移动焦点后，servlet没有接到GET请求

JSP代码段：
<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8"%>
<%
String path = request.getContextPath();
String basePath=request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">



Insert title here var xhr; function createXHR(){ if(window.XMLHttpRequest){ xhr =new XMLHttpRequest(); }else{ xhr = new ActiveXObject("Microsoft.XMLHTTP"); } } function checkUn(){ document.getElementById("unInfo").innerHTML=""; var un = document.getElementById("un").value; if(un==""){ document.getElementById("unInfo").innerHTML="用户名不能为空"; return false; } createXHR(); var url="<%=basePath %>CheckUserNameServlet?un="+un; xhr.open("GET",url); xhr.onreadystatechange=function(){ if(xhr.readyState == 4 && xhr.status == 200){ var txt = xhr.responseText; document.getElementById("unInfo").innerHTML = txt; } }; xhr.send(); }


用户名:请输入用户名
密码:



servlet代码段：
package ed.oracle.servlet;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class CheckUserNameServlet extends HttpServlet {
private static final long serialVersionUID = 1L;

public CheckUserNameServlet() {
    super();

}


protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

    System.out.println("GET");
    response.setContentType("text/html:charset=UTF-8");
    PrintWriter out= response.getWriter();
    String un=request.getParameter("un");
    if(un != null && un.equals("admin")){
        out.println("用户名已存在");
    }else{
        out.println("用户名可用");
    }

}


protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

}

}

首先确定 var url="<%=basePath %>CheckUserNameServlet?un="+un; 这个url能指定到CheckUserNameServlet，
还有你的web.xml中有没有配置这个Servlet

思路：
1，移动焦点代码在哪？
2，移动焦点能触发异步请求？