class);
+    $entity_reference->getValue()
+      ->willReturn(NULL)
+      ->shouldBeCalled();
+
+    $field_definition = $this->prophesize(FieldDefinitionInterface::class);
+    $field_definition->getSetting('target_type')
+      ->willReturn('test_type');
+
+    $this->fieldItem->getFieldDefinition()
+      ->willReturn($field_definition->reveal());
+
+    $this->fieldItem->get('entity')
+      ->willReturn($entity_reference->reveal())
+      ->shouldBeCalled();
+
+    $this->fieldItem->getProperties(TRUE)
+      ->willReturn(['target_id' => $this->getTypedDataProperty(FALSE)])
+      ->shouldBeCalled();
+
+    $normalized = $this->normalizer->normalize($this->fieldItem->reveal());
+
+    $expected = [
+      'target_id' => 'test',
+    ];
+    $this->assertSame($expected, $normalized);
+  }
+
+  /**
+   * @covers ::denormalize
+   */
+  public function testDenormalizeWithTypeAndUuid(): void {
+    $data = [
+      'target_id' => 'test',
+      'target_type' => 'test_type',
+      'target_uuid' => '080e3add-f9d5-41ac-9821-eea55b7b42fb',
+    ];
+
+    $entity = $this->prophesize(FieldableEntityInterface::class);
+    $entity->id()
+      ->willReturn('test')
+      ->shouldBeCalled();
+    $this->entityRepository
+      ->loadEntityByUuid($data['target_type'], $data['target_uuid'])
+      ->willReturn($entity)
+      ->shouldBeCalled();
+
+    $this->fieldItem->getProperties()->willReturn([
+      'target_id' => $this->prophesize(IntegerInterface::class),
+    ]);
+    $this->fieldItem->setValue(['target_id' => 'test'])->shouldBeCalled();
+
+    $this->assertDenormalize($data);
+  }
+
+  /**
+   * @covers ::denormalize
+   */
+  public function testDenormalizeWithUuidWithoutType(): void {
+    $data = [
+      'target_id' => 'test',
+      'target_uuid' => '080e3add-f9d5-41ac-9821-eea55b7b42fb',
+    ];
+
+    $entity = $this->prophesize(FieldableEntityInterface::class);
+    $entity->id()
+      ->willReturn('test')
+      ->shouldBeCalled();
+    $this->entityRepository
+      ->loadEntityByUuid('test_type', $data['target_uuid'])
+      ->willReturn($entity)
+      ->shouldBeCalled();
+
+    $this->fieldItem->getProperties()->willReturn([
+      'target_id' => $this->prophesize(IntegerInterface::class),
+    ]);
+    $this->fieldItem->setValue(['target_id' => 'test'])->shouldBeCalled();
+
+    $this->assertDenormalize($data);
+  }
+
+  /**
+   * @covers ::denormalize
+   */
+  public function testDenormalizeWithUuidWithIncorrectType(): void {
+    $this->expectException(UnexpectedValueException::class);
+    $this->expectExceptionMessage('The field "field_reference" property "target_type" must be set to "test_type" or omitted.');
+
+    $data = [
+      'target_id' => 'test',
+      'target_type' => 'wrong_type',
+      'target_uuid' => '080e3add-f9d5-41ac-9821-eea55b7b42fb',
+    ];
+
+    $this->fieldDefinition
+      ->getName()
+      ->willReturn('field_reference')
+      ->shouldBeCalled();
+
+    $this->assertDenormalize($data);
+  }
+
+  /**
+   * @covers ::denormalize
+   */
+  public function testDenormalizeWithTypeWithIncorrectUuid(): void {
+    $this->expectException(InvalidArgumentException::class);
+    $this->expectExceptionMessage('No "test_type" entity found with UUID "unique-but-none-non-existent" for field "field_reference"');
+
+    $data = [
+      'target_id' => 'test',
+      'target_type' => 'test_type',
+      'target_uuid' => 'unique-but-none-non-existent',
+    ];
+    $this->entityRepository
+      ->loadEntityByUuid($data['target_type'], $data['target_uuid'])
+      ->willReturn(NULL)
+      ->shouldBeCalled();
