DispatcherServlet作为前端核心控制器，作用接收用户请求，响应结果，相当于转发器，中央处理器。有了DispatcherServlet减少了其它组件之间的耦合度。
DispatcherServlet流程
第一步：发起请求到前端控制器(DispatcherServlet)
第二步：前端控制器请求HandlerMapping查找 Handler
可以根据xml配置、注解进行查找
第三步：处理器映射器HandlerMapping向前端控制器返回Handler
第四步：前端控制器调用处理器适配器去执行Handler
第五步：处理器适配器去执行Handler
第六步：Handler执行完成给适配器返回ModelAndView
第七步：处理器适配器向前端控制器返回ModelAndView
ModelAndView是springmvc框架的一个底层对象，包括 Model和view
第八步：前端控制器请求视图解析器去进行视图解析
根据逻辑视图名解析成真正的视图(jsp)
第九步：视图解析器向前端控制器返回View
第十步：前端控制器进行视图渲染
视图渲染将模型数据(在ModelAndView对象中)填充到request域
第十一步：前端控制器向用户响应结果
DispatcherServlet 逻辑处理源码解析
请求处理的入口定义在 HttpServlet，主要有以下几个方法：
当然，父类 HttpServlet 只是给出了定义，直接调用父类这些方法将会报错，所以 FrameworkServlet 将它们覆盖重写了处理逻辑：
可以看到 doGet 、doPost 这些方法，底层调用的都是 processRequest 方法进行处理，关键方法是委托给子类 DispatcherServlet 的 doServie() 方法
FrameworkServlet #processRequest
DispatcherServlet 重写了doServie方法
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
if (this.logger.isDebugEnabled()) { String resumed = WebAsyncUtils.getAsyncManager(request).hasConcurrentResult() ? " resumed" : ""; this.logger.debug("DispatcherServlet with name '" + this.getServletName() + "'" + resumed + " processing " + request.getMethod() + " request for [" + getRequestUri(request) + "]");}Map<String, Object> attributesSnapshot = null;if (WebUtils.isIncludeRequest(request)) { attributesSnapshot = new HashMap(); Enumeration attrNames = request.getAttributeNames(); label108: while(true) { String attrName; do { if (!attrNames.hasMoreElements()) { break label108; } attrName = (String)attrNames.nextElement(); } while(!this.cleanupAfterInclude && !attrName.startsWith("org.springframework.web.servlet")); attributesSnapshot.put(attrName, request.getAttribute(attrName)); }}request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.getWebApplicationContext());request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);request.setAttribute(THEME_SOURCE_ATTRIBUTE, this.getThemeSource());FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);if (inputFlashMap != null) { request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap));}request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());request.setAttribute(FLASH_MAP_MANAGER_ATTRIBUTE, this.flashMapManager);try { this.doDispatch(request, response);//进入请求处理过程
} finally { if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted() && attributesSnapshot != null) { this.restoreAttributesAfterInclude(request, attributesSnapshot); }}
}
第一步：请求分发和处理逻辑的核心是在 doDispatch(request, response) 方法中。
protected void doDispatch(HttpServletRequest request, HttpServletResponse response) {
HttpServletRequest processedRequest = request;
HandlerExecutionChain mappedHandler = null;
// 注释 10. 检查是否 MultipartContent 类型
processedRequest = checkMultipart(request);
// 根据 request 信息寻找对应的 Handler
mappedHandler = getHandler(processedRequest);
if (mappedHandler == null) {
// 没有找到 handler，通过 response 向用户返回错误信息
noHandlerFound(processedRequest, response);
return;
}
// 根据当前的 handler 找到对应的 HandlerAdapter 适配器
HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
// 如果当前 handler 支持 last-modified 头处理
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;
}
}
// 拦截器的 preHandler 方法的调用
if (!mappedHandler.applyPreHandle(processedRequest, response)) {
return;
}
// 真正激活 handler 进行处理，并返回视图
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
if (asyncManager.isConcurrentHandlingStarted()) {
return;
}
// 视图名称转换（有可能需要加上前后缀）
applyDefaultViewName(processedRequest, mv);
// 应用所有拦截器的 postHandle 方法
mappedHandler.applyPostHandle(processedRequest, response, mv);
// 处理分发的结果（如果有 mv，进行视图渲染和跳转）
processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
}
第二步：寻找处理器 mappedHandler
寻找处理器，就是根据 URL 找到对应的 Controller 方法
DispatcherServlet#getHandler
protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception { Iterator var2 = this.handlerMappings.iterator(); HandlerExecutionChain handler; //遍历注册的全部 handlerMapping do { if (!var2.hasNext()) { return null; } HandlerMapping hm = (HandlerMapping)var2.next(); if (this.logger.isTraceEnabled()) { this.logger.trace("Testing handler map [" + hm + "] in DispatcherServlet with name '" + this.getServletName() + "'"); } handler = hm.getHandler(request); } while(handler == null); return handler;}
实际上，在这一步遍历了所有注册的 HandlerMapping，然后委派它们去寻找处理器，如果找到了合适的，就不再往下寻找，直接返回。
具体寻找调用的方法：
AbstractHandlerMapping#getHandler
public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
// 根据 Request 获取对应的 handler
Object handler = getHandlerInternal(request);
// 将配置中的对应拦截器加入到执行链中，以保证这些拦截器可以有效地作用于目标对象
HandlerExecutionChain executionChain = getHandlerExecutionChain(handler, request);
if (hasCorsConfigurationSource(handler)) {
CorsConfiguration config = (this.corsConfigurationSource != null ? this.corsConfigurationSource.getCorsConfiguration(request) : null);
CorsConfiguration handlerConfig = getCorsConfiguration(handler, request);
config = (config != null ? config.combine(handlerConfig) : handlerConfig);
executionChain = getCorsHandlerExecutionChain(request, executionChain, config);
}
return executionChain;
}
(1) getHandlerInternal(request) 函数作用：
根据 request 信息获取对应的 Handler，也就是我们例子中的，通过 URL 找到匹配的 Controller 并返回。
(2) getHandlerExcetionChain 函数作用：
将适应该 URL 对应拦截器 MappedInterceptor 加入 addInterceptor() 到执行链 HandlerExecutionChain 中。
(3) CorsConfiguration
这个参数涉及到跨域设置
第三步：寻找适配器 HandlerAdapter
前面已经找到了对应的处理器了，下一步就得找到它对应的适配器
DispatcherServlet#getHandlerAdapter
protected getHandlerAdapter(Object handler) throws ServletException {
if (this.handlerAdapters != null) {
for (HandlerAdapter adapter : this.handlerAdapters) {
if (adapter.supports(handler)) {
return adapter;
}
}
}
}
第四步：调用处理器适配器执行Handler，得到执行结果ModelAndView
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
第五步：视图渲染，将model数据填充到request域。
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) {
this.logger.debug("ModelAndViewDefiningException encountered", exception);
mv = ((ModelAndViewDefiningException)exception).getModelAndView();
} else {
Object handler = mappedHandler != null ? mappedHandler.getHandler() : null;
mv = this.processHandlerException(request, response, handler, exception);
errorView = mv != null;
}
}
if (mv != null && !mv.wasCleared()) {
this.render(mv, request, response);//视图渲染
if (errorView) {
WebUtils.clearErrorRequestAttributes(request);
}
} else if (this.logger.isDebugEnabled()) {
this.logger.debug("Null ModelAndView returned to DispatcherServlet with name '" + this.getServletName() + "': assuming HandlerAdapter completed request handling");
}
if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
if (mappedHandler != null) {
mappedHandler.triggerAfterCompletion(request, response, (Exception)null);
}
}
}
1.视图解析，得到view，
2.调用view的渲染方法，将model数据填充到request域
protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception {
Locale locale = this.localeResolver.resolveLocale(request);
response.setLocale(locale);
View view;
if (mv.isReference()) {
view = this.resolveViewName(mv.getViewName(), mv.getModelInternal(), locale, request);//视图解析得到view对象
if (view == null) {
throw new ServletException("Could not resolve view with name '" + mv.getViewName() + "' in servlet with name '" + this.getServletName() + "'");
}
} else {
view = mv.getView();
if (view == null) {
throw new ServletException("ModelAndView [" + mv + "] neither contains a view name nor a " + "View object in servlet with name '" + this.getServletName() + "'");
}
}
if (this.logger.isDebugEnabled()) {
this.logger.debug("Rendering view [" + view + "] in DispatcherServlet with name '" + this.getServletName() + "'");
}
try {
view.render(mv.getModelInternal(), request, response);//调用view的渲染方法，将model数据填充到request域
} catch (Exception var7) {
if (this.logger.isDebugEnabled()) {
this.logger.debug("Error rendering view [" + view + "] in DispatcherServlet with name '" + this.getServletName() + "'", var7);
}
throw var7;
}
}