ajax + jsp+ selvert 实现数据的重复性

```java 导入：jqure 文件

编写js 文件

 <script type="text/javascript">
        window.onload  = function(){

            var nameV=	document.getElementsByName("userId")[0];
            nameV.onblur = function(){
                var name  = this.value;
                var re = new XMLHttpRequest();
                re.open("GET", "check?userId="+name);

                re.onreadystatechange = function(){
                    if(re.readyState==4)
                    {
                        if(re.status==200)
                        {

                            var tsy = 	document.getElementById("error");

                            if(re.responseText=="true")
                            {

                                tsy.innerHTML= "学号已经存在,请修改！";
                                tsy.style.color="red";

                            }
                            else if (re.responseText == "0"){
                                tsy.innerHTML= "学号为空,请输入！";
                                tsy.style.color="red";
                            }
                            else {
                                tsy.innerHTML= "√";
                                tsy.style.color="blue";
                            }

                        }
                    }
      }

                re.send(null);

            }
        }
    </script>

```## selvert 处理文件

package com.anbow.selvet;

import com.anbow.JDBC.JDBC;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.PrintWriter;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;


@WebServlet("/check")
public class CheckIDServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String id = request.getParameter("userId");
        Connection connection = JDBC.getConnection();
        System.out.println(id);
        String sql = "select * from userList where id = ?";


        try {
            PreparedStatement ps = connection.prepareStatement(sql);
            ps.setString(1,id);
            ResultSet rs = ps.executeQuery();
            response.setContentType("application/json;charset=utf-8");
            response.setHeader("Content-Type", "application/json;charset=UTF-8");//注意加上这一句
            PrintWriter writer = response.getWriter();
            StringBuffer stringBuffer = new StringBuffer();

            String userList = null;
            if (rs.next()){

                stringBuffer.append(rs.getString("id"));

                String names= stringBuffer.toString();

                String[] names2 = names.split(",");
                for(int i = 0;i<names2.length;i++)
                {

                    if(names2[i].equals(id))
                    {
                        writer.print(true);
                        break;
                    }
                    else if (names2[i].equals("") || names2[i] == null){
                        writer.println(0);
                    }

                }


            }else {
                writer.println("可以注册");


            }

        } catch (SQLException e) {
            e.printStackTrace();
        }

    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        this.doPost(request,response);
    }
}