function getXmlHttpRequest(){
		 var xhr=null;
		 if(window.XMLHttpRequest){
		 	//非ie
		 	xhr=new XMLHttpRequest();
		 }else{
		     //从这里也可以看出微软的嚣张跋扈
		 	xhr=new ActiveXObject("Microsoft.XMLHTTP");
		 }
		 return xhr;
}

function checkUsername() {

	//创建checkname.do request
	var xhr=getXmlHttpRequest();
				
	var username=document.getElementById("username").value;
	var url="checkusername?username="+username;
	xhr.open("get",true);//true,可以在响应回来之前进行其它操作;默认为true
				 
	//注册响应函数
	xhr.onreadystatechange=function(){
	//请求处理完毕后执行以下代码
	//xhr有5种状态,(4)请求处理完毕
	 if(xhr.readyState==4){
			//响应处理,获取服务器返回的Text
			var text=xhr.responseText;
			document.getElementById("name_msg").innerHTML=text;
		 }
	 }
				 
	//发送请求			
	 xhr.send(null);
}

<form method="post" action="checkusername">
	username:
	<input name="username" id="username" onblur="checkusername()" />
	<span id="name_msg" style="color:red"></span>
	<br />
	<input type="submit" value="OK" />
</form>

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 {

	public void service(HttpServletRequest request,HttpServletResponse response)
			throws ServletException,IOException {
		response.setContentType("text/plain;charset=utf-8");//text/plain 很重要
		request.setCharacterEncoding("utf-8");
		PrintWriter out=response.getWriter();
		String username = request.getParameter("username");
		System.out.println(username);
		
		if ("tom".equals(username)) {//为方便起见,直接给了一个
			out.print("此用户名已被占用");
		} else {
			out.print("可以使用");
		}
		out.flush();
		out.close();
	}
}