DispatcherServlet解读

Posted on 2024-03-20 In Spring

SpringMVC的核心就是DispatcherServlet，DispatcherServlet实质也是一个HttpServlet。DispatcherSevlet负责将请求分发，所有的请求都有经过它来统一分发。

大致看下SpringMVC请求处理的流程：

用户向服务器发送请求，请求会到DispatcherServlet，DispatcherServlet 对请求URL进行解析，得到请求资源标识符（URI），然后根据该URI，调用HandlerMapping获得该Handler配置的所有相关的对象（包括一个Handler处理器对象、多个HandlerInterceptor拦截器对象），最后以HandlerExecutionChain对象的形式返回。
DispatcherServlet 根据获得的Handler，选择一个合适的HandlerAdapter。提取Request中的模型数据，填充Handler入参，开始执行Handler（Controller)。在填充Handler的入参过程中，根据你的配置，Spring将帮你做一些额外的工作：

HttpMessageConveter：将请求消息（如Json、xml等数据）转换成一个对象，将对象转换为指定的响应信息

数据转换：对请求消息进行数据转换。如String转换成Integer、Double等

数据格式化：对请求消息进行数据格式化。如将字符串转换成格式化数字或格式化日期等

数据验证：验证数据的有效性（长度、格式等），验证结果存储到BindingResult或Error中

Handler执行完成后，向DispatcherServlet 返回一个ModelAndView对象；根据返回的ModelAndView，选择一个适合的ViewResolver返回给DispatcherServlet；ViewResolver 结合Model和View，来渲染视图，最后将渲染结果返回给客户端。

DispatcherSevlet中最重要的是doService方法：

   //获取请求，设置一些request的参数，然后分发给doDispatch
@Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
	if (logger.isDebugEnabled()) {
		String resumed = WebAsyncUtils.getAsyncManager(request).hasConcurrentResult() ? " resumed" : "";
		logger.debug("DispatcherServlet with name '" + getServletName() + "'" + resumed +
				" processing " + request.getMethod() + " request for [" + getRequestUri(request) + "]");
	}

	// Keep a snapshot of the request attributes in case of an include,
	// to be able to restore the original attributes after the include.
	Map<String, Object> attributesSnapshot = null;
	if (WebUtils.isIncludeRequest(request)) {
		attributesSnapshot = new HashMap<String, Object>();
		Enumeration<?> attrNames = request.getAttributeNames();
		while (attrNames.hasMoreElements()) {
			String attrName = (String) attrNames.nextElement();
			if (this.cleanupAfterInclude || attrName.startsWith("org.springframework.web.servlet")) {
				attributesSnapshot.put(attrName, request.getAttribute(attrName));
			}
		}
	}

	// Make framework objects available to handlers and view objects.
	/* 设置web应用上下文**/
	request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());
	/* 国际化本地**/
	request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
	/* 样式**/
	request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
	//设置样式资源
	request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());
	//请求刷新时保存属性
	FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);
	if (inputFlashMap != null) {
		request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap));
	}
	//Flash attributes 在对请求的重定向生效之前被临时存储（通常是在session)中，并且在重定向之后被立即移除
	request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());
	//FlashMap 被用来管理 flash attributes 而 FlashMapManager 则被用来存储，获取和管理 FlashMap 实体.
	request.setAttribute(FLASH_MAP_MANAGER_ATTRIBUTE, this.flashMapManager);

	try {
		doDispatch(request, response);
	}
	finally {
		if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
			// Restore the original attribute snapshot, in case of an include.
			if (attributesSnapshot != null) {
				restoreAttributesAfterInclude(request, attributesSnapshot);
			}
		}
	}
}

doService方法对Request设置了一些全局属性，最终接下来的操作是在doDispatcher：

    /**
 *将Handler进行分发，handler会被handlerMapping有序的获得
 *通过查询servlet安装的HandlerAdapters来获得HandlerAdapters来查找第一个支持handler的类
 *所有的HTTP的方法都会被这个方法掌控。取决于HandlerAdapters 或者handlers 他们自己去决定哪些方法是可用
 *@param request current HTTP request
 *@param response current HTTP response
 */
protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
	/* 当前HTTP请求**/
	HttpServletRequest processedRequest = request;
	HandlerExecutionChain mappedHandler = null;
	boolean multipartRequestParsed = false;

	WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

	try {
		ModelAndView mv = null;
		Exception dispatchException = null;

		try {
			//判断是否有文件上传
			processedRequest = checkMultipart(request);
			multipartRequestParsed = (processedRequest != request);

			// getHandler中遍历HandlerMapping，获取对应的HandlerExecutionChain
               // HandlerExecutionChain，包含HandlerIntercrptor和HandlerMethod
			mappedHandler = getHandler(processedRequest);
			if (mappedHandler == null || mappedHandler.getHandler() == null) {
				noHandlerFound(processedRequest, response);
				return;
			}

