流程图

摘自：https://www.tianxiaobo.com/2018/06/29/Spring-MVC-原理探秘-一个请求的旅行过程/

继承关系图

1.GenericServlet 只是一个抽象类提供一些基础的模板具体实现由以下子类实现

2.HTTPServlet提供了对service的实现

HttppServlet

service

javax.servlet.http.HttpServlet#service(javax.servlet.ServletRequest, javax.servlet.ServletResponse)

 public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException {
        if (req instanceof HttpServletRequest && res instanceof HttpServletResponse) {
            //转为HttpServletRequest
            HttpServletRequest request = (HttpServletRequest)req;
            //转为HttpServletResponse
            HttpServletResponse response = (HttpServletResponse)res;
            //<1>交给当前类的service方法处理 这里不是调用当前类的 而是调用子类FrameworkServlet的 重写了
            this.service(request, response);
        } else {
            throw new ServletException("non-HTTP request or response");
        }
    }

<2>service

javax.servlet.http.HttpServlet#service(javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse)

/**
     * 根据请求方式路由到对应的方法处理
     * @param req
     * @param resp
     * @throws ServletException
     * @throws IOException
     */
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //获取请求协议头的请求方式 如:Request Method: GET
        String method = req.getMethod();
        long lastModified;
        if (method.equals("GET")) {
            //-1则表示缓存绝对更新了  默认子类没有实现 直接返回的-1
            lastModified = this.getLastModified(req);
            if (lastModified == -1L) {
                //<3>根据协议版本 返回400 405空实现 由子类实现
                this.doGet(req, resp);
            } else {
                /**
                 *  <2>如果上一次请求客户端Last-Modified 如：Last-Modified: Thu, 20 Feb 2014 09:02:52 GMT
                 *  一般静态文件会默认带上
                 *  浏览器会将数据缓存本地
                 *  后续浏览器都会带上 每次跟服务器数据更新时间来做比较 如果没更新返回304 空响应
                 *  浏览器直接使用过本地数据
                 */
                long ifModifiedSince = req.getDateHeader("If-Modified-Since");
                //比较缓存是否过期
                if (ifModifiedSince < lastModified) {
                    this.maybeSetLastModified(resp, lastModified);
                    //<3>根据协议版本 返回400 405空实现 由子类实现
                    this.doGet(req, resp);
                } else {
                    //返回304
                    resp.setStatus(304);
                }
            }
        } else if (method.equals("HEAD")) {
            lastModified = this.getLastModified(req);
            this.maybeSetLastModified(resp, lastModified);
            this.doHead(req, resp);
        } else if (method.equals("POST")) {
            this.doPost(req, resp);
        } else if (method.equals("PUT")) {
            this.doPut(req, resp);
        } else if (method.equals("DELETE")) {
            this.doDelete(req, resp);
        } else if (method.equals("OPTIONS")) {
            this.doOptions(req, resp);
        } else if (method.equals("TRACE")) {
            this.doTrace(req, resp);
        } else {
            String errMsg = lStrings.getString("http.method_not_implemented");
            Object[] errArgs = new Object[]{method};
            errMsg = MessageFormat.format(errMsg, errArgs);
            resp.sendError(501, errMsg);
        }

    }

3.路由的方法都在HettpServlet都是返回405或者400的空实现由子类实现

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(405, msg);
        } else {
            resp.sendError(400, msg);
        }

    }

FrameworkServlet

<1>service

org.springframework.web.servlet.FrameworkServlet#service

 protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //如果请求方式是PATCH 则直接调用
        if (HttpMethod.PATCH.matches(request.getMethod())) {
            this.processRequest(request, response);
        } else {
            /**
             * 否则调用父类的路由方法 最终也是调用processRequest
             * <2>作用是是 比如父类的304缓存策略 以及路由的不同方法 处理方式 有所差异 如 option处理方式
             */
            super.service(request, response);
        }

    }

processRequest