文章目录

1. 1. Spring Boot集成HATEOAS
2. 2. Spring Boot集成Sring Data Rest
3. 3. 自定义MediaType的问题
   1. 3.1. 解决方案

Spring Boot集成HATEOAS

JacksonAutoConfiguration会注册Primary ObjectMapper，这个ObjectMapper通过spring.jackson.*来配置。
JacksonAutoConfiguration配置完后会注册HypermediaAutoConfiguration，HypermediaAutoConfiguration中会对Primary ObjectMapper注册Jackson2HalModule（由apply-to-primary-object-mapper决定）。

HypermediaConfiguration

private void registerHalModule(ObjectMapper objectMapper) {
	objectMapper.registerModule(new Jackson2HalModule());
	Jackson2HalModule.HalHandlerInstantiator instantiator = new Jackson2HalModule.HalHandlerInstantiator(
			HalObjectMapperConfiguration.this.relProvider,
			HalObjectMapperConfiguration.this.curieProvider);
	objectMapper.setHandlerInstantiator(instantiator);
}

Spring Boot使用Jackson2HalModule来把HAL相关的ObjectMapper配置适用到上下文环境中的Primary ObjectMapper中。 这允许HAL格式的响应能够发送给application/json的请求。这就导致了一个副作用，HAL特定的功能污染了Primary ObjectMapper。

Spring Boot 1.3.0中作了一系列修改来简化ObjectMapper的配置。

需改中提到

Previously, Boot used Jackson2HalModule to apply the HAL-related
ObjectMapper configuration to the context’s primary ObjectMapper. This
was to allow HAL-formatted responses to be sent for requests accepted
application/json (see gh-2147). This had the unwanted side-effect of
polluting the primary ObjectMapper with HAL-specific functionality.
Furthermore, Jackson2HalModule is an internal of Spring HATEOAS that
@olivergierke has asked us to avoid using.

This commit replaces the use of Jackson2HalModule with a new approach.
Now, the message converters of any RequestMappingHandlerAdapter beans
are examined and any TypeConstrainedMappingJackson2HttpMessageConverter
instances are modified to support application/json in addition to their
default support for application/hal+json. This behaviour can be disabled
by setting spring.hateoas.use-hal-as-default-json-media-type to false.
This property is named after Spring Data REST’s configuration option
which has the same effect when using Spring Data REST. The new property
replaces the old spring.hateoas.apply-to-primary-object-mapper property.

这次提交使用了一个新方法来替换Jackson2HalModule的使用。现在所有的RequestMappingHandlerAdapter的message converters会被检查，并且任何TypeConstrainedMappingJackson2HttpMessageConverter实例会被修改来支持application/json以及它们默认支持的application/hal+json。 这个行为可以通过设置spring.hateoas.use-hal-as-default-json-media-type为false来关闭。这个属性根据Spring Data REST的配置选项来命名，并且和使用Spring Data REST时有相同的作用。

Spring Data Rest中注册的TypeConstrainedMappingJackson2HttpMessageConverter也是这样的逻辑

@Bean
public TypeConstrainedMappingJackson2HttpMessageConverter halJacksonHttpMessageConverter() {

	ArrayList<MediaType> mediaTypes = new ArrayList<MediaType>();
	mediaTypes.add(MediaTypes.HAL_JSON);

	// Enable returning HAL if application/json is asked if it's configured to be the default type
	if (config().useHalAsDefaultJsonMediaType()) {
		mediaTypes.add(MediaType.APPLICATION_JSON);
	}

	int order = config().useHalAsDefaultJsonMediaType() ? Ordered.LOWEST_PRECEDENCE - 10
			: Ordered.LOWEST_PRECEDENCE - 1;

	TypeConstrainedMappingJackson2HttpMessageConverter converter = new ResourceSupportHttpMessageConverter(order);
	converter.setObjectMapper(halObjectMapper());
	converter.setSupportedMediaTypes(mediaTypes);

	return converter;
}

这样的话默认的ObjectMapper就不会被HAL特性污染。TypeConstrainedMappingJackson2HttpMessageConverter支持转换的HAL对象（Resource）不管请求的是application/json还是application/hal+json都能够被正确的转换。

Spring Boot集成Sring Data Rest

另外，此次修改中还提到

Previously, when Spring Data REST was on the classpath,
JacksonAutoConfiguration would be switched off resulting in the context
containing multiple ObjectMappers, none of which was primary.

