function fun() {
var xmlhttp;
if (window.XMLHttpRequest)
{
// code for IE7+, Firefox, Chrome, Opera, Safari
xmlhttp=new XMLHttpRequest();
}
else
{
// code for IE6, IE5
xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
}

    //2. 建立连接
    xmlhttp.open("POST","ajaxServlet?username=tom",false);// 同步或异步请求：true（异步）或 false（同步）
    //3.发送请求
    xmlhttp.send();
    //4.接受并处理来自服务器的响应结果
    //当xmlhttp对象的就绪状态改变时，触发事件onreadystatechange。
    xmlhttp.onreadystatechange=function()
    {
        //判断readyState就绪状态是否为4，判断status响应状态码是否为200
        if (xmlhttp.readyState==4 && xmlhttp.status==200)
        {
            //获取服务器的响应结果
            var responseText = xmlhttp.responseText;
            alert(responseText);//为什么原生js同步请求(false)拿不到响应的数据
        }
    }
}

---------------------***********Servlet************-------------------------------------------
@WebServlet("/ajaxServlet")
public class AjaxServlet extends HttpServlet {
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
//获取参数
String username = request.getParameter("username");
//模拟处理业务逻辑的耗时
Thread thread = new Thread();
try {
thread.sleep(3000);//让线程睡3秒
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(username);
//响应
response.getWriter().write("hello:"+username);
}

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    this.doPost(request, response);
}

}