MySQL Async Queue

Main.java

package de.fr3d63.main;

import de.fr3d63.mysql.Queue;
import org.bukkit.plugin.java.JavaPlugin;

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

public class Main extends JavaPlugin {


    private Queue queue;
    private Thread thread;

    @Override
    public void onEnable() {
        queue = new Queue();
        thread = new Thread(queue);

        thread.start();

        String a = "SELECT * from user";

        queue.put(a);

        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        ResultSet resultSet = queue.pop(a);

        try {
            while (resultSet.next()) {
                System.out.println(resultSet.getString("username"));
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }

    }

    @Override
    public void onDisable() {
        queue.stop();
        thread.interrupt();
    }
}

MysqlManager.java

package de.fr3d63.mysql;

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

public class MysqlManager {
    private static MysqlManager instance;
    private Connection connection;
    private final String host;
    private final String db;
    private final String user;
    private final String pw;

    private MysqlManager() {
        host = "IP";
        db = "DB";
        user = "USER";
        pw = "PW";

        try {
            openConnection();
        } catch (SQLException | ClassNotFoundException e) {
            e.printStackTrace();
        }
    }

    public static MysqlManager getInstance() {
        if (instance == null) {
            instance = new MysqlManager();
        }

        return instance;
    }

    private void openConnection() throws SQLException, ClassNotFoundException {
        if (connection != null && !connection.isClosed()) {
            return;
        }

        synchronized (this) {
            if (connection != null && !connection.isClosed()) {
                return;
            }
            Class.forName("com.mysql.jdbc.Driver");
            connection = DriverManager.getConnection("jdbc:mysql://" + this.host + ":" + 3306 + "/" + this.db, this.user, this.pw);
        }
    }

    public Connection getConnection() {
        return connection;
    }
}

Queue.java

package de.fr3d63.mysql;

import java.sql.Statement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.HashMap;
import java.util.Map;

public class Queue implements Runnable {

    private boolean runs;
    private Map<String, Boolean> statements;
    private Statement ps;
    private Map<String, ResultSet> resultSetMap;

    public Queue() {
        this.runs = true;
        statements = new HashMap<>();
        resultSetMap = new HashMap<>();
    }

    @Override
    public void run() {
        while (runs) {
            if (statements.size() > 0) {
                for (Map.Entry<String, Boolean> entry : statements.entrySet()) {
                    try {
                        if (!entry.getValue()) {
                            ps = MysqlManager.getInstance().getConnection().createStatement();
                            ps.execute(entry.getKey());
                            resultSetMap.put(entry.getKey(), ps.getResultSet());
                            statements.remove(entry.getKey());
                        }
                        Thread.sleep(400);
                    } catch (SQLException | InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        }
    }

    public void put(String statement) {
        statements.put(statement, false);
    }

    public ResultSet pop(String statement) {
        if (statements.containsKey(statement)) {
            return null;
        }

        ResultSet rs = resultSetMap.get(statement);
        resultSetMap.remove(statement);

        return rs;
    }

    public void stop() {
        this.runs = false;
        try {
            MysqlManager.getInstance().getConnection().close();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}