+    $this->fieldItem
+      ->getName()
+      ->willReturn('field_reference')
+      ->shouldBeCalled();
+
+    $this->assertDenormalize($data);
+  }
+
+  /**
+   * @covers ::denormalize
+   */
+  public function testDenormalizeWithEmptyUuid(): void {
+    $this->expectException(InvalidArgumentException::class);
+    $this->expectExceptionMessage('If provided "target_uuid" cannot be empty for field "field_reference".');
+
+    $data = [
+      'target_id' => 'test',
+      'target_type' => 'test_type',
+      'target_uuid' => '',
+    ];
+    $this->fieldItem
+      ->getName()
+      ->willReturn('field_reference')
+      ->shouldBeCalled();
+
+    $this->assertDenormalize($data);
+  }
+
+  /**
+   * @covers ::denormalize
+   */
+  public function testDenormalizeWithId(): void {
+    $data = [
+      'target_id' => 'test',
+    ];
+    $this->fieldItem->setValue($data)->shouldBeCalled();
+
+    $this->assertDenormalize($data);
+  }
+
+  /**
+   * Asserts denormalization process is correct for give data.
+   *
+   * @param array $data
+   *   The data to denormalize.
+   *
+   * @internal
+   */
+  protected function assertDenormalize(array $data): void {
+    $this->fieldItem->getParent()
+      ->willReturn($this->prophesize(FieldItemListInterface::class)->reveal());
+    $this->fieldItem->getFieldDefinition()->willReturn($this->fieldDefinition->reveal());
+    if (!empty($data['target_uuid'])) {
+      $this->fieldDefinition
+        ->getSetting('target_type')
+        ->willReturn('test_type')
+        ->shouldBeCalled();
+    }
+
+    // Avoid a static method call by returning dummy serialized property data.
+    $this->fieldDefinition
+      ->getFieldStorageDefinition()
+      ->willReturn()
+      ->shouldBeCalled();
+    $this->fieldDefinition
+      ->getName()
+      ->willReturn('field_reference')
+      ->shouldBeCalled();
+    $entity = $this->prophesize(EntityInterface::class);
+    $entity_type = $this->prophesize(EntityTypeInterface::class);
+    $entity->getEntityType()
+      ->willReturn($entity_type->reveal())
+      ->shouldBeCalled();
+    $this->fieldItem
+      ->getPluginDefinition()
+      ->willReturn([
+        'serialized_property_names' => [
+          'foo' => 'bar',
+        ],
+      ])
+      ->shouldBeCalled();
+    $this->fieldItem
+      ->getEntity()
+      ->willReturn($entity->reveal())
+      ->shouldBeCalled();
+
+    $context = ['target_instance' => $this->fieldItem->reveal()];
+    $denormalized = $this->normalizer->denormalize($data, EntityReferenceItem::class, 'json', $context);
+    $this->assertSame($context['target_instance'], $denormalized);
+  }
+
+  /**
+   * @covers ::constructValue
+   */
+  public function testConstructValueProperties(): void {
+    $data = [
+      'target_id' => 'test',
+      'target_type' => 'test_type',
+      'target_uuid' => '080e3add-f9d5-41ac-9821-eea55b7b42fb',
+      'extra_property' => 'extra_value',
+    ];
+
+    $entity = $this->prophesize(FieldableEntityInterface::class);
+    $entity->id()
+      ->willReturn('test')
+      ->shouldBeCalled();
+    $this->entityRepository
+      ->loadEntityByUuid($data['target_type'], $data['target_uuid'])
+      ->willReturn($entity)
+      ->shouldBeCalled();
+
+    $this->fieldItem->getProperties()->willReturn([
+      'target_id' => $this->prophesize(IntegerInterface::class),
+      'extra_property' => $this->prophesize(StringInterface::class),
+    ]);
+    $this->fieldItem->setValue([
+      'target_id' => 'test',
+      'extra_property' => 'extra_value',
+    ])->shouldBeCalled();
+
+    $this->assertDenormalize($data);
+  }
+
+}

