springboot第十一期 響應處理-ReturnValueHandler原理

文章目錄

• 給前端傳回一個json資料
• SpringMVC到底支援哪些傳回值
• 關于源碼

給前端傳回一個json資料

@Controller
public class ResponseTestController {
    @ResponseBody  //利用傳回值處理器裡面的消息轉換器進行處理
    @GetMapping(value = "/test/person")
    public Person getPerson(){
        Person person = new Person();
        person.setAge(28);
        person.setBirth(new Date());
        person.setUserName("zhangsan");
        return person;
    }
}
           

SpringMVC到底支援哪些傳回值

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

• 有 @ModelAttribute 且為對象類型的

  @ResponseBody 注解 —> RequestResponseBodyMethodProcessor；

關于源碼

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) {//<----關注點
				invocableMethod.setHandlerMethodReturnValueHandlers(this.returnValueHandlers);
			}

            ...

			invocableMethod.invokeAndHandle(webRequest, mavContainer);//看下塊代碼
			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 {
            //看下塊代碼
			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()實作在下面
		HandlerMethodReturnValueHandler handler = selectHandler(returnValue, returnType);
		if (handler == null) {
			throw new IllegalArgumentException("Unknown return value type: " + returnType.getParameterType().getName());
		}
        //開始處理
		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

，它實作

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);

        // 使用消息轉換器進行寫出操作，本方法下一章節介紹：
		// Try even with null return value. ResponseBodyAdvice could get involved.
		writeWithMessageConverters(returnValue, returnType, inputMessage, outputMessage);
	}

}