DispatcherServlet请求流程解析-doDispatch(三)

上篇文章我们主要看了DispatcherServlet在提供服务之间做的初始化工作，大部门工作都在WebApplicationContext中完成，然后WebApplicationContext是DispatcherServlet的一个属性。

在初始操作完成以后，DispatcherServlet可以提供健全的服务了，早先我们也提到了，真正的请求分发在doDispatcher这个方法之中，今天一起来看看这个方法到底做了什么操作。

看代码之前，先提前说明自己自己阅读源码过程中的一点简单思考：

• WebAsyncManager在普通的开发中用不到，在异步处理的时候有用到，这一块我们先忽略
• 关键部分：HandlerMappings,HandlerAdapter,HandlerExecutionChain对这几个熟悉之后基本上就懂了它到底在做什么

一起看代码吧

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
 HttpServletRequest processedRequest = request;
 HandlerExecutionChain mappedHandler = null;
 boolean multipartRequestParsed = false;

 WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

 try {
     ModelAndView mv = null;
     Exception dispatchException = null;

     try {
  processedRequest = checkMultipart(request);
  multipartRequestParsed = (processedRequest != request);

  // Determine handler for the current request.
  mappedHandler = getHandler(processedRequest);
  if (mappedHandler == null || mappedHandler.getHandler() == null) {
      noHandlerFound(processedRequest, response);
      return;
  }

  // Determine handler adapter for the current request.
  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 (logger.isDebugEnabled()) {
   logger.debug("Last-Modified value for [" + getRequestUri(request) + "] is: " + lastModified);
      }
      if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
   return;
      }
  }

  if (!mappedHandler.applyPreHandle(processedRequest, response)) {
      return;
  }

  // Actually invoke the handler.
  mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

  if (asyncManager.isConcurrentHandlingStarted()) {
      return;
  }

  applyDefaultViewName(processedRequest, mv);
  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);
     }
     processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
 }
 catch (Exception ex) {
     triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
 }
 catch (Throwable err) {
     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);
  }
     }
 }
    }

@Override
    public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
 Object handler = getHandlerInternal(request);
 if (handler == null) {
     handler = getDefaultHandler();
 }
 if (handler == null) {
     return null;
 }
 // Bean name or resolved handler?
 if (handler instanceof String) {
     String handlerName = (String) handler;
     handler = getApplicationContext().getBean(handlerName);
 }

      // 这一步的目的是就是，获取所有的interceptor的对象，然后与当前的handler组合在一起
 HandlerExecutionChain executionChain = getHandlerExecutionChain(handler, request);
 if (CorsUtils.isCorsRequest(request)) {
     CorsConfiguration globalConfig = this.globalCorsConfigSource.getCorsConfiguration(request);
     CorsConfiguration handlerConfig = getCorsConfiguration(handler, request);
     CorsConfiguration config = (globalConfig != null ? globalConfig.combine(handlerConfig) : handlerConfig);
     executionChain = getCorsHandlerExecutionChain(request, executionChain, config);
 }
 return executionChain;
    }

    @Override
    protected HandlerMethod getHandlerInternal(HttpServletRequest request) throws Exception {
 String lookupPath = getUrlPathHelper().getLookupPathForRequest(request);
 if (logger.isDebugEnabled()) {
     logger.debug("Looking up handler method for path " + lookupPath);
 }
 this.mappingRegistry.acquireReadLock();
 try {
     HandlerMethod handlerMethod = lookupHandlerMethod(lookupPath, request);
     if (logger.isDebugEnabled()) {
  if (handlerMethod != null) {
      logger.debug("Returning handler method [" + handlerMethod + "]");
  }
  else {
      logger.debug("Did not find handler method for [" + lookupPath + "]");
  }
     }
     return (handlerMethod != null ? handlerMethod.createWithResolvedBean() : null);
 }
 finally {
     this.mappingRegistry.releaseReadLock();
 }
    }

public interface HandlerAdapter {
   // 判断当前的HandlerAdpater是否支持handler的实例，也就是当前的adapter能否使用传过来的Handler
    boolean supports(Object handler);
    // 使用传过来的handler来处理请求
    ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception;

    //和Servlet的getLastModified功能基本一致
    long getLastModified(HttpServletRequest request, Object handler);

}

if (!mappedHandler.applyPreHandle(processedRequest, response)) {
    return;
}

boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response) throws Exception {
 HandlerInterceptor[] interceptors = getInterceptors();
 if (!ObjectUtils.isEmpty(interceptors)) {
     for (int i = 0; i < interceptors.length; i++) {
  HandlerInterceptor interceptor = interceptors[i];
  if (!interceptor.preHandle(request, response, this.handler)) {
      triggerAfterCompletion(request, response, null);
      return false;
  }
  this.interceptorIndex = i;
     }
 }
 return true;
    }

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) {
  logger.debug("ModelAndViewDefiningException encountered", exception);
  mv = ((ModelAndViewDefiningException) exception).getModelAndView();
     }
     else {
  Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
  mv = processHandlerException(request, response, handler, exception);
  errorView = (mv != null);
     }
 }

 // Did the handler return a view to render?
 if (mv != null && !mv.wasCleared()) {
     render(mv, request, response);
     if (errorView) {
  WebUtils.clearErrorRequestAttributes(request);
     }
 }
 else {
     if (logger.isDebugEnabled()) {
  logger.debug("Null ModelAndView returned to DispatcherServlet with name '" + getServletName() +
   "': assuming HandlerAdapter completed request handling");
     }
 }

 if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
     // Concurrent handling started during a forward
     return;
 }

 if (mappedHandler != null) {
     mappedHandler.triggerAfterCompletion(request, response, null);
 }
    }