core/modules/serialization/tests/src/Unit/Normalizer/InternalTypedDataTestTrait.php

@@ -0,0 +1,38 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\serialization\Unit\Normalizer;
+
+use Drupal\Core\TypedData\DataDefinitionInterface;
+use Drupal\Core\TypedData\TypedDataInterface;
+
+/**
+ * Trait that provides mocked typed data objects.
+ */
+trait InternalTypedDataTestTrait {
+
+  /**
+   * Gets a typed data property.
+   *
+   * @param bool $internal
+   *   Whether the typed data property is internal.
+   *
+   * @return \Drupal\Core\TypedData\TypedDataInterface
+   *   The typed data property.
+   */
+  protected function getTypedDataProperty($internal = TRUE) {
+    $definition = $this->prophesize(DataDefinitionInterface::class);
+    $definition->isInternal()
+      ->willReturn($internal)
+      ->shouldBeCalled();
+    $definition = $definition->reveal();
+
+    $property = $this->prophesize(TypedDataInterface::class);
+    $property->getDataDefinition()
+      ->willReturn($definition)
+      ->shouldBeCalled();
+    return $property->reveal();
+  }
+
+}

core/modules/serialization/tests/src/Unit/Normalizer/ListNormalizerTest.php

@@ -0,0 +1,103 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\serialization\Unit\Normalizer;
+
+use Drupal\Core\TypedData\DataDefinition;
+use Drupal\Core\TypedData\TypedDataManagerInterface;
+use Drupal\Tests\UnitTestCase;
+use Drupal\serialization\Normalizer\ListNormalizer;
+use Drupal\Core\TypedData\Plugin\DataType\ItemList;
+use Symfony\Component\Serializer\Serializer;
+
+/**
+ * @coversDefaultClass \Drupal\serialization\Normalizer\ListNormalizer
+ * @group serialization
+ */
+class ListNormalizerTest extends UnitTestCase {
+
+  /**
+   * The ListNormalizer instance.
+   *
+   * @var \Drupal\serialization\Normalizer\ListNormalizer
+   */
+  protected $normalizer;
+
+  /**
+   * The mock list instance.
+   *
+   * @var \Drupal\Core\TypedData\ListInterface|\PHPUnit\Framework\MockObject\MockObject
+   */
+  protected $list;
+
+  /**
+   * The expected list values to use for testing.
+   *
+   * @var array
+   */
+  protected $expectedListValues = ['test', 'test', 'test'];
+
+  /**
+   * The mocked typed data.
+   *
+   * @var \PHPUnit\Framework\MockObject\MockObject|\Drupal\Core\TypedData\TypedDataInterface
+   */
+  protected $typedData;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    // Mock the TypedDataManager to return a TypedDataInterface mock.
+    $this->typedData = $this->createMock('Drupal\Core\TypedData\TypedDataInterface');
+    $typed_data_manager = $this->createMock(TypedDataManagerInterface::class);
+    $typed_data_manager->expects($this->any())
+      ->method('getPropertyInstance')
+      ->willReturn($this->typedData);
+
+    // Set up a mock container as ItemList() will call for the 'typed_data_manager'
+    // service.
+    $container = $this->getMockBuilder('Symfony\Component\DependencyInjection\ContainerBuilder')
+      ->onlyMethods(['get'])
+      ->getMock();
+    $container->expects($this->any())
+      ->method('get')
+      ->with($this->equalTo('typed_data_manager'))
+      ->willReturn($typed_data_manager);
+
+    \Drupal::setContainer($container);
+
+    $this->normalizer = new ListNormalizer();
+
+    $this->list = new ItemList(new DataDefinition());
+    $this->list->setValue($this->expectedListValues);
+  }
+
+  /**
+   * Tests the supportsNormalization() method.
+   */
+  public function testSupportsNormalization(): void {
+    $this->assertTrue($this->normalizer->supportsNormalization($this->list));
+    $this->assertFalse($this->normalizer->supportsNormalization(new \stdClass()));
+  }
+
+  /**
+   * Tests the normalize() method.
+   */
+  public function testNormalize(): void {
+    $serializer = $this->prophesize(Serializer::class);
+    $serializer->normalize($this->typedData, 'json', ['mu' => 'nu'])
+      ->shouldBeCalledTimes(3)
+      ->willReturn('test');
+
+    $this->normalizer->setSerializer($serializer->reveal());
+
+    $normalized = $this->normalizer->normalize($this->list, 'json', ['mu' => 'nu']);
+
+    $this->assertEquals($this->expectedListValues, $normalized);
+  }
+
+}

