聊聊Spring MVC中 handlerMapping和handlerAdapter設計思想

先上一段Spring MVC核心類DispatcherServlet中最重要的方法doDispatch源碼

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);

         // 根據當前請求獲取對應的處理器映射器
         mappedHandler = getHandler(processedRequest);
         if (mappedHandler == null || mappedHandler.getHandler() == null) {
            noHandlerFound(processedRequest, response);
            return;
         }

         // 根據handler類型獲取對應的處理器適配器
         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);
         }
      }
   }
}

關注代碼中打中文註釋的兩個地方，一個獲取對應handler的處理器映射器，一個獲取對應handler的處理器適配器。那爲何須要這兩個東西，咱們直接在handler中寫映射邏輯，直接經過handler來執行處理器方法難道不行嗎？答案是否認的，但Spring爲何要這樣作？有如下幾個好處

1.將具體的handler與handlerMapping分離開，爲了符合單一職責

2.讓具體的處理器與DispatcherServlet解耦合，爲了符合開閉原則
咱們知道全部的處理器映射器都有共同的基類HandlerMapping，這個是能夠肯定的，也是不會改變的。而handler的類型在將來是極有可能變更或繼續追加的。當前版本Spring有如下幾種handler類型：
-HandlerMethod（經過RequestMappingHandlerMapping解析而來，咱們最常使用的）
-Servlet（繼承自Servlet接口的處理器）
-Controller（繼承自Controller接口的處理器）
它們以前並無共同的基類，也不可能有共同的基類，由於它們來自不一樣的包，來自不一樣的設計者。
Spring設計者將DispatcherServlet與HandlerMapping關聯，當追加新的handler類型之後，現有代碼並不須要改動，符合面向對象的又一原則：開閉原則。

處理器適配器與處理器映射器的做用相似，都是爲了解耦合。

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

若是咱們要新增一種處理器類型，必然要繼續追寫else if來進行處理，但使用處理器適配器後，DispatcherServlet不須要改動任何代碼，由於它只依賴HandlerAdapter，這樣DispatcherServlet與具體的Handler就解耦合了，它們以前能夠獨立發展。

優秀的程序員是藝術家，而藝術就是代碼。