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);
}
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。