當寫一般的servlet的時候，我們編寫一個servlet  去繼承 GenericServlet  ，那麼我們就得覆蓋它的一個抽象方法： abstract service(ServletRequest,ServletResponse)這個抽象方法。

  存在這個一個繼承關係：
                        interface  Servlet
                                      |---- abstract GenericServlet
                                                          |---abstract HttpServlet


我們在說一說Servlet的生命週期:init-----》service----》destory 
                          1.預設情況下，Servlet是在第一次被訪問的時候呼叫它的init()方法進行初始化的，執行Service方法                後，呼叫destory()方法消亡，生命週期完結。
                          2.可以進行配置，使得Servlet在容器（tomcat）載入的時候，進行初始化，在web.xml中進行下面的配置：

<servlet>
		<servlet-name>Servlet名字</servlet-name>
		<servlet-class>servlet完整類名</servlet-class>
		<load-on-startup>1（如果初始化的servlet多的話，這裡配置先後次序)</load-on-startup>
</servlet>

                        當寫一般的servlet的時候，我們編寫一個servlet  去繼承 GenericServlet  ，那麼我們就得覆蓋它的一個抽象方法： abstract service(ServletRequest,ServletResponse)這個抽象方法,執行業務，但是為什麼在寫有關Http協議的Servlet的時候，我們沒有重寫service方法？
                    下面是HttpServlet的原碼：

  /**
     * Receives standard HTTP requests from the public
     * <code>service</code> method and dispatches
     * them to the <code>do</code><i>XXX</i> methods defined in 
     * this class. This method is an HTTP-specific version of the 
     * {@link javax.servlet.Servlet#service} method. There's no
     * need to override this method.
     *
     * @param req   the {@link HttpServletRequest} object that
     *                  contains the request the client made of
     *                  the servlet
     *
     * @param resp  the {@link HttpServletResponse} object that
     *                  contains the response the servlet returns
     *                  to the client                                
     *
     * @exception IOException   if an input or output error occurs
     *                              while the servlet is handling the
     *                              HTTP request
     *
     * @exception ServletException  if the HTTP request
     *                                  cannot be handled
     * 
     * @see javax.servlet.Servlet#service
     */
    protected void service(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException {


        String method = req.getMethod();


        if (method.equals(METHOD_GET)) {
            long lastModified = getLastModified(req);
            if (lastModified == -1) {
                // servlet doesn't support if-modified-since, no reason
                // to go through further expensive logic
                doGet(req, resp);
            } else {
                long ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
                if (ifModifiedSince < (lastModified / 1000 * 1000)) {
                    // If the servlet mod time is later, call doGet()
                    // Round down to the nearest second for a proper compare
                    // A ifModifiedSince of -1 will always be less
                    maybeSetLastModified(resp, lastModified);
                    doGet(req, resp);
                } else {
                    resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
                }
            }


        } else if (method.equals(METHOD_HEAD)) {
            long lastModified = getLastModified(req);
            maybeSetLastModified(resp, lastModified);
            doHead(req, resp);


        } else if (method.equals(METHOD_POST)) {
            doPost(req, resp);
            
        } else if (method.equals(METHOD_PUT)) {
            doPut(req, resp);        
            
        } else if (method.equals(METHOD_DELETE)) {
            doDelete(req, resp);
            
        } else if (method.equals(METHOD_OPTIONS)) {
            doOptions(req,resp);
            
        } else if (method.equals(METHOD_TRACE)) {
            doTrace(req,resp);
            
        } else {
            //
            // Note that this means NO servlet supports whatever
            // method was requested, anywhere on this server.
            //


            String errMsg = lStrings.getString("http.method_not_implemented");
            Object[] errArgs = new Object[1];
            errArgs[0] = method;
            errMsg = MessageFormat.format(errMsg, errArgs);
            
            resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);
        }
    }
 

通過對原碼的閱讀我們發現，其實HttpServlet已經實現了GenericServlet類的service方法，並且對提交方式（get,post......）進行了一些列的判斷,執行相應的do....()方法.我們看一下HttpServlet中doget  dopost方法的實現：
  

protected void doGet(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException
    {
        String protocol = req.getProtocol();
        String msg = lStrings.getString("http.method_get_not_supported");
        if (protocol.endsWith("1.1")) {
            resp.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, msg);
        } else {
            resp.sendError(HttpServletResponse.SC_BAD_REQUEST, msg);
        }
    }


  protected void doPost(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException {


        String protocol = req.getProtocol();
        String msg = lStrings.getString("http.method_post_not_supported");
        if (protocol.endsWith("1.1")) {
            resp.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, msg);
        } else {
            resp.sendError(HttpServletResponse.SC_BAD_REQUEST, msg);
        }
    }
 

通過閱讀dopost和doget的原碼，我們可以看到，doget和dopost都有預設的實現，但是這個實現是“通知實現類去重寫  該方法的提示資訊”----也就是說，我們必須在HttpServlet的實現類裡實現doget和dopost方法，並且，不能去重寫service(httpServletRequest,HttpServletResponse)這個方法（實現後，就不能執行do....()方法了），所以不建議並且不能實現service方法。