#837 - Added section on registering a custom media type.

src/main/asciidoc/mediatypes.adoc

@@ -56,4 +56,89 @@ Since the purpose of the `CurieProvider` API is to allow for automatic curie cre
 [[mediatypes.custom]]
 == Registering a custom media type
 
-TODO
+Spring HATEOAS allows to integrate support for custom media types through a set of SPIs, that third parties can implement.
+The building blocks of an such an implementations are:
+
+1. Some form of Jackson ObjectMapper customization. In its most simple case that's a Jackson `Module` implementation.
+2. A `LinkDiscoverer` implementation so that the client side support is able to detect links in representations generated.
+3. Some configuration infrastructure that will allow Spring HATEOAS to find the custom implementation and pick up its configuration.
+
+[[mediatypes.custom.configuration]]
+=== Custom media type configuration
+
+Custom media type implementations are picked up through Spring's `SpringFactories` mechanism, similar to the Java `ServiceLoader` API.
+Each media type implementation needs to ship with a `spring.factories` in `META-INF` containing an implementation class entry for the `org.springframework.hateoas.config.MediaTypeConfigurationProvider` key:
+
+.An example `MediaTypeConfigurationProvider` declaration
+====
+[source]
+org.springframework.hateoas.config.MediaTypeConfigurationProvider=\
+  com.acme.mymediatype.MyMediaTypeConfigurationProvider
+====
+
+That implementation class could then look as follows:
+
+.An example `MediaTypeConfigurationProvider` implementation in `META-INF/spring.factories`
+====
+[source,java]
+----
+class MyMediaTypeConfigurationProvider
+  implements MediaTypeConfigurationProvider {
+
+  @Override
+  public Class<? extends HypermediaMappingInformation> getConfiguration() {
+    return MyMediaTypeConfiguration.class; <1>
+  }
+
+  @Override
+  public boolean supportsAny(Collection<MediaType> mediaTypes) {
+    return mediaTypes.contains(MediaTypes.HAL_JSON); <2>
+  }
+}
+----
+The configuration class needs to have a default constructor and expose two methods:
+
+<1> A method returning a Spring configuration class that will be included in the application bootstrap when Spring HATEOAS is activated (either implicitly via Spring Boot auto-configuration or via `@EnableHypermediaSupport`).
+<2> A callback method that will get the application selected media types to activate passed. This allows the media type implementation to control, when it it will be activated.
+====
+
+The configuration class has to implement `HypermediaMappingInformation`. It could look as simple as this:
+
+====
+[source, java]
+----
+@Configuration
+class MyMediaTypeConfiguration implements HypermediaMappingInformation {
+
+  @Override
+  public List<MediaType> getMediaTypes() {
+    return MediaType.parse("application/vnd-acme-media-type") <1>
+  }
+
+  @Override
+  public Module getJacksonModule() {
+    return new Jackson2MediaTypeModule(); <2>
+  }
+
+  @Bean
+  MyLinkDiscoverer myLinkDiscoverer() {
+    return new MyLinkDiscoverer(); <3>
+  }
+}
+----
+<1> The configuration class returns the media type it wants to get Spring MVC / Spring WebFlux support set up.
+<2> It overrides `getJacksonModule()` to provide custom serializers to create the media type specific representations.
+<3> It also declares a custom `LinkDiscoverer` implementation for client side support.
+====
+
+The Jackson module usually declares `Serializer` and `Deserializer` implementations for the representation model types `RepresentationModel`, `EntityModel`, `CollectionModel` and `PagedModel`.
+In case you need further customization of the Jackson `ObjectMapper` (like a custom `HandlerInstantiator`), you can alternatively override `configureObjectMapper(…)`.
+
+[[mediatypes.custom.recommendation]]
+=== Recommendations
+
+The preferred way to implement media type representations is by providing a type hierarchy that matches the expected format and can be serialized by Jackson as is.
+In the `Serializer` and `Deserializer` implementations registered for `RepresentationModel`, convert the instances into the media type specific model types and then lookup the Jackson serializer for those.
+
+The media types supported by default use the same configuration mechanism as third party implementations would.
+So it's worth studying the implementations in https://github.com/spring-projects/spring-hateoas/tree/master/src/main/java/org/springframework/hateoas/mediatype[the `mediatype` package].