core/modules/serialization/tests/src/Unit/Normalizer/NormalizerBaseTest.php

@@ -0,0 +1,92 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\serialization\Unit\Normalizer;
+
+use Drupal\Tests\UnitTestCase;
+use Drupal\serialization\Normalizer\NormalizerBase;
+
+/**
+ * @coversDefaultClass \Drupal\serialization\Normalizer\NormalizerBase
+ * @group serialization
+ */
+class NormalizerBaseTest extends UnitTestCase {
+
+  /**
+   * Tests the supportsNormalization method.
+   *
+   * @dataProvider providerTestSupportsNormalization
+   *
+   * @param bool $expected_return
+   *   The expected boolean return value from supportNormalization.
+   * @param mixed $data
+   *   The data passed to supportsNormalization.
+   * @param string $supported_types
+   *   (optional) The supported interface or class to set on the normalizer.
+   */
+  public function testSupportsNormalization($expected_return, $data, $supported_types = NULL): void {
+    $normalizer_base = $this->getMockForAbstractClass('Drupal\Tests\serialization\Unit\Normalizer\TestNormalizerBase');
+
+    if (isset($supported_types)) {
+      $normalizer_base->setSupportedTypes($supported_types);
+    }
+
+    $this->assertSame($expected_return, $normalizer_base->supportsNormalization($data));
+  }
+
+  /**
+   * Data provider for testSupportsNormalization.
+   *
+   * @return array
+   *   An array of provider data for testSupportsNormalization.
+   */
+  public static function providerTestSupportsNormalization() {
+    return [
+      // Something that is not an object should return FALSE immediately.
+      [FALSE, []],
+      // An object with no class set should return FALSE.
+      [FALSE, new \stdClass()],
+      // Set a supported Class.
+      [TRUE, new \stdClass(), 'stdClass'],
+      // Set a supported interface.
+      [TRUE, new \RecursiveArrayIterator(), 'RecursiveIterator'],
+      // Set a different class.
+      [FALSE, new \stdClass(), 'ArrayIterator'],
+      // Set a different interface.
+      [FALSE, new \stdClass(), 'RecursiveIterator'],
+    ];
+  }
+
+}
+
+/**
+ * Test class for NormalizerBase.
+ */
+abstract class TestNormalizerBase extends NormalizerBase {
+
+  /**
+   * The interface or class that this Normalizer supports.
+   *
+   * @var string[]
+   */
+  protected array $supportedTypes = ['*' => FALSE];
+
+  /**
+   * Sets the supported types.
+   *
+   * @param string $supported_types
+   *   The class name to set.
+   */
+  public function setSupportedTypes($supported_types): void {
+    $this->supportedTypes = [$supported_types => FALSE];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getSupportedTypes(?string $format): array {
+    return $this->supportedTypes;
+  }
+
+}

core/modules/serialization/tests/src/Unit/Normalizer/NullNormalizerTest.php

@@ -0,0 +1,58 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\serialization\Unit\Normalizer;
+
+use Drupal\serialization\Normalizer\NullNormalizer;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * @coversDefaultClass \Drupal\serialization\Normalizer\NullNormalizer
+ * @group serialization
+ */
+class NullNormalizerTest extends UnitTestCase {
+
+  /**
+   * The NullNormalizer instance.
+   *
+   * @var \Drupal\serialization\Normalizer\NullNormalizer
+   */
+  protected $normalizer;
+
+  /**
+   * The interface to use in testing.
+   *
+   * @var string
+   */
+  protected $interface = 'Drupal\Core\TypedData\TypedDataInterface';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $this->normalizer = new NullNormalizer($this->interface);
+  }
+
+  /**
+   * @covers ::__construct
+   * @covers ::supportsNormalization
+   */
+  public function testSupportsNormalization(): void {
+    $mock = $this->createMock('Drupal\Core\TypedData\TypedDataInterface');
+    $this->assertTrue($this->normalizer->supportsNormalization($mock));
+    // Also test that an object not implementing TypedDataInterface fails.
+    $this->assertFalse($this->normalizer->supportsNormalization(new \stdClass()));
+  }
+
+  /**
+   * @covers ::normalize
+   */
+  public function testNormalize(): void {
+    $mock = $this->createMock('Drupal\Core\TypedData\TypedDataInterface');
+    $this->assertNull($this->normalizer->normalize($mock));
+  }
+
+}

core/modules/serialization/tests/src/Unit/Normalizer/PrimitiveDataNormalizerTest.php

