.../jsonapi_test_data_type.info.yml                |   5 +
 .../jsonapi_test_data_type.services.yml            |   6 +
 .../src/Normalizer/StringNormalizer.php            |  25 ++++
 .../jsonapi_test_field_type.info.yml               |   5 +
 .../jsonapi_test_field_type.services.yml           |   6 +
 .../src/Normalizer/StringNormalizer.php            |  27 ++++
 tests/src/Functional/ExternalNormalizersTest.php   | 149 +++++++++++++++++++++
 7 files changed, 223 insertions(+)

diff --git a/tests/modules/jsonapi_test_data_type/jsonapi_test_data_type.info.yml b/tests/modules/jsonapi_test_data_type/jsonapi_test_data_type.info.yml
new file mode 100644
index 0000000..e61dde8
--- /dev/null
+++ b/tests/modules/jsonapi_test_data_type/jsonapi_test_data_type.info.yml
@@ -0,0 +1,5 @@
+name: 'JSON API test format-agnostic @DataType normalizers'
+type: module
+package: Testing
+version: VERSION
+core: 8.x
diff --git a/tests/modules/jsonapi_test_data_type/jsonapi_test_data_type.services.yml b/tests/modules/jsonapi_test_data_type/jsonapi_test_data_type.services.yml
new file mode 100644
index 0000000..b8ad331
--- /dev/null
+++ b/tests/modules/jsonapi_test_data_type/jsonapi_test_data_type.services.yml
@@ -0,0 +1,6 @@
+services:
+  serializer.normalizer.string.jsonapi_test_data_type:
+    class: Drupal\jsonapi_test_data_type\Normalizer\StringNormalizer
+    tags:
+      # The priority must be higher than serializer.normalizer.primitive_data.
+      - { name: normalizer , priority: 1000 }
diff --git a/tests/modules/jsonapi_test_data_type/src/Normalizer/StringNormalizer.php b/tests/modules/jsonapi_test_data_type/src/Normalizer/StringNormalizer.php
new file mode 100644
index 0000000..abb6610
--- /dev/null
+++ b/tests/modules/jsonapi_test_data_type/src/Normalizer/StringNormalizer.php
@@ -0,0 +1,25 @@
+<?php
+
+namespace Drupal\jsonapi_test_data_type\Normalizer;
+
+use Drupal\Core\TypedData\Plugin\DataType\StringData;
+use Drupal\serialization\Normalizer\NormalizerBase;
+
+/**
+ * Normalizes string data, with a twist: it replaces 'super' with 'NOT'.
+ */
+class StringNormalizer extends NormalizerBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $supportedInterfaceOrClass = StringData::class;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function normalize($object, $format = NULL, array $context = []) {
+    return str_replace('super', 'NOT', $object->getValue());
+  }
+
+}
diff --git a/tests/modules/jsonapi_test_field_type/jsonapi_test_field_type.info.yml b/tests/modules/jsonapi_test_field_type/jsonapi_test_field_type.info.yml
new file mode 100644
index 0000000..6a73aee
--- /dev/null
+++ b/tests/modules/jsonapi_test_field_type/jsonapi_test_field_type.info.yml
@@ -0,0 +1,5 @@
+name: 'JSON API test format-agnostic @FieldType normalizers'
+type: module
+package: Testing
+version: VERSION
+core: 8.x
diff --git a/tests/modules/jsonapi_test_field_type/jsonapi_test_field_type.services.yml b/tests/modules/jsonapi_test_field_type/jsonapi_test_field_type.services.yml
new file mode 100644
index 0000000..ebb51ed
--- /dev/null
+++ b/tests/modules/jsonapi_test_field_type/jsonapi_test_field_type.services.yml
@@ -0,0 +1,6 @@
+services:
+  serializer.normalizer.string.jsonapi_test_field_type:
+    class: Drupal\jsonapi_test_field_type\Normalizer\StringNormalizer
+    tags:
+      # The priority must be higher than serialization.normalizer.field_item.
+      - { name: normalizer , priority: 1000 }
diff --git a/tests/modules/jsonapi_test_field_type/src/Normalizer/StringNormalizer.php b/tests/modules/jsonapi_test_field_type/src/Normalizer/StringNormalizer.php
new file mode 100644
index 0000000..0376ca6
--- /dev/null
+++ b/tests/modules/jsonapi_test_field_type/src/Normalizer/StringNormalizer.php
@@ -0,0 +1,27 @@
+<?php
+
+namespace Drupal\jsonapi_test_field_type\Normalizer;
+
+use Drupal\Core\Field\Plugin\Field\FieldType\StringItem;
+use Drupal\serialization\Normalizer\FieldItemNormalizer;
+
+/**
+ * Normalizes string fields, with a twist: it replaces 'super' with 'NOT'.
+ */
+class StringNormalizer extends FieldItemNormalizer {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $supportedInterfaceOrClass = StringItem::class;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function normalize($object, $format = NULL, array $context = []) {
+    $data = parent::normalize($object, $format, $context);
+    $data['value'] = str_replace('super', 'NOT', $data['value']);
+    return $data;
+  }
+
+}
diff --git a/tests/src/Functional/ExternalNormalizersTest.php b/tests/src/Functional/ExternalNormalizersTest.php
new file mode 100644
index 0000000..dcd99c1
--- /dev/null
+++ b/tests/src/Functional/ExternalNormalizersTest.php
@@ -0,0 +1,149 @@
+<?php
+
+namespace Drupal\Tests\jsonapi\Functional;
+
+use Drupal\Component\Serialization\Json;
+use Drupal\Core\Url;
+use Drupal\entity_test\Entity\EntityTest;
+use Drupal\field\Entity\FieldConfig;
+use Drupal\field\Entity\FieldStorageConfig;
+use Drupal\Tests\BrowserTestBase;
+use Drupal\user\Entity\Role;
+use Drupal\user\RoleInterface;
+
+/**
+ * Asserts external normalizers are handled as expected by the JSON API module.
