springMVC原始碼分析--異常處理機制HandlerExceptionResolver執行原理（二）
阿新 • • 發佈：2019-01-05
上一篇部落格springMVC原始碼分析--異常處理機制HandlerExceptionResolver簡單示例（一）中我們簡單地實現了一個異常處理例項，接下來我們要介紹一下HandlerExceptionResolver是如何捕獲到Controller中丟擲的異常並展示到前臺頁面的。
DispatcherServlet是springMVC中最重要的一個類，之前我們已經有部落格對其進行介紹過，可以看看其實現機制。
HandlerExceptionResolver可以捕獲Controller中丟擲的異常，因此還是和Controller的執行機制有關，我們就想到了DispatcherServlet的doDispatch方法，其異常捕獲機制還是比較簡單的，就是通過一個try catch來完成的，簡直玩的六啊！
在doDispatch方法中我們看到了如何捕獲Controller執行過程中丟擲的異常，接下來就是異常處理機制了，在processDispatchResult中，紅色區域是異常處理的操作，最終操作還是在processHandlerException中了。protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception { try { ModelAndView mv = null; Exception dispatchException = null; //開始捕獲Controller執行過程中丟擲的異常 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); }//捕獲Controller執行過程中丟擲的異常 catch (Exception ex) { dispatchException = ex; }//異常處理操作，與HandlerExceptionResolver的操作有關 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); } } }
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中異常操作也是比較簡單的，springMVC啟動載入時會將所有的異常處理HandlerExceptionResolver實現類存放到handlerExceptionResolvers一個map結構中，當預設執行時，只要放回的exMv不為空就中止執行其他的異常處理實現類
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;
}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);
}
}最終獲得異常處理HandlerExceptionResolver執行後的結果ModelAndView物件，完成異常頁面的渲染工作。這個異常攔截處理的操作感覺設計的真是非常巧妙，希望能讓大家瞭解springMVC的異常處理機制。
HandlerExceptionResolver介面及實現類：
一個基於Spring MVC的Web應用程式中，可以存在多個實現了HandlerExceptionResolver的異常處理類，他們的執行順序，由其order屬性決定, order值越小，越是優先執行, 在執行到第一個返回不是null的ModelAndView的Resolver時，不再執行後續的尚未執行的Resolver的異常處理方法。