通过分析Service 源码我们可以发现，HttpServlet中写有Service和doPost/doGet等方法。即我们使用的Servlet继承于Httpservlet，就要重写doPost/doGet。

//请求会先访问这个service
public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException {
    if(req instanceof HttpServletRequest && res instanceof HttpServletResponse) {
        //将req 和res强制转换为带Http的request 和response
        HttpServletRequest request = (HttpServletRequest)req;
        HttpServletResponse response = (HttpServletResponse)res;
        //然后调用带Http req和res的service方法
        this.service((HttpServletRequest)request, (HttpServletResponse)response);
    } else {
        throw new ServletException("non-HTTP request or response");
    }
}


protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    //获得请求方法的类型        
    String method = req.getMethod();
    long errMsg;
    //如果是GET请求
    if(method.equals("GET")) {
        errMsg = this.getLastModified(req);
        if(errMsg == -1L) {
        //调用doGet方法
        this.doGet(req, resp);
        } else {
            long ifModifiedSince = req.getDateHeader("If-Modified-Since");
            if(ifModifiedSince < errMsg) {
                this.maybeSetLastModified(resp, errMsg);
                this.doGet(req, resp);
            } else {
                resp.setStatus(304);
            }
    }
    //如果是HEAD请求
    } else if(method.equals("HEAD")) {
        errMsg = this.getLastModified(req);
        this.maybeSetLastModified(resp, errMsg);
        this.doHead(req, resp);
    //如果是POST请求
    } else if(method.equals("POST")) {
        this.doPost(req, resp);
    //如果是PUT请求
    } else if(method.equals("PUT")) {
        this.doPut(req, resp);
    //如果是DELETE请求
    } else if(method.equals("DELETE")) {
        this.doDelete(req, resp);
    //如果是OPTIONS请求
    } else if(method.equals("OPTIONS")) {
        this.doOptions(req, resp);
    } else if(method.equals("TRACE")) {
        this.doTrace(req, resp);
    } else {
        String errMsg1 = lStrings.getString("http.method_not_implemented");
        Object[] errArgs = new Object[]{method};
        errMsg1 = MessageFormat.format(errMsg1, errArgs);
        resp.sendError(501, errMsg1);
    }

}

我们可以将自己对Http传递数据的处理写在重写后的doPost或者doGet方法当中，但是如果我们要重写Service方法，那么父类的Service方法会被覆盖。则重写后的doPost和doGet方法将不会生效，故我们需要加上

super.Service(request,response);

这样当前servlet下的doPost和doGet方法才会生效。