@@ -0,0 +1,105 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\serialization\Unit\Normalizer;
+
+use Drupal\Core\TypedData\DataDefinition;
+use Drupal\Core\TypedData\Plugin\DataType\BooleanData;
+use Drupal\Core\TypedData\Plugin\DataType\IntegerData;
+use Drupal\Core\TypedData\Plugin\DataType\StringData;
+use Drupal\Tests\UnitTestCase;
+use Drupal\serialization\Normalizer\PrimitiveDataNormalizer;
+
+/**
+ * @coversDefaultClass \Drupal\serialization\Normalizer\PrimitiveDataNormalizer
+ * @group serialization
+ */
+class PrimitiveDataNormalizerTest extends UnitTestCase {
+
+  /**
+   * The TypedDataNormalizer instance.
+   *
+   * @var \Drupal\serialization\Normalizer\TypedDataNormalizer
+   */
+  protected $normalizer;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $this->normalizer = new PrimitiveDataNormalizer();
+  }
+
+  /**
+   * @covers ::supportsNormalization
+   * @dataProvider dataProviderPrimitiveData
+   */
+  public function testSupportsNormalization($primitive_data, $expected): void {
+    $this->assertTrue($this->normalizer->supportsNormalization($primitive_data));
+  }
+
+  /**
+   * @covers ::supportsNormalization
+   */
+  public function testSupportsNormalizationFail(): void {
+    // Test that an object not implementing PrimitiveInterface fails.
+    $this->assertFalse($this->normalizer->supportsNormalization(new \stdClass()));
+  }
+
+  /**
+   * @covers ::normalize
+   * @dataProvider dataProviderPrimitiveData
+   */
+  public function testNormalize($primitive_data, $expected): void {
+    $this->assertSame($expected, $this->normalizer->normalize($primitive_data));
+  }
+
+  /**
+   * Data provider for testNormalize().
+   */
+  public static function dataProviderPrimitiveData() {
+    $data = [];
+
+    $definition = DataDefinition::createFromDataType('string');
+    $string = new StringData($definition, 'string');
+    $string->setValue('test');
+
+    $data['string'] = [$string, 'test'];
+
+    $definition = DataDefinition::createFromDataType('string');
+    $string = new StringData($definition, 'string');
+    $string->setValue(NULL);
+
+    $data['string-null'] = [$string, NULL];
+
+    $definition = DataDefinition::createFromDataType('integer');
+    $integer = new IntegerData($definition, 'integer');
+    $integer->setValue(5);
+
+    $data['integer'] = [$integer, 5];
+
+    $definition = DataDefinition::createFromDataType('integer');
+    $integer = new IntegerData($definition, 'integer');
+    $integer->setValue(NULL);
+
+    $data['integer-null'] = [$integer, NULL];
+
+    $definition = DataDefinition::createFromDataType('boolean');
+    $boolean = new BooleanData($definition, 'boolean');
+    $boolean->setValue(TRUE);
+
+    $data['boolean'] = [$boolean, TRUE];
+
+    $definition = DataDefinition::createFromDataType('boolean');
+    $boolean = new BooleanData($definition, 'boolean');
+    $boolean->setValue(NULL);
+
+    $data['boolean-null'] = [$boolean, NULL];
+
+    return $data;
+  }
+
+}

core/modules/serialization/tests/src/Unit/Normalizer/TimestampItemNormalizerTest.php

