//Servlet的方式

package web;

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;

import service.UserService;

public class SuggestServlet extends HttpServlet {
	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp)
			throws ServletException, IOException {
		PrintWriter out=resp.getWriter();
		String keyword=req.getParameter("keyword");
		//利用service来进行测试
		if(! "".equals(keyword.trim())){
			UserService userService=new UserService();
			String result=userService.queryUsernameByKeyword(keyword);
			if(result!=null){
				out.println(result);
			}else{
				out.println("");
			}
		}
		out.close();
	}
	@Override
	protected void doPost(HttpServletRequest req, HttpServletResponse resp)
			throws ServletException, IOException {
	this.doGet(req, resp);
	}
}

html页面内容：

<h3>利用Ajax实现搜索自动补齐提示!</h3>
    <div style="width:500px">
    	<input type="text" id="txtSearch" name="txtSearch"/>
    	<input type="submit" id="cmdFind" name="cmdFind" value="搜索"/>
    	<div id="suggest"></div>
    </div>

原生JS方式

var xhr;
		function createXhr(){
			try{
				return new XMLHttpRequest();
			}catch(e){
				return new ActiveXObject("Microsoft.XMLHTTP");
			}
		}
		//用户名文本框失去焦点的处理函数;
		function findSuggest(){
			xhr=createXhr();
			var keyword=document.getElementById("txtSearch").value;
			alert(keyword);
			xhr.onreadystatechange=callback;
			xhr.open("post","suggestAction",true);
			xhr.setRequestHeader("Content-type","application/x-www-form-urlencoded");
			xhr.send("keyword="+keyword);
		}
		function callback(){
			if(xhr.readyState==4){
				if(xhr.status==200){
					var sobj=document.getElementById("suggest");
					sobj.innerHTML="";
					var str=xhr.responseText.split(",");
					var suggest="";
					if(str.length>0 && str[0].length>0){
						for(i=0;i<str.length;i++){
							suggest+="<div οnclick='setSearch(this.innerHTML);'";
							suggest+=">"+str[i]+"</div>";
						}
						sobj.innerHTML=suggest;
						document.getElementById("suggest").style.display="block";
					}else{
						document.getElementById("suggest").style.display="none";
					}
				}
			}
		}
		function setSearch(obj){
			document.getElementById("txtSearch").value=obj;
			document.getElementById("suggest").style.display="none";
		}

2.使用jquery的方式

<script type="text/javascript" src="js/jquery-1.7.1.min.js"></script>
	<script type="text/javascript">
	//1.找对象；2.定事件；匿名事件；
		$(function(){
			$("#txtSearch").keyup(function(){
			  //发送post请求，设置请求url，请求数据，成功返回函数;
			  $.post("suggestAction",{keyword:$("#txtSearch").val()},callback);
			});
		});	
		function callback(data){
			var sobj=$("#suggest");
			sobj.innerHTML="";
			var suggest="";
			var str=data.split(",");alert(data);
			
			if(str.length>0 && str[0].length>0){
				for(i=0;i<str.length;i++){
					suggest+="<div οnclick='setSearch(this.innerHTML);'";
					suggest+=">"+str[i]+"</div>";
				}
				sobj.html(suggest).css("display","block");
			}else{
				sobj.css("display","none");			
			}
		}
		//将内容填充到搜索框中;
		function setSearch(obj){
			$("#txtSearch").val(obj);
			$("#suggest").css("display","none");
		}
	</script>