ajax传值,servlet得到为空

js代码:
var xhr = getXhr();
function getXhr(){
var xhr = null;
try {
xhr = new XMLHttpRequest();
} catch (trymicrosoft) {
try {
xhr = new ActiveXObject("Msxml2.XMLHTTP");
} catch (othermicrosoft) {
try {
xhr = new ActiveXObject("Microsoft.XMLHTTP");
} catch (failed) {
xhr = false;
}
}
}
if (!xhr) {
alert("err Happend!");
}
return xhr;
}
function check() {
var user = document.getElementById("user").value;
var url = "RegisterServlet";
if (xhr.readyState == 4 && xhr.status == 200) {
var str = xhr.responseText;
document.getElementById("checkname").innerHTML=str;
}
xhr.open('post',url,true);
alert(user);
xhr.send("id"+user);
}
servlet代码:
package com.Servlet;

import java.io.IOException;
import java.io.PrintWriter;
import java.util.Date;
import java.text.ParseException;
import java.text.SimpleDateFormat;

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

import com.Dao.LoginDao;
import com.Model.Login;

public class RegisterServlet extends HttpServlet {
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {

    response.setContentType("text/html");
    request.setCharacterEncoding("UTF-8");
    response.setCharacterEncoding("UTF-8");
    PrintWriter out = response.getWriter();
    String username=request.getParameter("id");
    System.out.println(username);
    LoginDao dao=new LoginDao();
    boolean mark=dao.RegisterCheck(username);
    System.out.println(mark);
    if(mark==true){
        out.println("用户名重复");
        System.out.println("用户名重复");
    }else if(mark==false){
        out.println("用户名可用");
        System.out.println("用户名可用");
    }
}

}
只截取了ajax传值部分，还有登陆的验证也写在doPost里先删了，
之前用get方法能行，改成post就出问题了；
大神帮看看，js里能得到user的值，servlet得到ajax传来的user值为null，

http://blog.csdn.net/rushkid02/article/details/7506787

根本你就没传参数，user没传

我看懂了。
我想省略写
还是应该写个变量
infor="id"+user;
xhr.setRequestHeader("Content-Type","application/x-www-form-urlencoded"); 前面再加个这个
xhr.send(infor);