web使用数据库时出问题

Question

问题遇到的现象和发生背景

最近在学web，在链接数据库时，使用搜索框搜索数据库的内容时报错Cannot invoke "java.sql.ResultSet.next()" because "rs" is null，不知道哪错了，

用代码块功能插入代码，请勿粘贴截图

ListYangWeb.java

package cn.edu.cidp;

import java.io.IOException;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class ListYangWeb
 */
@WebServlet("/ListYangWeb")
public class ListYangWeb extends HttpServlet {
  private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public ListYangWeb() {
        super();
        // TODO Auto-generated constructor stub
    }

  /**
   * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
   */
  protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    // TODO Auto-generated method stub
    //response.getWriter().append("Served at: ").append(request.getContextPath());
    //Connection conn = null;
    response.setContentType("text/html;utf-8");
    response.setCharacterEncoding("utf-8");
    String keyword = request.getParameter("keyword");
    //String sql;
    String sql = "select * from studentdata";
    //if(keyword != null )
    if(keyword != null && keyword.trim().length()>0)
    {
      sql += "where id like '%"+keyword+"%' and name like '%"+keyword+"%'";
    }
    
    Connection conn = null;
    conn = MySqlUtil.getConnection();
    ResultSet rs = MySqlUtil.executeSelect(conn, sql);
    
      
      try {
        response.getWriter().println("<form action = '#' method = 'post'>");
        response.getWriter().println("<input type = 'text' name = 'keyword'>");
        response.getWriter().println("<input type = 'submit' value = 'Search'/>");
        response.getWriter().println("</form>");
        response.getWriter().println("");
        while(rs.next())
        {
          
          StringID= rs.getString("id");
          StringName= rs.getString("name");
          StringPwd= rs.getString("pwd");
          response.getWriter().println("");
          response.getWriter().println("");
          response.getWriter().println("");
          response.getWriter().println("");
          response.getWriter().println("");
          response.getWriter().println("");
          
          //response.getWriter().println(id +","+Name+","+Pwd);
          
        }
        response.getWriter().println("
      
"+ID+"
"+Name+"
"+Pwd+"
">删除
");
      conn.close();
    } catch (SQLException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    }
      
    /*
      try {
        Class.forName("com.mysql.cj.jdbc.Driver");
        conn = DriverManager.getConnection(
                  MySqlUtil.URL,MySqlUtil.Login_User,MySqlUtil.Login_Pwd);
      Statement stmt = conn.createStatement();
        ResultSet rs = stmt.executeQuery(sql);
        response.getWriter().println("");
        while(rs.next())
        {
          
          String id = rs.getString("id");
          String Name = rs.getString("name");
          String Pwd = rs.getString("pwd");
          response.getWriter().println("");
          response.getWriter().println("");
          response.getWriter().println("");
          response.getWriter().println("");
          response.getWriter().println("");
          //response.getWriter().println(id +","+Name+","+Pwd);
          
        }
        response.getWriter().println("
"+id+"
"+Name+"
"+Pwd+"
");
        rs.close();
        stmt.close();
      } catch (SQLException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    } catch (ClassNotFoundException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    }
     */
}
      
  

  /**
   * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
   */
  protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    // TODO Auto-generated method stub
    doGet(request, response);
  }

}

MySqlUtil.java

package cn.edu.cidp;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

public class MySqlUtil {
    public static String URL = "jdbc:mysql://localhost:3306/demo";
    public static String Login_User = "root";
    public static String Login_Pwd = "YANG20020803";

    public static Connection getConnection()//获得数据库连接
    {
        Connection conn = null;
        try {
            Class.forName("com.mysql.jdbc.Driver");
            conn = DriverManager.getConnection(
                    MySqlUtil.URL,MySqlUtil.Login_User,MySqlUtil.Login_Pwd);
            return conn;
        } catch (ClassNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (SQLException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        
        return null;
    }
    public static ResultSet executeSelect(Connection conn,String sql) {
        //Connection conn = null;
        try {
          Class.forName("com.mysql.jdbc.Driver");
          conn = DriverManager.getConnection(
                    MySqlUtil.URL,MySqlUtil.Login_User,MySqlUtil.Login_Pwd);
          Statement stmt = conn.createStatement();
          ResultSet rs = stmt.executeQuery(sql);
          //int ret = stmt.executeQuery(sql);
          //stmt.close();  Operation not allowed after ResultSet closed
          
          return rs;
        } catch (ClassNotFoundException e) {
          // TODO Auto-generated catch block
          e.printStackTrace();
        } catch (SQLException e) {
          // TODO Auto-generated catch block
          e.printStackTrace();
        }
        //查找完不能马上关闭conn
        return null;
    }
    
    //insert delete update
    public static int executeUpdate(String sql)
    {
        Connection conn = null;
        try {
          Class.forName("com.mysql.jdbc.Driver");
          conn = DriverManager.getConnection(
                    MySqlUtil.URL,MySqlUtil.Login_User,MySqlUtil.Login_Pwd);
          Statement stmt = conn.createStatement();
          //ResultSet rs = stmt.executeQuery(sql);
          int ret = stmt.executeUpdate(sql);
          stmt.close();
          
          return ret;
        } catch (ClassNotFoundException e) {
          // TODO Auto-generated catch block
          e.printStackTrace();
        } catch (SQLException e) {
          // TODO Auto-generated catch block
          e.printStackTrace();
        }
        finally
        {
          try {
            conn.close();
          } catch (SQLException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
          }
        }
        return -1;
            
    }
    
}

运行结果及报错内容

You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'like '%1%'' at line 1

Cannot invoke "java.sql.ResultSet.next()" because "rs" is null

Answer 1

ResultSet为空,证明你的sql有问题,你把你的sql打印出来 放到数据库试试能不能运行?