Spring註解@ModelAttribute

當spring管理的對象的方法或者參數出現了註解@ModelAttribute時候，意味着這個屬性會被注入

@ModelAttribute
	public Book get(@RequestParam(required=false) String id) {
		Book entity = null;
		if (StringUtils.isNotBlank(id)){
			entity = bookService.get(id);
		}
		if (entity == null){
			entity = new Book();
		}
		return entity;
	}

當這個類中全部方法被調用以前這個方法都會被執行放入

ModelAndViewContainer

這個容器中去，當對應的方法被執行的時候若是沒有傳入參數會去這個容器中找到匹配參數繼而傳入到方法調用

@RequestMapping(value = "form")
	public String form(Book book, Model model) {
		model.addAttribute("book", book);
		return "ibook/book/bookForm";
	}

當這個方法再被調用時候以前查出來的Book對象會天然被傳入，而前臺並未傳入，此方法適合修改一個對象的操做如update，只須要傳入ModelAttribute對應方法的參數便可

請求流程圖以下：

 

當請求到RequestMappingHandlerAdapter對象的invokeHandleMethod方法

private ModelAndView invokeHandleMethod(HttpServletRequest request,
			HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {

		ServletWebRequest webRequest = new ServletWebRequest(request, response);

		WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod);
		ModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory);
		ServletInvocableHandlerMethod requestMappingMethod = createRequestMappingMethod(handlerMethod, binderFactory);

		ModelAndViewContainer mavContainer = new ModelAndViewContainer();
		mavContainer.addAllAttributes(RequestContextUtils.getInputFlashMap(request));
//初始化ModelAttribute註解方法
		modelFactory.initModel(webRequest, mavContainer, requestMappingMethod);
		mavContainer.setIgnoreDefaultModelOnRedirect(this.ignoreDefaultModelOnRedirect);

		AsyncWebRequest asyncWebRequest = WebAsyncUtils.createAsyncWebRequest(request, response);
		asyncWebRequest.setTimeout(this.asyncRequestTimeout);

		final WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
		asyncManager.setTaskExecutor(this.taskExecutor);
		asyncManager.setAsyncWebRequest(asyncWebRequest);
		asyncManager.registerCallableInterceptors(this.callableInterceptors);
		asyncManager.registerDeferredResultInterceptors(this.deferredResultInterceptors);

		if (asyncManager.hasConcurrentResult()) {
			Object result = asyncManager.getConcurrentResult();
			mavContainer = (ModelAndViewContainer) asyncManager.getConcurrentResultContext()[0];
			asyncManager.clearConcurrentResult();

			if (logger.isDebugEnabled()) {
				logger.debug("Found concurrent result value [" + result + "]");
			}
			requestMappingMethod = requestMappingMethod.wrapConcurrentResult(result);
		}

		requestMappingMethod.invokeAndHandle(webRequest, mavContainer);

		if (asyncManager.isConcurrentHandlingStarted()) {
			return null;
		}

		return getModelAndView(mavContainer, modelFactory, webRequest);
	}

 

ModelFactory的initModel以下

public void initModel(NativeWebRequest request, ModelAndViewContainer mavContainer, HandlerMethod handlerMethod)
			throws Exception {
        //抽取出目標類的全部方法
		Map<String, ?> attributesInSession = this.sessionAttributesHandler.retrieveAttributes(request);
		mavContainer.mergeAttributes(attributesInSession);
        //設置ModelAttribute註解的值
		invokeModelAttributeMethods(request, mavContainer);

		for (String name : findSessionAttributeArguments(handlerMethod)) {
			if (!mavContainer.containsAttribute(name)) {
				Object value = this.sessionAttributesHandler.retrieveAttribute(request, name);
				if (value == null) {
					throw new HttpSessionRequiredException("Expected session attribute '" + name + "'");
				}
				mavContainer.addAttribute(name, value);
			}
		}
	}

invokeModelAttributeMethods方法以下：

private void invokeModelAttributeMethods(NativeWebRequest request, ModelAndViewContainer mavContainer)
			throws Exception {

		for (InvocableHandlerMethod attrMethod : this.attributeMethods) {
            //獲取含有註解的對應方法名
			String modelName = attrMethod.getMethodAnnotation(ModelAttribute.class).value();
			if (mavContainer.containsAttribute(modelName)) {
				continue;
			}

			Object returnValue = attrMethod.invokeForRequest(request, mavContainer);

			if (!attrMethod.isVoid()){
				String returnValueName = getNameForReturnValue(returnValue, attrMethod.getReturnType());
				if (!mavContainer.containsAttribute(returnValueName)) {
					mavContainer.addAttribute(returnValueName, returnValue);
				}
			}
		}
	}

 

此時便把註解解析完成值都放入到了mavContainer容器中。

此時回到執行目標方法流程圖既是：RequestMappingHandlerAdapter的invokeHandleMethod方法

 

requestMappingMethod.invokeAndHandle(webRequest, mavContainer);

到ServletInvocableHandlerMethod的invokeAndHandle

public final void invokeAndHandle(ServletWebRequest webRequest,
      ModelAndViewContainer mavContainer, Object... providedArgs) throws Exception {
   //執行目標函數
   Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);

   setResponseStatus(webRequest);

   if (returnValue == null) {
      if (isRequestNotModified(webRequest) || hasResponseStatus() || mavContainer.isRequestHandled()) {
         mavContainer.setRequestHandled(true);
         return;
      }
   }
   else if (StringUtils.hasText(this.responseReason)) {
      mavContainer.setRequestHandled(true);
      return;
   }

   mavContainer.setRequestHandled(false);

   try {
      this.returnValueHandlers.handleReturnValue(returnValue, getReturnValueType(returnValue), mavContainer, webRequest);
   }
   catch (Exception ex) {
      if (logger.isTraceEnabled()) {
         logger.trace(getReturnValueHandlingErrorMessage("Error handling return value", returnValue), ex);
      }
      throw ex;
   }
}

 

到InvokableHandlerMethod的invokeForRequest

public final Object invokeForRequest(NativeWebRequest request, ModelAndViewContainer mavContainer,
      Object... providedArgs) throws Exception {

   Object[] args = getMethodArgumentValues(request, mavContainer, providedArgs);
   if (logger.isTraceEnabled()) {
      StringBuilder sb = new StringBuilder("Invoking [");
      sb.append(getBeanType().getSimpleName()).append(".");
      sb.append(getMethod().getName()).append("] method with arguments ");
      sb.append(Arrays.asList(args));
      logger.trace(sb.toString());
   }
   Object returnValue = invoke(args);
   if (logger.isTraceEnabled()) {
      logger.trace("Method [" + getMethod().getName() + "] returned [" + returnValue + "]");
   }
   return returnValue;

}