MySQL Netbeans Java 结果集失败

Posted 2023-02-24

【中文标题】MySQL Netbeans Java 结果集失败

【英文标题】：MySQL Netbeans Java ResultSet Failure

【发布时间】：2015-08-05 04:35:44

【问题描述】：

您好，我在 Netbeans 中安装了 mysql，并且一直在尝试将 Java 与 MySQL 一起使用，但是当我运行它时遇到了问题。我的数据库称为“测试”，我的表是“任务”。我拥有的两列是：“id”和“task”（我意识到将变量命名为与表相同可能不是一个好主意）。我在代码区域还有一个附带问题，询问它的作用。这是错误： 运行：

May 22, 2015 11:52:25 PM databasetest.DatabaseTest main
SEVERE: Operation not allowed after ResultSet closed
java.sql.SQLException: Operation not allowed after ResultSet closed
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1074)
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:988)
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:974)
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:919)
at com.mysql.jdbc.ResultSetImpl.checkClosed(ResultSetImpl.java:804)
at com.mysql.jdbc.ResultSetImpl.next(ResultSetImpl.java:6986)
at databasetest.DatabaseTest.main(DatabaseTest.java:43)

BUILD SUCCESSFUL (total time: 41 seconds)

这是我的代码：

package databasetest;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.logging.Level;
import java.util.logging.Logger;

public class DatabaseTest 

public static void main(String[] args) 

    Connection con = null;
    Statement st = null;
    ResultSet rs = null;
    PreparedStatement pst = null;

    String url = "jdbc:mysql://localhost:3306/test";
    String user = "root";
    String password = "cinder";

    try 

        String author = "Trygve Gulbranssen";

        con = DriverManager.getConnection(url, user, password);
        st = con.createStatement();
        rs = st.executeQuery("SELECT VERSION()");
          //^^ what is VERSION? What is this supposed to be doing?            

        for (int i=1; i<=1000; i++) 
            String query;
            query = "INSERT INTO task(task) VALUES(" + 2*i + ")";
            st.executeUpdate(query);
        

        if (rs.next()) 
            System.out.println(rs.getString(1));
        

     catch (SQLException ex) 
        Logger lgr = Logger.getLogger(DatabaseTest.class.getName());
        lgr.log(Level.SEVERE, ex.getMessage(), ex);

     finally 
        try 
            if (rs != null) 
                rs.close();
            
            if (st != null) 
                st.close();
            
            if (con != null) 
                con.close();
            

         catch (SQLException ex) 
            Logger lgr = Logger.getLogger(DatabaseTest.class.getName());
            lgr.log(Level.WARNING, ex.getMessage(), ex);
        
    

【参考方案1】：

SELECT VERSION() 旨在告诉您您的 MySQL 版本。首先，打印SELECT 的结果，然后运行其他查询。使用Statement 运行中间插入查询会隐式关闭ResultSet，因此会出现错误。移动

if (rs.next()) 
    System.out.println(rs.getString(1));

在你运行之前

for (int i=1; i<=1000; i++) 
    // String query;
    String query = "INSERT INTO task(task) VALUES(" + 2*i + ")";
    st.executeUpdate(query);

【讨论】：

参考：MySQL VERSION() function https://dev.mysql.com/doc/refman/5.5/en/information-functions.html#function_version