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Spring MVC
前置控制器DispatcherServlet
对请求进行处理的主要逻辑体现在其方法doDispatch
的实现,如下 :
/**
* Process the actual dispatching to the handler.
* The handler will be obtained by applying the servlet's HandlerMappings in order.
* The HandlerAdapter will be obtained by querying the servlet's installed HandlerAdapters
* to find the first that supports the handler class.
* All HTTP methods are handled by this method. It's up to HandlerAdapters or handlers
* themselves to decide which methods are acceptable.
* @param request current HTTP request
* @param response current HTTP response
* @throws Exception in case of any kind of processing failure
*/
protected void doDispatch(HttpServletRequest request, HttpServletResponse response)
throws Exception {
HttpServletRequest processedRequest = request;
// 用于记录针对该请求的请求处理器 handler 以及有关的 HandlerInteceptor , 封装成一个
// HandlerExecutionChain, 在随后的逻辑中会被使用
HandlerExecutionChain mappedHandler = null;
// 用于标记当前请求是否是一个文件上传请求
boolean multipartRequestParsed = false;
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
try {
ModelAndView mv = null;
Exception dispatchException = null;
try {
// 检查请求是否文件上传请求
processedRequest = checkMultipart(request);
// 如果 processedRequest 和 request 不同,说明检测到了文件上传请求过程中
// 才会出现的内容,现在把这个信息记录在 multipartRequestParsed
multipartRequestParsed = (processedRequest != request);
// Determine handler for the current request.
// 这里根据请求自身,和 DispatcherServlet 的 handlerMappings 查找能够处理该请求的
// handler, 记录在 mappedHandler , 具体返回的类型是 HandlerExecutionChain ,
// 相当于 n 个 HandlerInterceptor + 1 个 handler 。
mappedHandler = getHandler(processedRequest);
if (mappedHandler == null) {
// 如果没有找到可以处理该请求的 handler , 这里直接交给 noHandlerFound 处理,
// 根据配置决定 抛出异常 或者返回 404
noHandlerFound(processedRequest, response);
return;
}
// Determine handler adapter for the current request.
// 根据所找到的 handler 和 DispatcherServlet 的 handlerAdapters 看看哪个 handlerAdapter
// 可以执行这个 handler。
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) {
// 如果是 GET/HEAD 模式,并且发现资源从上次访问到现在没被修改过,则直接返回
return;
}
}
// 执行 mappedHandler 所有 HandlerInteceptor 的 preHandle 方法,
// 如果有执行失败,在这里直接返回不再继续处理该请求
if (!mappedHandler.applyPreHandle(processedRequest, response)) {
return;
}
// Actually invoke the handler.
// 现在才真正由所找到的 HandlerAdapter 执行 所对应的 handler,
// 返回结果是一个 ModelAndView 对象 mv, 或者 null
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
if (asyncManager.isConcurrentHandlingStarted()) {
return;
}
// 如果上面获取的 ModelAndView 对象 mv 不为 null, 并且没有含有 view,
// 并且配置了缺省 view,这里应用缺省 view
applyDefaultViewName(processedRequest, mv);
// 执行 mappedHandler 所有 HandlerInteceptor 的 postHandle 方法,
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);
}
// 处理 handler 结果 ModelAndView mv
// 1. 根据 view 名称 或者 异常找到最终要使用的 view
// 2. 渲染 view
// 3. 执行 mappedHandler 所有 HandlerInteceptor 的 afterCompletion 方法
processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
}
catch (Exception ex) {
// 异常情况下执行 mappedHandler 所有 HandlerInteceptor 的 afterCompletion 方法
triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
}
catch (Throwable err) {
// 异常情况下执行 mappedHandler 所有 HandlerInteceptor 的 afterCompletion 方法
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);
}
}
}
}