springmvc原始碼 ---DispatcherServlet 處理請求

概述

對於要給servlet 而言，處理請求相關最重要方法是 service()，但是HttpServlet 對service方法進行了封裝，並提供了一些子類重寫的方法 例如 doGet（），doPost（），doPut（）…
而 DispatcherServlet 繼承了 HttpServlet 所以只需要重寫HttpServlet暴露出來的方法就好
其實HttpServlet 暴露出來的方法最終呼叫的都是 doGet() 和 doPost()

doGet（）和 doPost（）

DispatcherServlet 的 doGet（）和 doPost（）是在 FrameworkServlet

@Override
	protected final void doGet(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {

		processRequest(request, response);
	}

	@Override
	protected final void doPost(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {

		processRequest(request, response);
	}

可以看到不論是 doGet() 還是 doPost() 最終呼叫的都是 processRequest(request, response)

直接追溯到核心程式碼
FrameworkServlet.processRequest()>DispatcherServlet .doService()>DispatcherServlet .doDispatch()

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 {
				//檔案上傳相關，如果傳過來的物件是 MultipartContent
				// 那麼就會轉換 request =>  MultipartHttpServletRequest
				processedRequest = checkMultipart(request);
				multipartRequestParsed = (processedRequest != request);

				// Determine handler for the current request.
				// 根據request 資訊找到對應的Handler
				mappedHandler = getHandler(processedRequest);
				if (mappedHandler == null) {
					// 如果沒有找到對應的Handler ，那麼通過response 反饋錯誤資訊
					noHandlerFound(processedRequest, response);
					return;
				}

				// Determine handler adapter for the current request.
				// 獲取介面卡
				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)) {
					// 處理lastModified
					long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
					if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
						return;
					}
				}
				// 處理攔截前的邏輯
				if (!mappedHandler.applyPreHandle(processedRequest, response)) {//也就說前攔截如果執行失敗了 就會return

					return;
				}

				// Actually invoke the handler.
				// 業務處理 ，根據具體的介面卡執行處理器
				mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

				if (asyncManager.isConcurrentHandlingStarted()) {
					return;
				}
				//注入一個預設的檢視名
				applyDefaultViewName(processedRequest, mv);
				//執行攔截中
				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);
			}

			//處理返回值  渲染檢視   裡面會執行攔截後
			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);
				}
			}
		}
	}

1. 首先處理檔案上傳，如果reques 是MultipartContext 型別的 request 裝換為 MultipartHttpServletRequest
2. 根據request 資訊找到對應的 Handler ，並且這裡的Handler 是一個鏈 ，是 處理器+攔截器鏈
3. 如果沒有找到對應的Handler ，那麼通過response 反饋錯誤資訊
4. 獲取介面卡 處理 GET 和 Head 請求的 lastModified 瀏覽器快取 (用來記錄頁面的最後修改時間。當客戶端訪問頁面時，伺服器會將
   頁面最後修改時間通過 Last-Modified 標識由伺服器發往客戶端，客戶端記錄修改時間，再次請求本地存在的cache頁面時)
5. 處理攔截前的邏輯
6. 業務處理 ，根據具體的介面卡執行處理器
7. 注入一個預設的檢視名
8. 執行攔截中
9. 處理返回值 ,渲染檢視 ,執行攔截後