理解SpringMVC-------DispatchServlet

在web.xml中,可以找到这么一段配置文件,它的作用是将DispatchServlet作为一个servlet加载到servlet容器中
其中,是将某一类请求转交给这个servlet进行处理


    SpringMVC
    org.springframework.web.servlet.DispatcherServlet
    
        contextConfigLocation
        classpath:spring-mvc.xml
    
    1
     //是否开启异步支持
    


    SpringMVC
    /

然后我们开始解析一下DispatchServlet具体做了些什么事情

1)加载DispatcherServlet.properties中的属性

private static final String DEFAULT_STRATEGIES_PATH = "DispatcherServlet.properties";

...

static {
  // Load default strategy implementations from properties file.
  // This is currently strictly internal and not meant to be customized
  // by application developers.
  try {
      ClassPathResource resource = new ClassPathResource(DEFAULT_STRATEGIES_PATH, DispatcherServlet.class);
      defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);
  }
  catch (IOException ex) {
      throw new IllegalStateException("Could not load 'DispatcherServlet.properties': " + ex.getMessage());
  }
}

找到DispatcherServlet.properties,其中内容如下

# Default implementation classes for DispatcherServlet's strategy interfaces.
# Used as fallback when no matching beans are found in the DispatcherServlet context.
# Not meant to be customized by application developers.

org.springframework.web.servlet.LocaleResolver=org.springframework.web.servlet.i18n.AcceptHeaderLocaleResolver

org.springframework.web.servlet.ThemeResolver=org.springframework.web.servlet.theme.FixedThemeResolver

org.springframework.web.servlet.HandlerMapping=org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping,\
  org.springframework.web.servlet.mvc.annotation.DefaultAnnotationHandlerMapping

org.springframework.web.servlet.HandlerAdapter=org.springframework.web.servlet.mvc.HttpRequestHandlerAdapter,\
  org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter,\
  org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter

org.springframework.web.servlet.HandlerExceptionResolver=org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerExceptionResolver,\
  org.springframework.web.servlet.mvc.annotation.ResponseStatusExceptionResolver,\
  org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolver

org.springframework.web.servlet.RequestToViewNameTranslator=org.springframework.web.servlet.view.DefaultRequestToViewNameTranslator

org.springframework.web.servlet.ViewResolver=org.springframework.web.servlet.view.InternalResourceViewResolver

org.springframework.web.servlet.FlashMapManager=org.springframework.web.servlet.support.SessionFlashMapManager

所以我们可以知道,这个DispatcherServlet.properties的作用是将默认实现类的路径以键值对的形式加载进来
LocaleResolver(本地化解析器,AcceptHeaderLocaleResolver)
ThemeResolver(主题解析器,FixedThemeResolver)
HandlerMapping(映射处理器,BeanNameUrlHandlerMapping)
HandlerAdapter(处理适配器,多个)
ViewResolver(视图解析器,InternalResourceViewResolver)
RequestToViewNameTranslator(将请求名称转化为视图名称,DefaultRequestToViewNameTranslator)

2)初始化加载SpringMVC框架下需要配置的基本的bean

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

这里的作用是将SpringMVC框架所需的各个基本的bean初始化,并加载到容器中

3)重写doService()方法

DispatchServlet实际上继承于FrameworkServlet,并且在其具体实现中重写了doService()方法。
doService()主要是将一些全局属性set到request中,具体的请求处理则调用并在doDispatch()中处理

4)doDipatch()将请求处理后转发到Controller

/**
* 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 { //获取当前要处理的request HttpServletRequest processedRequest = request; HandlerExecutionChain mappedHandler = null; boolean multipartRequestParsed = false; WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request); try { ModelAndView mv = null; Exception dispatchException = null; try { //判断是否有文件上传 processedRequest = checkMultipart(request); multipartRequestParsed = (processedRequest != request); // Determine handler for the current request. //获取HandlerExecutionChain,其中包含了private HandlerInterceptor[] interceptors,即拦截器的一个数组 mappedHandler = getHandler(processedRequest); if (mappedHandler == null || mappedHandler.getHandler() == null) { noHandlerFound(processedRequest, response); return; } // Determine handler adapter for the current request. //获取HandlerAdapter HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler()); // Process last-modified header, if supported by the handler. //从request中获取HTTP请求方法 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; } } //如果有拦截器,则处理其前置方法preHandle if (!mappedHandler.applyPreHandle(processedRequest, response)) { return; } // Actually invoke the handler. //返回处理后的modelView mv = ha.handle(processedRequest, response, mappedHandler.getHandler()); //判断是否是异步请求 if (asyncManager.isConcurrentHandlingStarted()) { return; } //如果view为空,返回一个默认的view applyDefaultViewName(processedRequest, mv); //处理拦截器的后置方法postHandle 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 { //判断是否是异步请求 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. //删除上传资源 if (multipartRequestParsed) { cleanupMultipart(processedRequest); } } } }

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