SpringMVC 流程分析

SpringMVC 流程

服务器启动阶段（`DispatcherServlet`初始化阶段）

启动后ContextLoaderListener首先会在初始化方法contextInitialized(ServletContextEvent event)中调用initWebApplicationContext(event.getServletContext())来在创建一个WebApplicationContext。并将其作为ServletContext的一个属性设置到Servlet环境中。这个上下文对象为跟上下文对象。
ContextLoader#initWebApplicationContext(ServletContext servletContext)
```
servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);
```

DispatcherServlet初始化过程中中也会创建一个WebApplicationContext对象，并将ContextLoaderListener创建的上下文对象作为父上下文。同样也会将其作为ServletContext的一个属性。
FrameworkServlet#initWebApplicationContext()

String attrName = getServletContextAttributeName();      getServletContext().setAttribute(attrName, wac);

创建完上下文对象后还会调用onRefresh进行一系列初始化工作。
DispatcherServlet#initStrategies(ApplicationContext context)

initMultipartResolver(context);   initLocaleResolver(context);  initThemeResolver(context);  initHandlerMappings(context);  initHandlerAdapters(context);  initHandlerExceptionResolvers(context);  initRequestToViewNameTranslator(context);  initViewResolvers(context);  initFlashMapManager(context);

这几个init方法的实现思路基本相同，首先从Bean容器中过的特定类型的类的实例。如果没有找到，则使用默认配置。即DispatcherServlet.properties中的配置。
DispatcherServlet#initHandlerAdapters(ApplicationContext context)

private void initHandlerAdapters(ApplicationContext context) {      this.handlerAdapters = null;        if (this.detectAllHandlerAdapters) {          // Find all HandlerAdapters in the ApplicationContext, including ancestor contexts.          Map<String, HandlerAdapter> matchingBeans =                  BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerAdapter.class, true, false);          if (!matchingBeans.isEmpty()) {              this.handlerAdapters = new ArrayList<HandlerAdapter>(matchingBeans.values());              // We keep HandlerAdapters in sorted order.              // 排序后对后来的getHandlerAdapter的结果会有所影响              OrderComparator.sort(this.handlerAdapters);          }      }      else {          try {              HandlerAdapter ha = context.getBean(HANDLER_ADAPTER_BEAN_NAME, HandlerAdapter.class);              this.handlerAdapters = Collections.singletonList(ha);          }          catch (NoSuchBeanDefinitionException ex) {              // Ignore, we'll add a default HandlerAdapter later.          }      }        // Ensure we have at least some HandlerAdapters, by registering      // default HandlerAdapters if no other adapters are found.      if (this.handlerAdapters == null) {          this.handlerAdapters = getDefaultStrategies(context, HandlerAdapter.class);          if (logger.isDebugEnabled()) {              logger.debug("No HandlerAdapters found in servlet '" + getServletName() + "': using default");          }      }  }

经过一系列的init方法后DispatcherServlet正式初始化完成。

请求-响应阶段

发起请求
DispatcherServlet是一个标准的Servlet。当我们发送一个GET请求后，会有DispatcherServlet的doGet(HttpServletRequest request, HttpServletResponse response)来处理。
FrameworkServlet#doGet(HttpServletRequest request, HttpServletResponse response)
```
processRequest(request, response);
```
在processRequest这一阶段主要调用doService,然后是doDispatch。
处理请求
在doDispatch主要有一下几步操作。
1) 首先调用getHandler首先获得一个HandlerMapping对象，然后调用其getHandler方法返回一个HandlerExecutionChain对象。具体查看HandlerMapping实现类，例如RequestMappingHandlerMapping
DispatcherServlet#getHandler(HttpServletRequest request)
```
protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {      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;  }
```
以RequestMethodHandlerMapping为例，此对象中主要包含一个HandlerMethod实例及处理此请求的拦截器。
2) 调用getHandlerAdapter来得到第一个supports(handler)为true的HandlerAdapter。
DispatcherServlet#getHandlerAdapter(Object handler)
```
protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {      for (HandlerAdapter ha : this.handlerAdapters) {          if (logger.isTraceEnabled()) {              logger.trace("Testing handler adapter [" + ha + "]");          }          if (ha.supports(handler)) {              return ha;          }      }      throw new ServletException("No adapter for handler [" + handler +              "]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler");  }
```
3) 然后调用HandlerExecutionChain#applyPreHandle(...)开始应用拦截器，一个一个调用其preHandle方法。
HandlerExecutionChain#applyPreHandle(HttpServletRequest request, HttpServletResponse response)
```
boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response) throws Exception {      if (getInterceptors() != null) {          for (int i = 0; i < getInterceptors().length; i++) {              HandlerInterceptor interceptor = getInterceptors()[i];              if (!interceptor.preHandle(request, response, this.handler)) {                  triggerAfterCompletion(request, response, null);                  return false;              }              this.interceptorIndex = i;          }      }      return true;  }
```
4) 接着调用HandlerAdapter.handle(...)得到一个ModelAndView对象,里面封装了数据对象及具体的View对象。具体实现需要查看HandlerAdapter实现类。例如：RequestMappingHandlerAdapter。
5) 然后调用HandlerExecutionChain#applyPostHandle(...)再次应用拦截器，调用其postHandle方法。
HandlerExecutionChain#applyPreHandleapplyPostHandle(HttpServletRequest request, HttpServletResponse response, ModelAndView mv)
```
void applyPostHandle(HttpServletRequest request, HttpServletResponse response, ModelAndView mv) throws Exception {      if (getInterceptors() == null) {          return;      }      for (int i = getInterceptors().length - 1; i >= 0; i--) {          HandlerInterceptor interceptor = getInterceptors()[i];          interceptor.postHandle(request, response, this.handler, mv);      }  }
```
6) 最后一步processDispatchResult处理结果，相应给客户端。在processDispatchResult首先调用了render方法
DispatcherServlet#processDispatchResult(HttpServletRequest request, HttpServletResponse response, HandlerExecutionChain mappedHandler, ModelAndView mv, Exception exception)
```
render(mv, request, response);
```
最后是HandlerExecutionChain#triggerAfterCompletion(...),调用拦截器的afterCompletion方法。拦截器处理流程至此结束。
分析一下reader方法。首先调用resolveViewName，在里面先遍历所有的ViewResolver实例,调用其resolveViewName方法，返回第一不为空的View实例。
DispatcherServlet#resolveViewName(String viewName, Map model, Locale locale, HttpServletRequest request)
```
protected View resolveViewName(String viewName, Map<String, Object> model, Locale locale,          HttpServletRequest request) throws Exception {        for (ViewResolver viewResolver : this.viewResolvers) {          View view = viewResolver.resolveViewName(viewName, locale);          if (view != null) {              return view;          }      }      return null;  }
```
ViewResolver#resolveViewName具体可参看InternalResourceViewResolver实现。
最后调用View的render方法。render(Map<String, ?> model, HttpServletRequest request, HttpServletResponse response)方法的具体实现可参考View实现类JstlView。
在JstlView中主要是调用renderMergedOutputModel，首先Model对象数据放置到request中，然后得到一个RequestDispatcher,最后调用其forward方法响应给客户端。