@@ -0,0 +1,184 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\serialization\Unit\Normalizer;
+
+use Drupal\Core\Entity\EntityInterface;
+use Drupal\Core\Entity\EntityTypeInterface;
+use Drupal\Core\Field\FieldDefinitionInterface;
+use Drupal\Core\Field\Plugin\Field\FieldType\CreatedItem;
+use Drupal\Core\Field\Plugin\Field\FieldType\EntityReferenceItem;
+use Drupal\Core\Field\Plugin\Field\FieldType\TimestampItem;
+use Drupal\Core\TypedData\DataDefinitionInterface;
+use Drupal\Core\TypedData\Plugin\DataType\Timestamp;
+use Drupal\serialization\Normalizer\TimestampItemNormalizer;
+use Drupal\Tests\UnitTestCase;
+use Symfony\Component\Serializer\Serializer;
+
+/**
+ * Tests that TimestampItem (de)normalization uses Timestamp (de)normalization.
+ *
+ * @group serialization
+ * @coversDefaultClass \Drupal\serialization\Normalizer\TimestampItemNormalizer
+ * @see \Drupal\serialization\Normalizer\TimestampNormalizer
+ */
+class TimestampItemNormalizerTest extends UnitTestCase {
+
+  /**
+   * @var \Drupal\serialization\Normalizer\TimestampItemNormalizer
+   */
+  protected $normalizer;
+
+  /**
+   * The test TimestampItem.
+   *
+   * @var \Drupal\Core\Field\Plugin\Field\FieldType\TimestampItem
+   */
+  protected $item;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $this->normalizer = new TimestampItemNormalizer();
+  }
+
+  /**
+   * @covers ::supportsNormalization
+   */
+  public function testSupportsNormalization(): void {
+    $timestamp_item = $this->createTimestampItemProphecy();
+    $this->assertTrue($this->normalizer->supportsNormalization($timestamp_item->reveal()));
+
+    $entity_ref_item = $this->prophesize(EntityReferenceItem::class);
+    $this->assertFalse($this->normalizer->supportsNormalization($entity_ref_item->reveal()));
+  }
+
+  /**
+   * @covers ::supportsDenormalization
+   */
+  public function testSupportsDenormalization(): void {
+    $timestamp_item = $this->createTimestampItemProphecy();
+    $this->assertTrue($this->normalizer->supportsDenormalization($timestamp_item->reveal(), TimestampItem::class));
+
+    // CreatedItem extends regular TimestampItem.
+    $timestamp_item = $this->prophesize(CreatedItem::class);
+    $this->assertTrue($this->normalizer->supportsDenormalization($timestamp_item->reveal(), TimestampItem::class));
+
+    $entity_ref_item = $this->prophesize(EntityReferenceItem::class);
+    $this->assertFalse($this->normalizer->supportsNormalization($entity_ref_item->reveal(), TimestampItem::class));
+  }
+
+  /**
+   * @covers ::normalize
+   * @see \Drupal\Tests\serialization\Unit\Normalizer\TimestampNormalizerTest
+   */
+  public function testNormalize(): void {
+    // Mock TimestampItem @FieldType, which contains a Timestamp @DataType,
+    // which has a DataDefinition.
+    $data_definition = $this->prophesize(DataDefinitionInterface::class);
+    $data_definition->isInternal()
+      ->willReturn(FALSE)
+      ->shouldBeCalled();
+    $timestamp = $this->prophesize(Timestamp::class);
+    $timestamp->getDataDefinition()
+      ->willReturn($data_definition->reveal())
+      ->shouldBeCalled();
+    $timestamp = $timestamp->reveal();
+    $timestamp_item = $this->createTimestampItemProphecy();
+    $timestamp_item->getProperties(TRUE)
+      ->willReturn(['value' => $timestamp])
+      ->shouldBeCalled();
+
+    // Mock Serializer service, to assert that the Timestamp @DataType
+    // normalizer would be called.
+    $timestamp_datetype_normalization = $this->randomMachineName();
+    $serializer_prophecy = $this->prophesize(Serializer::class);
+    // This is where \Drupal\serialization\Normalizer\TimestampNormalizer would
+    // be called.
+    $serializer_prophecy->normalize($timestamp, NULL, [])
+      ->willReturn($timestamp_datetype_normalization)
+      ->shouldBeCalled();
+
+    $this->normalizer->setSerializer($serializer_prophecy->reveal());
+
+    $normalized = $this->normalizer->normalize($timestamp_item->reveal());
+    $this->assertSame(['value' => $timestamp_datetype_normalization, 'format' => \DateTime::RFC3339], $normalized);
+  }
+
+  /**
+   * @covers ::denormalize
+   */
+  public function testDenormalize(): void {
+    $timestamp_item_normalization = [
+      'value' => $this->randomMachineName(),
+      'format' => \DateTime::RFC3339,
+    ];
+    $timestamp_data_denormalization = $this->randomMachineName();
+
+    $timestamp_item = $this->createTimestampItemProphecy();
+    // The field item should get the Timestamp @DataType denormalization set as
+    // a value, in FieldItemNormalizer::denormalize().
+    $timestamp_item->setValue(['value' => $timestamp_data_denormalization])
+      ->shouldBeCalled();
+
+    // Avoid a static method call by returning dummy serialized property data.
+    $field_definition = $this->prophesize(FieldDefinitionInterface::class);
+    $timestamp_item
+      ->getFieldDefinition()
+      ->willReturn($field_definition->reveal())
+      ->shouldBeCalled();
+    $timestamp_item->getPluginDefinition()
+      ->willReturn([
+        'serialized_property_names' => [
+          'foo' => 'bar',
+        ],
+      ])
+      ->shouldBeCalled();
+    $entity = $this->prophesize(EntityInterface::class);
+    $entity_type = $this->prophesize(EntityTypeInterface::class);
+    $entity->getEntityType()
+      ->willReturn($entity_type->reveal())
+      ->shouldBeCalled();
+    $timestamp_item
+      ->getEntity()
+      ->willReturn($entity->reveal())
+      ->shouldBeCalled();
+
+    $context = [
+      'target_instance' => $timestamp_item->reveal(),
+      'datetime_allowed_formats' => [\DateTime::RFC3339],
+    ];
+
+    // Mock Serializer service, to assert that the Timestamp @DataType
+    // denormalizer would be called.
+    $serializer_prophecy = $this->prophesize(Serializer::class);
+    // This is where \Drupal\serialization\Normalizer\TimestampNormalizer would
+    // be called.
+    $serializer_prophecy->denormalize($timestamp_item_normalization['value'], Timestamp::class, NULL, $context)
+      ->willReturn($timestamp_data_denormalization)
+      ->shouldBeCalled();
+
+    $this->normalizer->setSerializer($serializer_prophecy->reveal());
+
+    $denormalized = $this->normalizer->denormalize($timestamp_item_normalization, TimestampItem::class, NULL, $context);
+    $this->assertInstanceOf(TimestampItem::class, $denormalized);
+  }
+
+  /**
+   * Creates a TimestampItem prophecy.
+   *
+   * @return \Prophecy\Prophecy\ObjectProphecy|\Drupal\Core\Field\Plugin\Field\FieldType\TimestampItem
+   */
+  protected function createTimestampItemProphecy() {
+    $timestamp_item = $this->prophesize(TimestampItem::class);
+    $timestamp_item->getParent()
+      ->willReturn(TRUE);
+
+    return $timestamp_item;
+  }
+
+}

