springboot第十一期 响应处理-ReturnValueHandler原理

文章目录

• 给前端返回一个json数据
• SpringMVC到底支持哪些返回值
• 关于源码

给前端返回一个json数据

@Controller
public class ResponseTestController {
    @ResponseBody  //利用返回值处理器里面的消息转换器进行处理
    @GetMapping(value = "/test/person")
    public Person getPerson(){
        Person person = new Person();
        person.setAge(28);
        person.setBirth(new Date());
        person.setUserName("zhangsan");
        return person;
    }
}
           

SpringMVC到底支持哪些返回值

• ModelAndView
• Model
• View
• ResponseEntity
• ResponseBodyEmitter
• StreamingResponseBody
• HttpEntity
• HttpHeaders
• Callable
• DeferredResult
• ListenableFuture
• CompletionStage
• WebAsyncTask

• 有 @ModelAttribute 且为对象类型的

  @ResponseBody 注解 —> RequestResponseBodyMethodProcessor；

关于源码

public class RequestMappingHandlerAdapter extends AbstractHandlerMethodAdapter
		implements BeanFactoryAware, InitializingBean {

    ...
    
	@Nullable
	protected ModelAndView invokeHandlerMethod(HttpServletRequest request,
			HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {

		ServletWebRequest webRequest = new ServletWebRequest(request, response);
		try {
			
            ...
            
            ServletInvocableHandlerMethod invocableMethod = createInvocableHandlerMethod(handlerMethod);
                
			if (this.argumentResolvers != null) {
				invocableMethod.setHandlerMethodArgumentResolvers(this.argumentResolvers);
			}
			if (this.returnValueHandlers != null) {//<----关注点
				invocableMethod.setHandlerMethodReturnValueHandlers(this.returnValueHandlers);
			}

            ...

			invocableMethod.invokeAndHandle(webRequest, mavContainer);//看下块代码
			if (asyncManager.isConcurrentHandlingStarted()) {
				return null;
			}

			return getModelAndView(mavContainer, modelFactory, webRequest);
		}
		finally {
			webRequest.requestCompleted();
		}
	}
           

public class ServletInvocableHandlerMethod extends InvocableHandlerMethod {
    
	public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer,
			Object... providedArgs) throws Exception {

		Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);
		
        ...
        
		try {
            //看下块代码
			this.returnValueHandlers.handleReturnValue(
					returnValue, getReturnValueType(returnValue), mavContainer, webRequest);
		}
		catch (Exception ex) {
			...
		}
	}
           

public class HandlerMethodReturnValueHandlerComposite implements HandlerMethodReturnValueHandler {
    
    ...
    
	@Override
	public void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,
			ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {

        //selectHandler()实现在下面
		HandlerMethodReturnValueHandler handler = selectHandler(returnValue, returnType);
		if (handler == null) {
			throw new IllegalArgumentException("Unknown return value type: " + returnType.getParameterType().getName());
		}
        //开始处理
		handler.handleReturnValue(returnValue, returnType, mavContainer, webRequest);
	}
    
   	@Nullable
	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;
	}
           

@ResponseBody

注解，即

RequestResponseBodyMethodProcessor

，它实现

HandlerMethodReturnValueHandler

接口

public class RequestResponseBodyMethodProcessor extends AbstractMessageConverterMethodProcessor {

    ...
    
	@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);

        // 使用消息转换器进行写出操作，本方法下一章节介绍：
		// Try even with null return value. ResponseBodyAdvice could get involved.
		writeWithMessageConverters(returnValue, returnType, inputMessage, outputMessage);
	}

}