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

上一篇博客​​springMVC源码分析--异常处理机制HandlerExceptionResolver简单示例（一）​​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);

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);
      }
    }

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;
      }
    }

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：

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。