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This version is still in development and is not considered stable yet. For the latest stable version, please use Spring Data REST 4.4.0! |
Customizing the JSON Output
Sometimes in your application, you need to provide links to other resources from a particular entity. For example, a Customer response might be enriched with links to a current shopping cart or links to manage resources related to that entity. Spring Data REST provides integration with Spring HATEOAS and provides an extension hook that lets you alter the representation of resources that go out to the client.
The RepresentationModelProcessor Interface
Spring HATEOAS defines a RepresentationModelProcessor<> interface for processing entities. All beans of type RepresentationModelProcessor<EntityModel<T>> are automatically picked up by the Spring Data REST exporter and triggered when serializing an entity of type T.
For example, to define a processor for a Person entity, add a @Bean similar to the following (which is taken from the Spring Data REST tests) to your ApplicationContext:
@Bean
public RepresentationModelProcessor<EntityModel<Person>> personProcessor() {
return new RepresentationModelProcessor<EntityModel<Person>>() {
@Override
public EntityModel<Person> process(EntityModel<Person> model) {
model.add(new Link("http://localhost:8080/people", "added-link"));
return model;
}
};
}
The preceding example hard codes a link to localhost:8080/people. If you have a Spring MVC endpoint inside your app to which you wish to link, consider using Spring HATEOAS’s linkTo(…) method to avoid managing the URL.
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Adding Links
You can add links to the default representation of an entity by calling model.add(Link), as the preceding example shows. Any links you add to the EntityModel are added to the final output.
Customizing the Representation
The Spring Data REST exporter runs any discovered RepresentationModelProcessor instances before it creates the output representation. It does so by registering a Converter<Entity, EntityModel> instance with an internal ConversionService. This is the component responsible for creating the links to referenced entities (such as those objects under the _links property in the object’s JSON representation). It takes an @Entity and iterates over its properties, creating links for those properties that are managed by a Repository and copying across any embedded or simple properties.
If your project needs to have output in a different format, however, you can completely replace the default outgoing JSON representation with your own. If you register your own ConversionService in the ApplicationContext and register your own Converter<Entity, EntityModel>, you can return a EntityModel implementation of your choosing.