			
			//获得HandlerAdapter
			HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

			//获得HTTP请求方法
			String method = request.getMethod();
			boolean isGet = "GET".equals(method);
			if (isGet || "HEAD".equals(method)) {
				long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
				if (logger.isDebugEnabled()) {
					logger.debug("Last-Modified value for [" + getRequestUri(request) + "] is: " + lastModified);
				}
				if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
					return;
				}
			}
			//如果有拦截器的话，会执行拦截器的preHandler方法
			if (!mappedHandler.applyPreHandle(processedRequest, response)) {
				return;
			}

			//返回ModelAndView
			mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

			if (asyncManager.isConcurrentHandlingStarted()) {
				return;
			}
			//当view为空时，，根据request设置默认view
			applyDefaultViewName(processedRequest, mv);
			//执行拦截器的postHandle
			mappedHandler.applyPostHandle(processedRequest, response, mv);
		}
		catch (Exception ex) {
			dispatchException = ex;
		}
		processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
	}
	catch (Exception ex) {
		triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
	}
	catch (Error err) {
		triggerAfterCompletionWithError(processedRequest, response, mappedHandler, err);
	}
	finally {
		//判断是否是异步请求
		if (asyncManager.isConcurrentHandlingStarted()) {
			// Instead of postHandle and afterCompletion
			if (mappedHandler != null) {
				mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
			}
		}
		else {
			// Clean up any resources used by a multipart request.
			//删除上传资源
			if (multipartRequestParsed) {
				cleanupMultipart(processedRequest);
			}
		}
	}
}

HandlerInterceptor拦截器

在doDispatcher方法中：

1	mappedHandler = getHandler(processedRequest);

protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
   for (HandlerMapping hm : this.handlerMappings) {
      if (logger.isTraceEnabled()) {
         logger.trace(
               "Testing handler map [" + hm + "] in DispatcherServlet with name '" + getServletName() + "'");
      }
      HandlerExecutionChain handler = hm.getHandler(request);
      if (handler != null) {
         return handler;
      }
   }
   return null;
}

遍历HandlerMapping，获取对应的HandlerExecutionChain，HandlerExecutionChain中有3个变量:

1
2
3

private final Object handler; // HandlerMethod
private HandlerInterceptor[] interceptors; //所有的HandlerInterceptor的数组
private List<HandlerInterceptor> interceptorList; //所有的HandlerInterceptor的链表

在执行Controller的方法调用之前先执行：

1
2
3

if (!mappedHandler.applyPreHandle(processedRequest, response)) {
   return;
}

HandlerExecutionChain#applyPreHandle即调用HandlerInterceptor# preHandler方法。在Controller的方法调用之后：

1	mappedHandler.applyPostHandle(processedRequest, response, mv);

HandlerExecutionChain#applyPostHandle即调用HandlerInterceptor# postHandle方法。最后

1	processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);

方法会调用：

1	mappedHandler.triggerAfterCompletion(request, response, null);

调用HandlerInterceptor#afterCompletion方法。
详细用法参考 https://blog.csdn.net/zero__007/article/details/80028720

HandlerExceptionResolver异常处理

在doDispatcher方法中会catch住Controller方法调用的异常，之后再处理：

1	processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);

private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
      HandlerExecutionChain mappedHandler, ModelAndView mv, Exception exception) throws Exception {

   boolean errorView = false;

   if (exception != null) {
      if (exception instanceof ModelAndViewDefiningException) {
         logger.debug("ModelAndViewDefiningException encountered", exception);
         mv = ((ModelAndViewDefiningException) exception).getModelAndView();
      }
      else {
         Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
         mv = processHandlerException(request, response, handler, exception);
         errorView = (mv != null);
      }
   }
   // …
}

如果有exception，调用processHandlerException：

protected ModelAndView processHandlerException(HttpServletRequest request, HttpServletResponse response,
      Object handler, Exception ex) throws Exception {

   // Check registered HandlerExceptionResolvers...
   ModelAndView exMv = null;
   for (HandlerExceptionResolver handlerExceptionResolver : this.handlerExceptionResolvers) {
      exMv = handlerExceptionResolver.resolveException(request, response, handler, ex);
      if (exMv != null) {
         break;
      }
   }
   if (exMv != null) {
      if (exMv.isEmpty()) {
         request.setAttribute(EXCEPTION_ATTRIBUTE, ex);
         return null;
      }
      // We might still need view name translation for a plain error model...
      if (!exMv.hasView()) {
         exMv.setViewName(getDefaultViewName(request));
      }
      if (logger.isDebugEnabled()) {
         logger.debug("Handler execution resulted in exception - forwarding to resolved error view: " + exMv, ex);
      }
      WebUtils.exposeErrorRequestAttributes(request, ex, getServletName());
      return exMv;
   }

   throw ex;
}

SpringMVC启动加载时会将所有的异常处理HandlerExceptionResolver实现类存放到handlerExceptionResolvers中，当默认执行时，只要返回的exMv不为空就中止执行其他的异常处理实现类。
可以实现多个HandlerExceptionResolver的异常处理类，执行顺序由其order属性决定, order值越小，越是优先执行, 在执行到第一个返回不是null的ModelAndView的Resolver时，不再执行后续的尚未执行的Resolver的异常处理方法。

