Java JDBC MySQL异常：“结果集关闭后不允许操作”

Question

我已经为此工作了将近一天半，我似乎无法解决这个错误。我不知道为什么结果集被关闭。也许你们中的一些人可以帮我。

MySQL数据库：

package net.gielinor.network.sql;

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

public abstract class MySQLDatabase {

    private String host;
    private String database;
    private String username;
    private String password;
    private Connection connection = null;
    private Statement statement;

    public MySQLDatabase(String host, String database, String username, String password) {
        this.host = host;
        this.database = database;
        this.username = username;
        this.password = password;
    }

    public abstract void cycle() throws SQLException;

    public abstract void ping();

    public void connect() {
        try {
            Class.forName("com.mysql.jdbc.Driver").newInstance();
            connection = DriverManager.getConnection(String.format("jdbc:mysql://%s/%s", host, database), username, password);
            statement = connection.createStatement();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public void ping(String table, String variable) {
        try {
            statement.executeQuery(String.format("SELECT * FROM `%s` WHERE `%s` = 'null'", table, variable));
        } catch (Exception e) {
            connect();
        }
    }

    public ResultSet query(String query) throws SQLException {
        if (query.toLowerCase().startsWith("select")) {
            return statement.executeQuery(query);
        } else {
            statement.executeUpdate(query);
        }
        return null;
    }

    public Connection getConnection() {
        return connection;
    }

}

MySQLHandler

package net.gielinor.network.sql;

import java.io.FileInputStream;
import java.util.ArrayList;
import java.util.List;
import java.util.Properties;

import net.gielinor.network.sql.impl.MySQLDonation;

public class MySQLHandler extends Thread {

    private static final MySQLHandler mysqlHandler = new MySQLHandler();

    public static MySQLHandler getMySQLHandler() {
        return mysqlHandler;
    }

    private static List<MySQLDatabase> updateList;
    private static String host;
    private static String database;
    private static String username;
    private static String password;

    @Override
    public void run() {
        while (true) {
            for (MySQLDatabase database : updateList) {
                try {
                    if (database.getConnection() == null) {
                        database.connect();
                    } else {
                        database.ping();
                    }
                    database.cycle();
                } catch (Exception ex) {
                    ex.printStackTrace();
                }

                try {
                    Thread.sleep(10000);
                } catch (Exception ex) {
                }
            }
        }
    }

    private static void loadProperties() {
        Properties p = new Properties();
        try {
            p.load(new FileInputStream("./sql.ini"));
            host = p.getProperty("host");
            database = p.getProperty("database");
            username = p.getProperty("username");
            password = p.getProperty("password");   
        } catch (Exception ex) {
            System.out.println("Error loading MySQL properties.");
        }
    }

    public static String getHost() {
        return host;
    }

    static {
        loadProperties();
        updateList = new ArrayList<MySQLDatabase>();
        updateList.add(new MySQLDonation(host, database, username, password));
    }

}

MySQLDonation

package net.gielinor.network.sql.impl;

import java.sql.ResultSet;
import java.sql.SQLException;

import net.gielinor.game.model.player.Client;
import net.gielinor.game.model.player.PlayerHandler;
import net.gielinor.game.model.player.PlayerSave;
import net.gielinor.network.sql.MySQLDatabase;

public final class MySQLDonation extends MySQLDatabase {

    public MySQLDonation(String host, String database, String username, String password) {
        super(host, database, username, password);
    }

    @Override
    public void cycle() throws SQLException {
        ResultSet results = query("SELECT * FROM `gieli436_purchases`.`donations`");
        if (results == null) {
            return;
        }
        while (results.next()) {
            String username = results.getString("username").replace("_", " ");
            System.out.println("name=" + username);
            Client client = (Client) PlayerHandler.getPlayer(username.toLowerCase());
            System.out.println(client == null);
            if (client != null && !client.disconnected) {
                int creditamount = results.getInt("creditamount");
                if (creditamount <= 0) {
                    continue;
                }
                handleDonation(client, creditamount);
                query(String.format("DELETE FROM `gieli436_purchases`.`donations` WHERE `donations`.`username`='%s' LIMIT 1", client.playerName.replaceAll(" ", "_")));
            }
        }
    }

    @Override
    public void ping() {
        super.ping("donations", "username");
    }

    private void handleDonation(Client client, int creditamount) throws SQLException {
        client.credits = (client.credits + creditamount);
        client.sendMessage("Thank you for your purchase. You have received " + creditamount + " store credits.");
        PlayerSave.save(client);
    }

}

异常发生在这里：在MySQLDonation和实际堆栈跟踪的同时循环中：

java.sql.SQLException: Operation not allowed after ResultSet closed
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1055)
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:956)
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:926)
    at com.mysql.jdbc.ResultSetImpl.checkClosed(ResultSetImpl.java:794)
    at com.mysql.jdbc.ResultSetImpl.next(ResultSetImpl.java:7077)
    at net.gielinor.network.sql.impl.MySQLDonation.cycle(Unknown Source)
    at net.gielinor.network.sql.MySQLHandler.run(Unknown Source)

有了这些信息，让我说这确实有效，我得到了我的信息和游戏中没有的东西，但它会重复，就像用户永远不会从查询中删除，所以它会给他们无限的奖励。如果您需要更多信息，请随时询问。

Answer 1

这个错误是当我们对不同的类型使用相同的语句对象时发生的。类型

检查语句对象；

Answer 2

运行Delete查询时，使用的语句与Select查询中使用的语句相同。在同一语句上重新执行时，上一个结果集将关闭。

为了避免这种情况，每次执行查询时都应该创建一个新的语句。所以删除语句=连接。createStatement（） 从MySQLDatabase类中的connect（）方法，并将该类中的所有语句替换为连接。createStatement（）。您也可以选择删除私有变量语句。

你可以在这里了解更多。