core/modules/serialization/tests/src/Unit/Normalizer/TimestampNormalizerTest.php

@@ -0,0 +1,152 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\serialization\Unit\Normalizer;
+
+use Drupal\Core\Config\ConfigFactoryInterface;
+use Drupal\Core\Datetime\DrupalDateTime;
+use Drupal\Core\TypedData\Plugin\DataType\IntegerData;
+use Drupal\Core\TypedData\Plugin\DataType\Timestamp;
+use Drupal\Core\TypedData\Type\DateTimeInterface;
+use Drupal\serialization\Normalizer\TimestampNormalizer;
+use Drupal\Tests\UnitTestCase;
+use Symfony\Component\Serializer\Exception\UnexpectedValueException;
+
+/**
+ * Unit test coverage for the "Timestamp" @DataType.
+ *
+ * @group serialization
+ * @coversDefaultClass \Drupal\serialization\Normalizer\TimestampNormalizer
+ * @see \Drupal\Core\TypedData\Plugin\DataType\Timestamp
+ */
+class TimestampNormalizerTest extends UnitTestCase {
+
+  /**
+   * The tested data type's normalizer.
+   *
+   * @var \Drupal\serialization\Normalizer\TimestampNormalizer
+   */
+  protected $normalizer;
+
+  /**
+   * The tested data type.
+   *
+   * @var \Drupal\Core\Field\Plugin\Field\FieldType\TimestampItem
+   */
+  protected $data;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $this->normalizer = new TimestampNormalizer($this->prophesize(ConfigFactoryInterface::class)->reveal());
+    $this->data = $this->prophesize(Timestamp::class);
+  }
+
+  /**
+   * @covers ::supportsNormalization
+   */
+  public function testSupportsNormalization(): void {
+    $this->assertTrue($this->normalizer->supportsNormalization($this->data->reveal()));
+
+    $integer = $this->prophesize(IntegerData::class);
+    $this->assertFalse($this->normalizer->supportsNormalization($integer->reveal()));
+
+    $datetime = $this->prophesize(DateTimeInterface::class);
+    $this->assertFalse($this->normalizer->supportsNormalization($datetime->reveal()));
+  }
+
+  /**
+   * @covers ::supportsDenormalization
+   */
+  public function testSupportsDenormalization(): void {
+    $this->assertTrue($this->normalizer->supportsDenormalization($this->data->reveal(), Timestamp::class));
+  }
+
+  /**
+   * @covers ::normalize
+   */
+  public function testNormalize(): void {
+    $random_rfc_3339_string = $this->randomMachineName();
+
+    $drupal_date_time = $this->prophesize(TimestampNormalizerTestDrupalDateTime::class);
+    $drupal_date_time->setTimezone(new \DateTimeZone('UTC'))
+      ->willReturn($drupal_date_time->reveal());
+    $drupal_date_time->format(\DateTime::RFC3339)
+      ->willReturn($random_rfc_3339_string);
+
+    $this->data->getDateTime()
+      ->willReturn($drupal_date_time->reveal());
+
+    $normalized = $this->normalizer->normalize($this->data->reveal());
+    $this->assertSame($random_rfc_3339_string, $normalized);
+  }
+
+  /**
+   * Tests the denormalize function with good data.
+   *
+   * @covers ::denormalize
+   * @dataProvider providerTestDenormalizeValidFormats
+   */
+  public function testDenormalizeValidFormats($normalized, $expected): void {
+    $denormalized = $this->normalizer->denormalize($normalized, Timestamp::class, NULL, []);
+    $this->assertSame($expected, $denormalized);
+  }
+
+  /**
+   * Data provider for testDenormalizeValidFormats.
+   *
+   * @return array
+   */
+  public static function providerTestDenormalizeValidFormats() {
+    $expected_stamp = 1478422920;
+
+    $data = [];
+
+    $data['U'] = [$expected_stamp, $expected_stamp];
+    $data['RFC3339'] = ['2016-11-06T09:02:00+00:00', $expected_stamp];
+    $data['RFC3339 +0100'] = ['2016-11-06T09:02:00+01:00', $expected_stamp - 1 * 3600];
+    $data['RFC3339 -0600'] = ['2016-11-06T09:02:00-06:00', $expected_stamp + 6 * 3600];
+
+    $data['ISO8601'] = ['2016-11-06T09:02:00+0000', $expected_stamp];
+    $data['ISO8601 +0100'] = ['2016-11-06T09:02:00+0100', $expected_stamp - 1 * 3600];
+    $data['ISO8601 -0600'] = ['2016-11-06T09:02:00-0600', $expected_stamp + 6 * 3600];
+
+    return $data;
+  }
+
+  /**
+   * Tests the denormalize function with bad data.
+   *
+   * @covers ::denormalize
+   */
+  public function testDenormalizeException(): void {
+    $this->expectException(UnexpectedValueException::class);
+    $this->expectExceptionMessage('The specified date "2016/11/06 09:02am GMT" is not in an accepted format: "U" (UNIX timestamp), "Y-m-d\TH:i:sO" (ISO 8601), "Y-m-d\TH:i:sP" (RFC 3339).');
+
+    $normalized = '2016/11/06 09:02am GMT';
+
+    $this->normalizer->denormalize($normalized, Timestamp::class, NULL, []);
+  }
+
+}
+
+/**
+ * Provides a test class for testing DrupalDateTime.
+ *
+ * Note: Prophecy does not support magic methods. By subclassing and specifying
+ * an explicit method, Prophecy works.
+ * @see https://github.com/phpspec/prophecy/issues/338
+ * @see https://github.com/phpspec/prophecy/issues/34
+ * @see https://github.com/phpspec/prophecy/issues/80
+ */
+class TimestampNormalizerTestDrupalDateTime extends DrupalDateTime {
+
+  public function setTimezone(\DateTimeZone $timezone) {
+    parent::setTimezone($timezone);
+  }
+
+}

