Java作为编程界的常青树，有自己生存的独到之处，小猿圈java讲师今天就分享一个关于从代码看spring mvc请求处理过程，通过示例代码介绍的非常详细，对大家的学习或者工作具有一定的参考学习价值。
请求处理的过程：
1.DispatcherServelt作为前端控制器，拦截request对象。
2.DispatcherServlet接收到request对象之后，查询HandlerMapping,得到一个HandlerExecutionChain对象。
3.DispatcherServlet将Handler对象交由HandlerAdapter(适配器模式的典型应用)，调用相应的controller方法。
4.Controller方法返回ModelAndView对象，DispatcherServlet将view交由ViewResolver进行解析，得到相应的视图。用model渲染视图。
5.返回响应结果。
整个过程的流程其实就是DispatcherServelt中doDispatch()方法的调用过程。
protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
//第一步拦截request对象，doDispatch()方法在doService()方法中被调用，request对象是经过处理的。
HttpServletRequest processedRequest = request;
HandlerExecutionChain mappedHandler = null;
boolean multipartRequestParsed = false;
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
try {
ModelAndView mv = null;
Exception dispatchException = null;
try {
//和文件的上传和下载有关系，判断请求的类型是否是multipart类型
processedRequest = checkMultipart(request);
multipartRequestParsed = (processedRequest != request);
// Determine handler for the current request.
//主要看这里，这里是得到HandlerExecutionChain的方法。关于Handler()方法向下看
mappedHandler = getHandler(processedRequest);
if (mappedHandler == null || mappedHandler.getHandler() == null) {
noHandlerFound(processedRequest, response);
return;
}
// Determine handler adapter for the current request.
//这里已经获取到HandlerExecutionChain对象，接下来就要获取HandlerAdapter对象，调用Handler对象的方法。
HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
// Process last-modified header, if supported by the handler
//有关浏览器缓存的设定（304）
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;
}
}
//pan'du
if (!mappedHandler.applyPreHandle(processedRequest, response)) {
return;
}
// Actually invoke the handler.
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
if (asyncManager.isConcurrentHandlingStarted()) {
return;
}
//解析视图，数据渲染
applyDefaultViewName(request, mv);
mappedHandler.applyPostHandle(processedRequest, response, mv);
}
catch (Exception ex) {
dispatchException = ex;
}
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);
}
}
}
}
protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
//遍历HandlerMappingList对象（存储若干个HandlerMapping对象），不断调用，直到不为空为止，否则返回null
for (HandlerMapping hm : this.handlerMappings) {
if (logger.isTraceEnabled()) {
logger.trace(
"Testing handler map [" + hm + "] in DispatcherServlet with name '" + getServletName() + "'");
}
HandlerExecutionChain handler = hm.getHandler(request);
if (handler != null) {
return handler;
}
}
return null;
}
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