掌握@ControllerAdvice配合RequestBodyAdvice/ResponseBodyAdvice使用，让你的选择不仅仅只有拦截器【享学Spring MVC】(中)

public interface RequestBodyAdvice {
  // 第一个调用的。判断当前的拦截器（advice是否支持） 
  // 注意它的入参有：方法参数、目标类型、所使用的消息转换器等等
  boolean supports(MethodParameter methodParameter, Type targetType, Class<? extends HttpMessageConverter<?>> converterType);
  // 如果body体木有内容就执行这个方法（后面的就不会再执行喽）
  Object handleEmptyBody(Object body, HttpInputMessage inputMessage, MethodParameter parameter, Type targetType, Class<? extends HttpMessageConverter<?>> converterType);
  // 重点：它在body被read读/转换**之前**进行调用的
  HttpInputMessage beforeBodyRead(HttpInputMessage inputMessage, MethodParameter parameter, Type targetType, Class<? extends HttpMessageConverter<?>> converterType) throws IOException;
  // 它在body体已经转换为Object后执行。so此时都不抛出IOException了嘛~
  Object afterBodyRead(Object body, HttpInputMessage inputMessage, MethodParameter parameter, Type targetType, Class<? extends HttpMessageConverter<?>> converterType);
}

// @since 4.2
public class JsonViewRequestBodyAdvice extends RequestBodyAdviceAdapter {
  // 处理使用的消息转换器是AbstractJackson2HttpMessageConverter类型
  // 并且入参上标注有@JsonView注解的
  @Override
  public boolean supports(MethodParameter methodParameter, Type targetType, Class<? extends HttpMessageConverter<?>> converterType) {
    return (AbstractJackson2HttpMessageConverter.class.isAssignableFrom(converterType) &&
        methodParameter.getParameterAnnotation(JsonView.class) != null);
  }
  // 显然这里实现的beforeBodyRead这个方法：
  // 它把body最终交给了MappingJacksonInputMessage来反序列处理消息体
  // 注意：@JsonView能处理这个注解。也就是说能指定把消息体转换成指定的类型，还是比较实用的
  // 可以看到当标注有@jsonView注解后 targetType就没啥卵用了
  @Override
  public HttpInputMessage beforeBodyRead(HttpInputMessage inputMessage, MethodParameter methodParameter, Type targetType, Class<? extends HttpMessageConverter<?>> selectedConverterType) throws IOException {
    JsonView ann = methodParameter.getParameterAnnotation(JsonView.class);
    Assert.state(ann != null, "No JsonView annotation");
    Class<?>[] classes = ann.value();
    // 必须指定class类型，并且有且只能指定一个类型
    if (classes.length != 1) {
      throw new IllegalArgumentException("@JsonView only supported for request body advice with exactly 1 class argument: " + methodParameter);
    }
    // 它是一个InputMessage的实现
    return new MappingJacksonInputMessage(inputMessage.getBody(), inputMessage.getHeaders(), classes[0]);
  }
}

WebMvcConfigurationSupport：
  @Bean
  public RequestMappingHandlerAdapter requestMappingHandlerAdapter() {
    ...
    if (jackson2Present) {
      adapter.setRequestBodyAdvice(Collections.singletonList(new JsonViewRequestBodyAdvice()));
      adapter.setResponseBodyAdvice(Collections.singletonList(new JsonViewResponseBodyAdvice()));
    }
    ...
  }

@Getter
@Setter
@ToString
public static class User {
    @JsonView({Simple.class, Complex.class})
    private Long id;
    @JsonView({Simple.class, Complex.class})
    private String name;
    @JsonView({Complex.class})
    private Integer age;
}
// 准备两个view类型（使用接口、类均可）
interface Simple {}
interface Complex {}

@ResponseBody
@PostMapping("/test/requestbody")
public String testRequestBodyAdvice(@JsonView(Simple.class) @RequestBody User user) {
    System.out.println(user);
    return "hello world";
}

HelloController.User(id=1, name=fsx, age=18)

AbstractJackson2HttpMessageConverter（实际为MappingJackson2HttpMessageConverter）：
  @Override
  public Object read(Type type, @Nullable Class<?> contextClass, HttpInputMessage inputMessage) throws IOException, HttpMessageNotReadableException {
    JavaType javaType = getJavaType(type, contextClass);
    // 把body内的东西转换为java对象
    return readJavaType(javaType, inputMessage);
  }
  private Object readJavaType(JavaType javaType, HttpInputMessage inputMessage) throws IOException {
    if (inputMessage instanceof MappingJacksonInputMessage) {
      Class<?> deserializationView = ((MappingJacksonInputMessage) inputMessage).getDeserializationView();
      if (deserializationView != null) {
        return this.objectMapper.readerWithView(deserializationView).forType(javaType).readValue(inputMessage.getBody());
      }
    }
    return this.objectMapper.readValue(inputMessage.getBody(), javaType);
  }

// @since 4.1 泛型T：body类型
public interface ResponseBodyAdvice<T> {
  boolean supports(MethodParameter returnType, Class<? extends HttpMessageConverter<?>> converterType);
  @Nullable
  T beforeBodyWrite(@Nullable T body, MethodParameter returnType, MediaType selectedContentType, Class<? extends HttpMessageConverter<?>> selectedConverterType, ServerHttpRequest request, ServerHttpResponse response);
}

public abstract class AbstractMappingJacksonResponseBodyAdvice implements ResponseBodyAdvice<Object> {
  @Override
  public boolean supports(MethodParameter returnType, Class<? extends HttpMessageConverter<?>> converterType) {
    return AbstractJackson2HttpMessageConverter.class.isAssignableFrom(converterType);
  }
  // 最终使用MappingJacksonValue来序列化body体
  @Override
  @Nullable
  public final Object beforeBodyWrite(@Nullable Object body, MethodParameter returnType, MediaType contentType, Class<? extends HttpMessageConverter<?>> converterType, ServerHttpRequest request, ServerHttpResponse response) {
    if (body == null) {
      return null;
    }
    MappingJacksonValue container = getOrCreateContainer(body);
    beforeBodyWriteInternal(container, contentType, returnType, request, response);
    return container;
  }
}