聊聊Spring MVC中 handlerMapping和handlerAdapter设计思想

先上一段Spring MVC核心类DispatcherServlet中最重要的方法doDispatch源码

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);

         // 根据当前请求获取对应的处理器映射器
         mappedHandler = getHandler(processedRequest);
         if (mappedHandler == null || mappedHandler.getHandler() == null) {
            noHandlerFound(processedRequest, response);
            return;
         }

         // 根据handler类型获取对应的处理器适配器
         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);
         }
      }
   }
}

关注代码中打中文注释的两个地方，一个获取对应handler的处理器映射器，一个获取对应handler的处理器适配器。那为何须要这两个东西，咱们直接在handler中写映射逻辑，直接经过handler来执行处理器方法难道不行吗？答案是否认的，但Spring为何要这样作？有如下几个好处

1.将具体的handler与handlerMapping分离开，为了符合单一职责

2.让具体的处理器与DispatcherServlet解耦合，为了符合开闭原则
咱们知道全部的处理器映射器都有共同的基类HandlerMapping，这个是能够肯定的，也是不会改变的。而handler的类型在将来是极有可能变更或继续追加的。当前版本Spring有如下几种handler类型：
-HandlerMethod（经过RequestMappingHandlerMapping解析而来，咱们最常使用的）
-Servlet（继承自Servlet接口的处理器）
-Controller（继承自Controller接口的处理器）
它们以前并无共同的基类，也不可能有共同的基类，由于它们来自不一样的包，来自不一样的设计者。
Spring设计者将DispatcherServlet与HandlerMapping关联，当追加新的handler类型之后，现有代码并不须要改动，符合面向对象的又一原则：开闭原则。

处理器适配器与处理器映射器的做用相似，都是为了解耦合。

if(handler instanceof Servlet){
    (Servlet)handler.service();
}else if(handler instanceof HandlerMethod){
    (ServletInvocableHandlerMethod)handler.invokeAndHandle();
}else if (handler instanceof Controller){
    ((Controller) handler).handleRequest();
}

若是咱们要新增一种处理器类型，必然要继续追写else if来进行处理，但使用处理器适配器后，DispatcherServlet不须要改动任何代码，由于它只依赖HandlerAdapter，这样DispatcherServlet与具体的Handler就解耦合了，它们以前能够独立发展。

优秀的程序员是艺术家，而艺术就是代码。