阅读下面代码片段:Servlet1:request.setAttribute("username", "张三");String username=(String) request.getAttribute("username");System.out.println("username:"+username);request.setAttribute("username", "四");username=(String) request.getAttribute("username");System.out.println("username:"+username);request.removeAttribute("username");username=(String) request.getAttribute("username");System.out.println("username:"+username);当使用浏览器访问Servlet1时,写出控制台的输出结果:第一行输出:【 】第二行输出:【 】第三行输出:【 】