单选题 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", null); username=(String) request.getAttribute("username"); System.out.println("username:"+username); 当使用浏览器访问Servlet1时控制台会输出什么结果( )