ajax对注册名进行验证检测是否存在于数据库中

使用ajax对注册名进行验证判断它是否在数据库中存在，具体的实现如下，感性的朋友可以参考下，希望对大家有所帮助

这个是我在网上看到的一个有错的ajax 我改正确了，却找不到那个帖子了。
用servlet验证：

 
package Servlet; 
import java.io.*; 
import java.sql.*; 
import java.io.IOException; 
import java.io.PrintWriter; 
import javax.servlet.*; 
import javax.servlet.http.HttpServletRequest; 
import javax.servlet.http.HttpServletResponse; 
import javax.servlet.http.HttpServlet; 
public class CheckServlet extends HttpServlet { 
static HttpServletRequest request = null; 
static HttpServletResponse response = null; 
public static void main(String[] args) throws Exception, IOException{ 
CheckServlet c = new CheckServlet(); 
c.doPost(request, response); 
} 

public static final String DBDRIVER = "com.mysql.jdbc.Driver"; 
public static final String DBURL = "jdbc:mysql://localhost:3306/spring"; 
public static final String DBUSER = "root"; 
public static final String DBPASS = "liangke"; 
public void doGet(HttpServletRequest request, HttpServletResponse response) 
throws ServletException, IOException { 
this.doPost(request, response); 
} 
public void doPost(HttpServletRequest request, HttpServletResponse response) 
throws ServletException, IOException { 
request.setCharacterEncoding("gbk"); 
response.setContentType("text/html"); 
Connection conn = null; 
PreparedStatement pstmt = null; 
ResultSet rs = null; 
PrintWriter out = response.getWriter(); 
String userid = request.getParameter("userid");// 接受验证的用户名 
try { 
Class.forName(DBDRIVER); 
conn = DriverManager.getConnection(DBURL, DBUSER, DBPASS); 
String sql = " select * from user where username=?"; 
pstmt = conn.prepareStatement(sql); 
pstmt.setString(1, userid); 
rs = pstmt.executeQuery(); 
StringBuffer jsonStr=new StringBuffer(); 
jsonStr.append("[{flag:'"); 
if(rs.next()){ 
jsonStr.append("true',alertTest:'aaa'}]"); 
out.println(jsonStr.toString()); 
}else{ 
jsonStr.append("false',alertTest:'bbb'}]"); 
out.println(jsonStr.toString()); 

} 

out.close(); 
} catch (Exception e) { 
e.printStackTrace(); 
} finally { 
try { 
conn.close(); 
} catch (SQLException e) { 
e.printStackTrace(); 
} 
} 
} 
} 