深入理解SpringMVC DispatcherServlet源码及全流程原理

DispatcherServlet概述

DispatcherServlet是SpringMVC的前端控制器（Front Controller），它负责拦截所有进入应用的HTTP请求，分发请求给相应的处理器，并处理返回的响应。DispatcherServlet的主要职责包括：

初始化WebApplicationContext
加载各种组件（如HandlerMapping、HandlerAdapter等）
分发请求
视图解析和渲染

DispatcherServlet初始化

DispatcherServlet的初始化过程包括创建和配置Spring的WebApplicationContext以及加载各种必要的组件。下面是 DispatcherServlet初始化的主要步骤：

初始化WebApplicationContext：

protected WebApplicationContext initWebApplicationContext() {
// 获取现有的WebApplicationContext
WebApplicationContext rootContext =
WebApplicationContextUtils.getWebApplicationContext(getServletContext());
复制代码
```
 WebApplicationContext wac = createWebApplicationContext(rootContext);
 // 配置WebApplicationContext
 configureAndRefreshWebApplicationContext(wac);
 return wac;
```
}
加载配置文件和初始化组件：

protected void initStrategies(ApplicationContext context) {
initMultipartResolver(context);
initLocaleResolver(context);
initThemeResolver(context);
initHandlerMappings(context);
initHandlerAdapters(context);
initHandlerExceptionResolvers(context);
initRequestToViewNameTranslator(context);
initViewResolvers(context);
initFlashMapManager(context);
}

请求处理流程

当一个HTTP请求到达时，DispatcherServlet将按以下步骤处理请求：

请求接收：

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
HttpServletRequest processedRequest = request;
HandlerExecutionChain mappedHandler = null;
ModelAndView mv = null;
Exception dispatchException = null;

复制代码

 try {
     // 检查并包装Multipart请求
     processedRequest = checkMultipart(request);

     // 获得处理器
     mappedHandler = getHandler(processedRequest);
     if (mappedHandler == null) {
         noHandlerFound(processedRequest, response);
         return;
     }

     // 获得处理器适配器
     HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

     // 执行处理器
     mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

     // 处理视图
     processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
 }
 catch (Exception ex) {
     dispatchException = ex;
     processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
 }

}

检查Multipart请求：

protected HttpServletRequest checkMultipart(HttpServletRequest request) throws MultipartException {
if (this.multipartResolver != null && this.multipartResolver.isMultipart(request)) {
return this.multipartResolver.resolveMultipart(request);
}
return request;
}
获取处理器：

protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
for (HandlerMapping hm : this.handlerMappings) {
HandlerExecutionChain handler = hm.getHandler(request);
if (handler != null) {
return handler;
}
}
return null;
}
获取处理器适配器：

protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
for (HandlerAdapter ha : this.handlerAdapters) {
if (ha.supports(handler)) {
return ha;
}
}
throw new ServletException("No adapter for handler [" + handler + "]");
}
处理视图：

protected void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
HandlerExecutionChain mappedHandler, ModelAndView mv, Exception exception) throws Exception {
if (exception != null) {
if (mv == null) {
throw exception;
}
mv.addObject("exception", exception);
}
复制代码
```
 if (mv != null) {
     render(mv, request, response);
 }
```
}

处理器映射

处理器映射（Handler Mapping）负责将请求URL映射到相应的处理器。常用的处理器映射有：

RequestMappingHandlerMapping：基于注解的处理器映射。
BeanNameUrlHandlerMapping：基于Bean名称的处理器映射。

public HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
String lookupPath = getUrlPathHelper().getLookupPathForRequest(request);
HandlerMethod handlerMethod = lookupHandlerMethod(lookupPath, request);
return handlerMethod != null ? new HandlerExecutionChain(handlerMethod) : null;
}

处理器适配

处理器适配（Handler Adapter）负责调用处理器方法。常用的处理器适配有：

RequestMappingHandlerAdapter：适配使用@RequestMapping注解的方法。
HttpRequestHandlerAdapter：适配实现HttpRequestHandler接口的处理器。

public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
return ((HandlerMethod) handler).invokeForRequest(request, response);
}

视图解析

视图解析（View Resolver）负责将逻辑视图名称解析为实际视图对象，并渲染视图。常用的视图解析器有：

InternalResourceViewResolver：解析JSP视图。
ThymeleafViewResolver：解析Thymeleaf视图。

public View resolveViewName(String viewName, Locale locale) throws Exception {
return getView(viewName, locale);
}

protected View getView(String viewName, Locale locale) throws Exception {
return (viewName != null ? new InternalResourceView(viewName) : null);
}