+ *
+ * @see jsonapi.normalizers
+ *
+ * @group jsonapi
+ */
+class ExternalNormalizersTest extends BrowserTestBase {
+
+  /**
+   * The original value for the test field.
+   *
+   * @var string
+   */
+  const VALUE_ORIGINAL = 'Llamas are super awesome!';
+
+  /**
+   * The expected overridden value for the test field.
+   *
+   * @see \Drupal\jsonapi_test_field_type\Normalizer\StringNormalizer
+   * @see \Drupal\jsonapi_test_data_type\Normalizer\StringNormalizer
+   */
+  const VALUE_OVERRIDDEN = 'Llamas are NOT awesome!';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = [
+    'jsonapi',
+    'entity_test',
+  ];
+
+  /**
+   * The test entity.
+   *
+   * @var \Drupal\entity_test\Entity\EntityTest
+   */
+  protected $entity;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setUp() {
+    parent::setUp();
+
+    // This test is not about access control at all, so allow anonymous users to
+    // view the test entities.
+    Role::load(RoleInterface::ANONYMOUS_ID)
+      ->grantPermission('view test entity')
+      ->save();
+
+    FieldStorageConfig::create([
+      'field_name' => 'field_test',
+      'type' => 'string',
+      'entity_type' => 'entity_test',
+    ])
+      ->save();
+    FieldConfig::create([
+      'field_name' => 'field_test',
+      'entity_type' => 'entity_test',
+      'bundle' => 'entity_test',
+    ])
+      ->save();
+
+    $this->entity = EntityTest::create([
+      'name' => 'Llama',
+      'type' => 'entity_test',
+      'field_test' => static::VALUE_ORIGINAL,
+    ]);
+    $this->entity->save();
+  }
+
+  /**
+   * Tests a format-agnostic normalizer.
+   *
+   * @param string $test_module
+   *   The test module to install, which comes with a high-priority normalizer.
+   * @param string $expected_value_jsonapi_normalization
+   *   The expected JSON API normalization of the tested field. Must be either
+   *   - static::VALUE_ORIGINAL (normalizer IS NOT expected to override)
+   *   - static::VALUE_OVERRIDDEN (normalizer IS expected to override)
+   *
+   * @dataProvider providerTestFormatAgnosticNormalizers
+   */
+  public function testFormatAgnosticNormalizers($test_module, $expected_value_jsonapi_normalization) {
+    assert(in_array($expected_value_jsonapi_normalization, [static::VALUE_ORIGINAL, static::VALUE_OVERRIDDEN], TRUE));
+
+    // Asserts the entity contains the value we set.
+    $this->assertSame(static::VALUE_ORIGINAL, $this->entity->field_test->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);
+    $this->assertSame(static::VALUE_ORIGINAL, $core_normalization['field_test'][0]['value']);
+
+    // Install test module that contains a high-priority alternative normalizer.
+    $this->container->get('module_installer')->install([$test_module]);
+    $this->rebuildContainer();
+
+    // Asserts normalizing the entity using core's 'serializer' service DOES NOT
+    // ANYMORE yield the value we set.
+    $core_normalization = $this->container->get('serializer')->normalize($this->entity);
+    $this->assertSame(static::VALUE_OVERRIDDEN, $core_normalization['field_test'][0]['value']);
+
+    // Asserts that this does NOT affect the JSON API normalization.
+    // @todo Remove line below in favor of commented line in https://www.drupal.org/project/jsonapi/issues/2878463.
+    $url = Url::fromRoute('jsonapi.entity_test--entity_test.individual', ['entity_test' => $this->entity->uuid()]);
+    /* $url = $this->entity->toUrl('jsonapi'); */
+    $client = $this->getSession()->getDriver()->getClient()->getClient();
+    $response = $client->request('GET', $url->setAbsolute(TRUE)->toString());
+    $document = Json::decode((string) $response->getBody());
+    $this->assertSame($expected_value_jsonapi_normalization, $document['data']['attributes']['field_test']);
+  }
+
+  /**
+   * Data provider.
+   *
+   * @return array
+   *   Test cases.
+   */
+  public function providerTestFormatAgnosticNormalizers() {
+    return [
+      'Format-agnostic @FieldType-level normalizers SHOULD NOT be able to affect the JSON API normalization' => [
+        // \Drupal\jsonapi_test_field_type\Normalizer\StringNormalizer::normalize()
+        'jsonapi_test_field_type',
+        static::VALUE_ORIGINAL,
+      ],
+      'Format-agnostic @DataType-level normalizers SHOULD be able to affect the JSON API normalization' => [
+        // \Drupal\jsonapi_test_field_type\Normalizer\StringNormalizer::normalize()
+        'jsonapi_test_data_type',
+        static::VALUE_OVERRIDDEN,
+      ],
+    ];
+  }
+
+}