core/modules/serialization/tests/src/Unit/Normalizer/TypedDataNormalizerTest.php

@@ -0,0 +1,60 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\serialization\Unit\Normalizer;
+
+use Drupal\Tests\UnitTestCase;
+use Drupal\serialization\Normalizer\TypedDataNormalizer;
+
+/**
+ * @coversDefaultClass \Drupal\serialization\Normalizer\TypedDataNormalizer
+ * @group serialization
+ */
+class TypedDataNormalizerTest extends UnitTestCase {
+
+  /**
+   * The TypedDataNormalizer instance.
+   *
+   * @var \Drupal\serialization\Normalizer\TypedDataNormalizer
+   */
+  protected $normalizer;
+
+  /**
+   * The mock typed data instance.
+   *
+   * @var \Drupal\Core\TypedData\TypedDataInterface|\PHPUnit\Framework\MockObject\MockObject
+   */
+  protected $typedData;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $this->normalizer = new TypedDataNormalizer();
+    $this->typedData = $this->createMock('Drupal\Core\TypedData\TypedDataInterface');
+  }
+
+  /**
+   * Tests the supportsNormalization() method.
+   */
+  public function testSupportsNormalization(): void {
+    $this->assertTrue($this->normalizer->supportsNormalization($this->typedData));
+    // Also test that an object not implementing TypedDataInterface fails.
+    $this->assertFalse($this->normalizer->supportsNormalization(new \stdClass()));
+  }
+
+  /**
+   * Tests the normalize() method.
+   */
+  public function testNormalize(): void {
+    $this->typedData->expects($this->once())
+      ->method('getValue')
+      ->willReturn('test');
+
+    $this->assertEquals('test', $this->normalizer->normalize($this->typedData));
+  }
+
+}