既是一个参数解析器，也是一个返回结果处理器。
1.持有消息转换器的集合

protected final List> messageConverters;

2.作为参数解析器，例如对@RequestBody标识的参数进行解析

• 判断是否支持当前类型的参数

@Override
public boolean supportsParameter(MethodParameter parameter) {return parameter.hasParameterAnnotation(RequestBody.class);
}

• 进行解析
  RequestResponseBodyMethodProcessor根据请求的content-type和我们的参数以及Controller对象类型等信息挨个询问每个消息转化器是否支持解析当前HTTP消息。

@Override
public Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception {parameter = parameter.nestedIfOptional();//核心的参数解析Object arg = readWithMessageConverters(webRequest, parameter, parameter.getNestedGenericParameterType());//...return adaptArgumentIfNecessary(arg, parameter);
}

readWithMessageConverters()函数会将HTTP请求体中的JSON转为我们的Java对象，主要用到转换器的**canRead（）和Read（）**方法，其源码如下：

@Nullable
protected  Object readWithMessageConverters(HttpInputMessage inputMessage, MethodParameter parameter,Type targetType) throws IOException, HttpMediaTypeNotSupportedException, HttpMessageNotReadableException {//拿到请求类型MediaType contentType;boolean noContentType = false;try {contentType = inputMessage.getHeaders().getContentType();}//...//拿到我们的参数类型和Controller类型Class contextClass = parameter.getContainingClass();Class targetClass = (targetType instanceof Class ? (Class) targetType : null);//...//拿到HTTP请求类型HttpMethod httpMethod = (inputMessage instanceof HttpRequest ? ((HttpRequest) inputMessage).getMethod() : null);Object body = NO_VALUE;EmptyBodyCheckingHttpInputMessage message = null;try {message = new EmptyBodyCheckingHttpInputMessage(inputMessage);//开始遍历所有的HttpMessageConverter，看看谁支持将当前http请求内容转为我们的参数for (HttpMessageConverter converter : this.messageConverters) {Class> converterType = (Class>) converter.getClass();GenericHttpMessageConverter genericConverter =(converter instanceof GenericHttpMessageConverter ? (GenericHttpMessageConverter) converter : null);if (genericConverter != null ? genericConverter.canRead(targetType, contextClass, contentType) ://核心就是canRead，与我们参数解析器的support()功能一致(targetClass != null && converter.canRead(targetClass, contentType))) {if (message.hasBody()) {HttpInputMessage msgToUse =getAdvice().beforeBodyRead(message, parameter, targetType, converterType);//找到HttpMessageConverter后调用read方法进行转化body = (genericConverter != null ? genericConverter.read(targetType, contextClass, msgToUse) :((HttpMessageConverter) converter).read(targetClass, msgToUse));body = getAdvice().afterBodyRead(body, msgToUse, parameter, targetType, converterType);}else {body = getAdvice().handleEmptyBody(null, message, parameter, targetType, converterType);}break;}}}//...异常捕捉与日志记录return body;
}

3.作为返回值处理器，可以处理@ResponseeBody所标识方法的返回值

• 返回值处理器处理返回值：

this.returnValueHandlers.handleReturnValue(returnValue, getReturnValueType(returnValue), mavContainer, webRequest);

public void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {HandlerMethodReturnValueHandler handler = selectHandler(returnValue, returnType);if (handler == null) {throw new IllegalArgumentException("Unknown return value type: " + returnType.getParameterType().getName());}//处理返回值handler.handleReturnValue(returnValue, returnType, mavContainer, webRequest);
}private HandlerMethodReturnValueHandler selectHandler(@Nullable Object value, MethodParameter returnType) {boolean isAsyncValue = isAsyncReturnValue(value, returnType);for (HandlerMethodReturnValueHandler handler : this.returnValueHandlers) {if (isAsyncValue && !(handler instanceof AsyncHandlerMethodReturnValueHandler)) {continue;}if (handler.supportsReturnType(returnType)) {return handler;}}return null;
}

supportsReturnType()是判断是否支持当前返回值，源码如下:

@Override
public boolean supportsReturnType(MethodParameter returnType) {return (AnnotatedElementUtils.hasAnnotation(returnType.getContainingClass(), ResponseBody.class) ||returnType.hasMethodAnnotation(ResponseBody.class));
}

• 处理返回值
  通过内容协商确定输出信息的类型，遍历消息转换器，寻找能够支持该类型的转换器，并进行转换。主要用到的方法是canWirte（）和Write（）。最终可以将JaveBean对象转换为Json。