用ajax实现检测注册用户名是否重复的完整例子
ValidateName.java代码如下所示，并且采用userIsExist查找数据库看是否存在相同的用户名。

package com.wuliu.test;
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 com.wuliu.dao.LoginDAO;

public class ValidateName extends HttpServlet {
	public ValidateName(){
		super();
	}

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

		response.setContentType("text/html");
		LoginDAO dao = new LoginDAO();
		boolean flag = false;
		String loginName=request.getParameter("loginName").toString();
		flag = dao.userIsExist(loginName);
		if(true == flag)
		{
			response.getWriter().write("true");//此值jquery可以接收到  
		}
	}

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

		this.doGet(request, response); 
		
	}

}

public boolean userIsExist(String loginId){
			 	System.out.println("Enter userIsExist");
				this.dao = new DBConnection();
				this.cn = this.dao.getConnection();
		        // 根据指定用户名查询用户信息
				String sql = "select * from LoginTable where LoginId='"+loginId+"'";
				System.out.println("logid:"+loginId);
				try {
		            // 获取PreparedStatement对象
		        	this.ps = this.cn.prepareStatement(sql);
		            // 对用户对象属性赋值
		           // ps.setString(1, loginId);
		            // 执行查询获取结果集
		            rs = this.ps.executeQuery();
		            // 判断结果集是否有效
		           // System.out.println("rs.next()= "+rs.next());
		            if(false == rs.next()){
		                // 如果无效则证明此用户名可用
		            	System.out.println("用户名可用");
		                return true;
		            }
		            // 释放此 ResultSet 对象的数据库和 JDBC 资源
		            rs.close();
		            // 释放此 PreparedStatement 对象的数据库和 JDBC 资源
		            ps.close();
		        } catch (SQLException e) {
		            e.printStackTrace();
		        }finally{
		            // 关闭数据库连接
		        	this.dao.closeConnection(cn);
		        }
		        System.out.println("用户名不可用");
		        return false;
		    }

	<form action="register.do?action=add" onsubmit="return submessage(this)" method="post" name="form1">
			<table border="1" width="500" cellspacing="1" cellpadding="3" align="left" bordercolor="#326598" >
				<tr>
					<td colspan="7" bgcolor="#FEA817">
						[align=center]
							<font color="#FFFFFF"><b>用户注册</b> </font>
						[/align]
					</td>
				</tr>
				<tr>
					<td>
						用户名
					</td>
					<td>
						<input name="uname" id="username" type="text" class="form_text" size="20" onblur="validatorloginName()">      
					</td>
				</tr>
				
				<tr>
					<td>
						登陆密码
					</td>
					<td>
						<input type="password" name="upwd">
					</td>
				</tr>
				<tr>
					<td>
						确认密码
					</td>
					<td>
						<input type="password" name="upwd1">
					</td>
				</tr>
				<tr>
					<td colspan="2" align="center">
						<input type="submit" value="提交">
						<input type="reset" value="重置">
					</td>
				</tr>
			</table>
					</form>

通过ajax将注册用户名发送到ValidateName.do进行校验。
页面上添加的ajax组件：

<script src="/js/jquery/jquery-1.3.2.js" type="text/javascript"></script>  
    <script type="text/javascript" language="javascript">  
function validatorloginName(){
		 var loginName=document.getElementById("uname").value;
		 if(loginName == "")
		 {
		 	alert("用户名不能为空!");
		 	return;
		 }
		 $.ajax({
		 		type: "POST",