HttpServlet中的service和doGet的区别

我们首先看下service的代码, 注释直接和代码写在一块, 方便理解.

protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String method = req.getMethod(); //判断用户的提交方式
        long errMsg;
        if(method.equals("GET")) { //根据方法的不同,调用对应的处理方法. 比如这里是get, 那么就调用doGet方法.
            errMsg = this.getLastModified(req);
            if(errMsg == -1L) {
                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);
                }
            }
        } else if(method.equals("HEAD")) {
            errMsg = this.getLastModified(req);
            this.maybeSetLastModified(resp, errMsg);
            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 errMsg1 = lStrings.getString("http.method_not_implemented");
            Object[] errArgs = new Object[]{method};
            errMsg1 = MessageFormat.format(errMsg1, errArgs);
            resp.sendError(501, errMsg1);
        }

    }

默认doGet实现如下:

    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String protocol = req.getProtocol(); //获取协议版本,是http1.1还是1.0
        String msg = lStrings.getString("http.method_get_not_supported");
        if(protocol.endsWith("1.1")) {
            resp.sendError(405, msg);
        } else {
            resp.sendError(400, msg);
        }

    }

可见默认的doGet实现只是把错误发送出去. 所以, 为了使我们的web正常展示, 应该重写我们的doGet方法, 在doGet中完成相应逻辑. 这样当用户访问时, 调用默认service函数, service函数根据用户的get调用方法, 调用我们重写的doGet实现.