所以说这段代码并不难理解，它的核心在于 doDispatch 方法，所以接下来我们就来看看 doDispatch 方法。
doDispatch
doDispatch 方法所做的事情就比较多了，我们来看下：
protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
HttpServletRequest processedRequest = request;
HandlerExecutionChain mappedHandler = null;
boolean multipartRequestParsed = false;
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
try {
ModelAndView mv = null;
Exception dispatchException = null;
try {
processedRequest = checkMultipart(request);
multipartRequestParsed = (processedRequest != request);
// Determine handler for the current request.
mappedHandler = getHandler(processedRequest);
if (mappedHandler == 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 (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);
}
catch (Exception ex) {
dispatchException = ex;
}
catch (Throwable err) {
// As of 4.3, we're processing Errors thrown from handler methods as well,
// making them available for @ExceptionHandler methods and other scenarios.
dispatchException = new NestedServletException("Handler dispatch failed", err);
}
processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
}
catch (Exception ex) {
triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
}
catch (Throwable err) {
triggerAfterCompletion(processedRequest, response, mappedHandler,
new NestedServletException("Handler processing failed", 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);
}
}
}
}
这个方法比较长，涉及到很多组件的处理，这里松哥先和大家把思路梳理畅通，各个组件的详细用法松哥将在以后的文章中和大家仔细分享。
doDispatch 方法其实主要做了两方面的事情：请求处理以及页面渲染，我们先来看看初始变量的含义：
接下来再来看看具体的处理逻辑：
这是 doDispatch 方法的一个大致执行逻辑，doDispatch 里边的 try-catch 有两层，最里边那一层，抛出来的异常会被赋值给 dispatchException 变量，这些异常最终在 processDispatchResult 方法中被处理掉，外面的异常则是 processDispatchResult 方法在执行的过程中抛出的异常，一般来说主要是页面渲染时候的异常。
processDispatchResult
最后我们再来看下 processDispatchResult 方法的执行逻辑：
private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
@Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv,
@Nullable Exception exception) throws Exception {
boolean errorView = false;
if (exception != null) {
if (exception instanceof ModelAndViewDefiningException) {
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 (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
// Concurrent handling started during a forward
return;
}
if (mappedHandler != null) {
// Exception (if any) is already handled..
mappedHandler.triggerAfterCompletion(request, response, null);
}
}
可以看到，在 processDispatchResult 方法中首先对异常进行了处理，配置好异常对应的 ModelAndView，然后调用 render 方法对页面进行渲染，最后通过 triggerAfterCompletion 方法去触发拦截器的 afterCompletion 方法。
小结
至此，我们就把一个请求的大致流程和大家梳理完了，松哥画了一张流程图我们一起来看下：
这下相信大家对 doDispatch 方法比较熟悉了，当然这里还涉及到很多组件，这些组件松哥将在后面的文章中和大家逐一进行分析。
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