SpringMVC的@ResponseBody与@RequestBody

内容参考自https://my.oschina.net/lichhao/blog/172562

在SpringMVC中，可以使用@RequestBody与@ResponseBody完成报文到对象、对象到报文的转换。这就是依靠Spring3.0引入的消息转换器机制实现的：

HttpMessageConverter

/**
 * Strategy interface that specifies a converter that can convert from and to HTTP requests and responses.
 *
 * @author Arjen Poutsma
 * @author Juergen Hoeller
 * @since 3.0
 * @param  the converted object type
 */
public interface HttpMessageConverter {


	boolean canRead(Class clazz, @Nullable MediaType mediaType);


	boolean canWrite(Class clazz, @Nullable MediaType mediaType);


	T read(Class clazz, HttpInputMessage inputMessage)
			throws IOException, HttpMessageNotReadableException;

	void write(T t, @Nullable MediaType contentType, HttpOutputMessage outputMessage)
			throws IOException, HttpMessageNotWritableException;

}

在HttpServlet接口处理请求时会调用service()方法：

其中的参数ServletRequest与ServletResponse封装了请求和结果。

    public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException {
        HttpServletRequest request;
        HttpServletResponse response;
        try {
            request = (HttpServletRequest)req;
            response = (HttpServletResponse)res;
        } catch (ClassCastException var6) {
            throw new ServletException(lStrings.getString("http.non_http"));
        }

        this.service(request, response);
    }

在ServletRequest接口中，通过getInputStream()获取请求信息。

    ServletInputStream getInputStream() throws IOException;

在ServletResponse接口中，通过getOutputStream()返回结果信息。

    ServletOutputStream getOutputStream() throws IOException;

 可以理解成Http的交互就是流的交互，在SpringMVC中，对内部的流进行了抽象，顶层接口就是HttpMessage。他的子类分别是

public interface HttpOutputMessage extends HttpMessage {

	/**
	 * Return the body of the message as an output stream.
	 * @return the output stream body (never {@code null})
	 * @throws IOException in case of I/O errors
	 */
	OutputStream getBody() throws IOException;

}

public interface HttpInputMessage extends HttpMessage {

	/**
	 * Return the body of the message as an input stream.
	 * @return the input stream body (never {@code null})
	 * @throws IOException in case of I/O errors
	 */
	InputStream getBody() throws IOException;

}

 一次http请求与返回跟SpringMVC的过程简述为下图

Http发送接收的是流，HttpMessageConverter操作的是HttpMessage，这整个过程用一个类完成，那就是RequestResponseBodyMethodProcessor。它同时负责请求的参数解析和返回值处理，判断是否有@RequestBody、@ResponseBody,然后调用converter(HttpMessageConverter的各种子类，比如StringHttpMessageConverter处理返回值是String的)进行读写。

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

	@Override
	public boolean supportsReturnType(MethodParameter returnType) {
		return (AnnotatedElementUtils.hasAnnotation(returnType.getContainingClass(), ResponseBody.class) ||
				returnType.hasMethodAnnotation(ResponseBody.class));
	}
        @Override
	public Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,
			NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception {

		parameter = parameter.nestedIfOptional();
		Object arg = readWithMessageConverters(webRequest, parameter, parameter.getNestedGenericParameterType());
		String name = Conventions.getVariableNameForParameter(parameter);

		if (binderFactory != null) {
			WebDataBinder binder = binderFactory.createBinder(webRequest, arg, name);
			if (arg != null) {
				validateIfApplicable(binder, parameter);
				if (binder.getBindingResult().hasErrors() && isBindExceptionRequired(binder, parameter)) {
					throw new MethodArgumentNotValidException(parameter, binder.getBindingResult());
				}
			}
			if (mavContainer != null) {
				mavContainer.addAttribute(BindingResult.MODEL_KEY_PREFIX + name, binder.getBindingResult());
			}
		}

		return adaptArgumentIfNecessary(arg, parameter);
	}
	@Override
	public void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,
			ModelAndViewContainer mavContainer, NativeWebRequest webRequest)
			throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {

		mavContainer.setRequestHandled(true);
		ServletServerHttpRequest inputMessage = createInputMessage(webRequest);
		ServletServerHttpResponse outputMessage = createOutputMessage(webRequest);

		// Try even with null return value. ResponseBodyAdvice could get involved.
		writeWithMessageConverters(returnValue, returnType, inputMessage, outputMessage);
	}