How to enable HAL rendering without using @EnableHypermediaSupport with spring hateoas?

Question

With @EnableHypermediaSupport(type = HypermediaType.HAL) Spring hateoas provides a simple and convenient way to enable HAL rendering.

The annotations triggers some configuration magic which is explained here: https://github.com/spring-projects/spring-hateoas#enablehypermediasupport

However if you are working on a given xml-config based spring application, it is not easy to integrate @EnableHypermediaSupport. I tried a lot of different ways to enable HAL rendering but no single solution was working correctly:

• Including a configuration class in existing mvc servlet xml configuration.
• Using different versions of spring hateos (0.70.0, 0.80.0, 0.9.0.BUILD-SNAPSHOT), hoping that 0.90.0 will produce HAL out of the box, since, according to a jira issue, it should become the default rendering (at least for spring data rest).
• Using a custom MappingJackson2HttpMessageConverter to register a new Jackson2HalModule() also did not work. Although the converter was used for rendering (for example to render Dates), the output was not rendered in HAL.

Therefore my question: How can I enable HAL rendering without using EnableHypermediaSupport?

I intentionally left out any code snippets, because I don't think that it would help much.

Answer 1

Based on @WaldemarSchneider answer, here is a concrete how-to:

Create an HTTP message converter:

public class HalHttpMessageConverter extends AbstractJackson2HttpMessageConverter {

  public HalHttpMessageConverter() {
    super(new ObjectMapper(), new MediaType("application", "hal+json", DEFAULT_CHARSET));
    objectMapper.registerModule(new Jackson2HalModule());
    objectMapper.setHandlerInstantiator(new Jackson2HalModule.HalHandlerInstantiator(new DefaultRelProvider(), null));
    // customize your mapper if needed
    objectMapper.enable(SerializationFeature.INDENT_OUTPUT);
  }

  @Override
  protected boolean supports(Class<?> clazz) {
    return ResourceSupport.class.isAssignableFrom(clazz);
  }

}

Register it in your servlet context:

<mvc:annotation-driven>
  <mvc:message-converters>
    <bean class="package.to.HalHttpMessageConverter" />
  </mvc:message-converters>
</mvc:annotation-driven>

And don't forget to make your rest controllers produce application/hal+json:

@RestController
@RequestMapping(value = "/articles", produces = "application/hal+json")
public class ArticleRestController {
  ...
}

Answer 2

Maybe this question is a little bit old but i want to provide my solution for the following users.

I had similiar circumstances (Spring MVC 3.2.10.RELEASE, Spring HATEAOS 0.16.0.RELEASE) to configure spring hateoas to use hypermedia without java configuration.

My solution was the following:

Create a custom HalJacksonHttpMessageConverter (which acts similiar as the Spring MVC standard MappingJacksonHttpMessageConverter, it also have to have a method which exposes the ObjectMapper) but register application/hal+json as the supported hypermediatypes. The supports method should return true if the given class is assignable from ResourceSupport.

Create a RelProvider (the DefaultRelProvider fulfilled my needs in this case) in your xml configuration.

I created a BeanPostProcessorBean (implemented interfaces: BeanPostProcessor and BeanFactoryAware). Enrich the existing ObjectMapper from my HalJachsonHttpMessageConverter with the following settings:

// retrieve the object mapper from the messageConverter
RelProvider provider = beanFactory.getBean("relProvider", RelProvider.class);
ObjectMapper mapper = (ObjectMapper) objectMapper;
mapper.registerModule(new Jackson2HalModule());
mapper.setHandlerInstantiator(new Jackson2HalModule.HalHandlerInstantiator(provider, null));
mapper.enable(SerializationFeature.INDENT_OUTPUT);

Register these beans in your spring bean xml. I hope this helps. If someone has a better configuration approach, please let me know.