springMVC源码分析--异常处理机制HandlerExceptionResolver执行原理（二）

ゝ一纸荒年。 2022-07-13 00:51 135阅读 0赞

上一篇博客[springMVC源码分析--异常处理机制HandlerExceptionResolver简单示例（一）][springMVC_--_HandlerExceptionResolver]中我们简单地实现了一个异常处理实例，接下来我们要介绍一下HandlerExceptionResolver是如何捕获到Controller中抛出的异常并展示到前台页面的。

DispatcherServlet是springMVC中最重要的一个类，之前我们已经有博客对其进行介绍过，可以看看其实现机制。

HandlerExceptionResolver可以捕获Controller中抛出的异常，因此还是和Controller的运行机制有关，我们就想到了DispatcherServlet的doDispatch方法，其异常捕获机制还是比较简单的，就是通过一个try catch来完成的，简直玩的六啊！

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
    		
    
    		try {
    			ModelAndView mv = null;
    			Exception dispatchException = null;
    			//开始捕获Controller执行过程中抛出的异常
    			try { 
    				processedRequest = checkMultipart(request);
    				multipartRequestParsed = (processedRequest != request);
    
    				// Determine handler for the current request.
    				mappedHandler = getHandler(processedRequest);
    				if (mappedHandler == null || mappedHandler.getHandler() == null) {
    					noHandlerFound(processedRequest, response);
    					return;
    				}
    
    				// Determine handler adapter for the current request.
    				HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
    
    				// Process last-modified header, if supported by the handler.
    				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;
    					}
    				}
    
    				if (!mappedHandler.applyPreHandle(processedRequest, response)) {
    					return;
    				}
    
    				// Actually invoke the handler.
    				mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
    
    				if (asyncManager.isConcurrentHandlingStarted()) {
    					return;
    				}
    
    				applyDefaultViewName(processedRequest, mv);
    				mappedHandler.applyPostHandle(processedRequest, response, mv);
    			}//捕获Controller执行过程中抛出的异常 catch (Exception ex) { dispatchException = ex; }
    			//异常处理操作，与HandlerExceptionResolver的操作有关 processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
    		}
    		catch (Exception ex) {
    			triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
    		}
    		catch (Error err) {
    			triggerAfterCompletionWithError(processedRequest, response, mappedHandler, err);
    		}
    		finally {
    			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.
    				if (multipartRequestParsed) {
    					cleanupMultipart(processedRequest);
    				}
    			}
    		}

在doDispatch方法中我们看到了如何捕获Controller执行过程中抛出的异常，接下来就是异常处理机制了，在processDispatchResult中，红色区域是异常处理的操作，最终操作还是在processHandlerException中了。

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); } }
    
    		// Did the handler return a view to render?
    		if (mv != null && !mv.wasCleared()) {
    			render(mv, request, response);
    			if (errorView) {
    				WebUtils.clearErrorRequestAttributes(request);
    			}
    		}
    		else {
    			if (logger.isDebugEnabled()) {
    				logger.debug("Null ModelAndView returned to DispatcherServlet with name '" + getServletName() +
    						"': assuming HandlerAdapter completed request handling");
    			}
    		}
    
    		if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
    			// Concurrent handling started during a forward
    			return;
    		}
    
    		if (mappedHandler != null) {
    			mappedHandler.triggerAfterCompletion(request, response, null);
    		}
    	}

processHandlerException中异常操作也是比较简单的，springMVC启动加载时会将所有的异常处理HandlerExceptionResolver实现类存放到handlerExceptionResolvers一个map结构中，当默认执行时，只要放回的exMv不为空就中止执行其他的异常处理实现类

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;
    	}

最终结果就放回ModelAndView变量exMV， processDispatchResult方法执行接下来的操作

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);
    			}
    		}
    
    		// Did the handler return a view to render? if (mv != null && !mv.wasCleared()) { render(mv, request, response); if (errorView) { WebUtils.clearErrorRequestAttributes(request); } } else { if (logger.isDebugEnabled()) { logger.debug("Null ModelAndView returned to DispatcherServlet with name '" + getServletName() + "': assuming HandlerAdapter completed request handling"); } } if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) { // Concurrent handling started during a forward return; } if (mappedHandler != null) { mappedHandler.triggerAfterCompletion(request, response, null); }
    	}

最终获得异常处理HandlerExceptionResolver执行后的结果ModelAndView对象，完成异常页面的渲染工作。这个异常拦截处理的操作感觉设计的真是非常巧妙，希望能让大家了解springMVC的异常处理机制。

HandlerExceptionResolver接口及实现类：

![70][]

一个基于Spring MVC的Web应用程序中，可以存在多个实现了HandlerExceptionResolver的异常处理类，他们的执行顺序，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。

[springMVC_--_HandlerExceptionResolver]: http://blog.csdn.net/qq924862077/article/details/53711682
[70]: /images/20220713/94216e17c0924114ac173ee75f7a2940.png