This commit configures RepositoryRestMvcAutoConfiguration to run after
JacksonAutoConfiguration. This gives the latter a chance to create its
primary ObjectMapper before the former adds its ObjectMapper beans to
the context.

之前当Spring Data REST在classpath中时，JacksonAutoConfiguration会被自动关闭，导致上下文环境包含多个ObjectMapper，并且没有一个是primary的。也就是说RepositoryRestMvcConfiguration中注册的ObjectMapper同样也会被JacksonHttpMessageConvertersConfiguration中配置的MessageConverter使用，同样可能会导致HAL污染的问题。

这次提交修改成配置RepositoryRestMvcAutoConfiguration在JacksonAutoConfiguration之后执行。这给了后者在前者添加它的ObjectMapper之前，创建primary ObjectMapper的机会。最终达到了隔离ObjectMapper的目录。

另外Spring Boot中对于ObjectMapper配置的属性也会套用到RepositoryRestMvcAutoConfiguration中定义的ObjectMapper

class SpringBootRepositoryRestConfigurer extends RepositoryRestConfigurerAdapter {

	@Autowired(required = false)
	private Jackson2ObjectMapperBuilder objectMapperBuilder;

	@Override
	public void configureJacksonObjectMapper(ObjectMapper objectMapper) {
		if (this.objectMapperBuilder != null) {
			this.objectMapperBuilder.configure(objectMapper);
		}
	}
}

自定义MediaType的问题

在1.3.0之前，请求application/vnd.xxx.v2.5.hal+json这样的自定义头，由于HAL污染的原因，并且MappingJackson2HttpMessageConverter默认支持application/json以及application/\+json是能够正确返回HAL格式的。 现在虽然TypeConstrainedMappingJackson2HttpMessageConverter也支持application/json*，但是由于自定义的头并不符合，所以并不能正确返回HAL格式的响应。

解决方案

注册一个带有通配符的MediaType到TypeConstrainedMappingJackson2HttpMessageConverter中，让其能够处理自定义版本头。另外，由于HalMessageConverterSupportedMediaTypeCustomizer会添加application/json的头兼容JSON请求，需要再其之后添加，否则会被覆盖。

@Bean
@DependsOn("halMessageConverterSupportedMediaTypeCustomizer")
public HalVersionMessageConverterSupportedMediaTypesCustomizer registerCustomMediaType(@Qualifier("halJacksonHttpMessageConverter") TypeConstrainedMappingJackson2HttpMessageConverter halJacksonHttpMessageConverter) {
  return new HalVersionMessageConverterSupportedMediaTypesCustomizer();
}

private class HalVersionMessageConverterSupportedMediaTypesCustomizer implements BeanFactoryAware {

    private static final String HAL_JACKSON_HTTP_MESSAGE_CONVERTER_BEAN_NAME = "halJacksonHttpMessageConverter";
    private volatile BeanFactory beanFactory;

    @PostConstruct
    public void customizedSupportedMediaTypes() {
        TypeConstrainedMappingJackson2HttpMessageConverter halJacksonHttpMessageConverter = beanFactory.getBean(HAL_JACKSON_HTTP_MESSAGE_CONVERTER_BEAN_NAME, TypeConstrainedMappingJackson2HttpMessageConverter.class);
        List<MediaType> supportedMediaTypes = new ArrayList<>(halJacksonHttpMessageConverter.getSupportedMediaTypes());
        supportedMediaTypes.add(new MediaType("application", "*+hal+json"));
        halJacksonHttpMessageConverter.setSupportedMediaTypes(supportedMediaTypes);
    }

    @Override
    public void setBeanFactory(BeanFactory beanFactory) throws BeansException {
        this.beanFactory = beanFactory;
    }
}