如何配置由spring-hateoas注册的MappingJackson2HttpMessageConverter

Question

I like to use spring-hateoas in my project and configured it with @EnableHypermediaSupport . The problem now is, that this magic config annotation registers its own MappingJackson2HttpMessageConverter and my own customized converter will be ignored.

Background: I added some Jackson modules (like the JodaModule ) to my project and I want them to get registered using objectMapper.findAndRegisterModules(); . This is done by overriding configureMessageConverters(List<HttpMessageConverter<?>> converters) in WebMvcConfigurationSupport or WebMvcConfigurer .

My current configuration looks like this:

@Configuration
@EnableHypermediaSupport(type = HAL)
public class WebMvcConfiguration extends WebMvcConfigurationSupport {

  @Override
  protected void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
    final MappingJackson2HttpMessageConverter converter = new MappingJackson2HttpMessageConverter();
    converter.getObjectMapper().findAndRegisterModules();

    converters.add(converter);
  }
}

Is there a way to customize the MappingJackson2HttpMessageConverter or the ObjectMapper that is used by spring-hateoas?

Answer 1

I had to do the same thing. With HATEOAS .16 i was able to ge this working..but really really ugly.

The key was that in the HypermediaSupportBeanDefinitionRegistrar the part that registers the HAL converter checks to see if there already is a HAL converter before it tries to add another one. So I just added a HAL Converter in my WebMVCConfig::configureMessageConverters myself.

Something like:

private static final String DELEGATING_REL_PROVIDER_BEAN_NAME = "_relProvider";
private static final String LINK_DISCOVERER_REGISTRY_BEAN_NAME = "_linkDiscovererRegistry";
private static final String HAL_OBJECT_MAPPER_BEAN_NAME = "_halObjectMapper";

@Autowired
private ListableBeanFactory beanFactory;

private static CurieProvider getCurieProvider(BeanFactory factory) {

    try {
        return factory.getBean(CurieProvider.class);
    } catch (NoSuchBeanDefinitionException e) {
        return null;
    }
}

@Override
    public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
        List<HttpMessageConverter<?>> baseConverters = new ArrayList<HttpMessageConverter<?>>();
        super.configureMessageConverters(baseConverters);

        //Need to override some behaviour in the HAL Serializer...so let's make our own
        CurieProvider curieProvider = getCurieProvider(beanFactory);
        RelProvider relProvider = beanFactory.getBean(DELEGATING_REL_PROVIDER_BEAN_NAME, RelProvider.class);
        ObjectMapper halObjectMapper = beanFactory.getBean(HAL_OBJECT_MAPPER_BEAN_NAME, ObjectMapper.class);

        halObjectMapper.registerModule(new Jackson2HalModule());
        halObjectMapper.setHandlerInstantiator(new Jackson2HalModule.HalHandlerInstantiator(relProvider, curieProvider));

        MappingJackson2HttpMessageConverter halConverter = new TypeConstrainedMappingJackson2HttpMessageConverter(
            ResourceSupport.class);
        halConverter.setSupportedMediaTypes(Arrays.asList(HAL_JSON));
        halConverter.setObjectMapper(halObjectMapper);

        converters.add(halConverter);
    }

This is clearly implementation dependent, uses implementation details, and really doesn't let you modify the one that @EnableHyperMediaSupport build for you....but it works for now.

Answer 2

I use the following approach

@Configuration
@EnableHypermediaSupport(type = HypermediaType.HAL)
public class MvcConfig {    

  @Bean
  public MappingJackson2HttpMessageConverter mappingJacksonHttpMessageConverter() {

Answer 3

I found an ugly solution for my problem:

I'm using a BeanPostProcessor and a lot of reflection magic, to replace Spring HATEOAS' internal ConversionService with my own one, that has been added to the Spring context before. This way I make sure, that Spring HATEOAS uses exactly the same ConversionService as Spring MVC.

/**
 * This is a HACK to work around a not yet implemented feature. At the moment Spring Hateoas uses a
 * {@link ConversionService}, which is hold in a private static final field and hence cannot be accessed to add more
 * Converters<br/>
 *
 * <ul>
 *   <li><a href="https://github.com/spring-projects/spring-hateoas/issues/118">Spring Hateoas Issue</a></li>
 *   <li><a
 *     href="http://stackoverflow.com/questions/22240155/converter-from-pathvariable-domainobject-to-string-using-controllerlinkbuilde">
 *     Solution on Stackoverflow</a></li>
 * </ul>
 */
public static class HateoasConversionServicePostProcessor implements BeanPostProcessor {

    @Override
    public Object postProcessBeforeInitialization(final Object bean, final String beanName) throws BeansException {
        if (bean instanceof ConversionService) {
            try {
                Class<?> clazz = Class.forName(
                        "org.springframework.hateoas.mvc.AnnotatedParametersParameterAccessor$BoundMethodParameter");
                Field field = clazz.getDeclaredField("CONVERSION_SERVICE");
                field.setAccessible(true);

                Field modifiersField = Field.class.getDeclaredField("modifiers");
                modifiersField.setAccessible(true);
                modifiersField.setInt(field, field.getModifiers() & ~Modifier.FINAL);

                field.set(null, bean);

                modifiersField.setInt(field, field.getModifiers() & Modifier.FINAL);
            } catch (Exception ex) {
                throw new RuntimeException(ex);
            }
        }

        return bean;
    }

    @Override
    public Object postProcessAfterInitialization(final Object bean, final String beanName) throws BeansException {
        return bean;
    }
}