HandlerMethodArgumentResolver参数解析器

HandlerMethodArgumentResolver是用来为处理器解析参数的，主要用在HandlerMethod中，每个Resolver对应一种类型的参数，其实现类特别的多。HandlerMethodArgumentResolver的接口定义如下：

supportsParameter 用于判断是否支持对某种参数的解析

resolveArgument 将请求中的参数值解析为某种对象

参数的处理是在InvocableHandlerMethod类中，当执行具体的请求处理时执行invokeForRequest：

public Object invokeForRequest(NativeWebRequest request, ModelAndViewContainer mavContainer,
      Object... providedArgs) throws Exception {
   //获取执行Controller的函数的参数对象
   Object[] args = getMethodArgumentValues(request, mavContainer, providedArgs);
   if (logger.isTraceEnabled()) {
      logger.trace("Invoking '" + ClassUtils.getQualifiedMethodName(getMethod(), getBeanType()) +
            "' with arguments " + Arrays.toString(args));
   }
   Object returnValue = doInvoke(args);
   if (logger.isTraceEnabled()) {
      logger.trace("Method [" + ClassUtils.getQualifiedMethodName(getMethod(), getBeanType()) +
            "] returned [" + returnValue + "]");
   }
   return returnValue;
}

具体的参数解析器的选择和使用参数解析器去解析参数的实现都在getMethodArgumentValues中。SpringMVC在启动时会将所有的参数解析器放到HandlerMethodArgumentResolverComposite，然后从HandlerMethodArgumentResolverComposite参数解析器集合中选择一个支持对parameter解析的参数解析器，之后就使用支持参数解析的解析器进行参数解析。

private Object[] getMethodArgumentValues(NativeWebRequest request, ModelAndViewContainer mavContainer,
			Object... providedArgs) throws Exception {
		//获取执行的具体函数的参数
		MethodParameter[] parameters = getMethodParameters();
		Object[] args = new Object[parameters.length];
		for (int i = 0; i < parameters.length; i++) {
			MethodParameter parameter = parameters[i];
			parameter.initParameterNameDiscovery(this.parameterNameDiscoverer);
			GenericTypeResolver.resolveParameterType(parameter, getBean().getClass());
			//获取参数值对象
			args[i] = resolveProvidedArgument(parameter, providedArgs);
			if (args[i] != null) {
				continue;
			}
			//首先判断是否有参数解析器支持参数parameter，采用职责链的设计模式
			if (this.argumentResolvers.supportsParameter(parameter)) {
				try {
					//如果参数解析器支持解析参数parameter，那么解析参数成Controller的函数需要的格式
					args[i] = this.argumentResolvers.resolveArgument(
							parameter, mavContainer, request, this.dataBinderFactory);
					continue;
				}
				catch (Exception ex) {
					if (logger.isDebugEnabled()) {
						logger.debug(getArgumentResolutionErrorMessage("Error resolving argument", i), ex);
					}
					throw ex;
				}
			}
			if (args[i] == null) {
				String msg = getArgumentResolutionErrorMessage("No suitable resolver for argument", i);
				throw new IllegalStateException(msg);
			}
		}
		//返回参数值
		return args;
	}

以上代码会将URL链接提交的参数值转换为对象值。

HandlerMethodReturnValueHandler返回值解析器

HandlerMethodReturnValueHandler是用于对Controller中函数执行的返回值进行处理操作的。接口HandlerMethodReturnValueHandler定义的抽象方法如下：

supportsReturnType用于判断是否支持对返回值的处理。

handleReturnValue实现对返回值的处理操作。

执行过程是在ServletInvocableHandlerMethod的invokeAndHandle中：

public void invokeAndHandle(ServletWebRequest webRequest,
			ModelAndViewContainer mavContainer, Object... providedArgs) throws Exception {
		// 获取返回值
		Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);
		.......
		//对返回值进行处理操作
		this.returnValueHandlers.handleReturnValue(
					returnValue, getReturnValueType(returnValue), mavContainer, webRequest);
		.........
	}

具体实现是在HandlerMethodReturnValueHandlerComposite中，HandlerMethodReturnValueHandlerComposite中包含了所有springMVC提供的返回值处理器。

public void handleReturnValue(Object returnValue, MethodParameter returnType,
      ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {
   //选择可以处理返回值的处理器
   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(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;
}