Java async MySQL queries

Question

First of all, I don't have much experience with thread safe programming.

I have a MySQL class, and I want to use one instance in multiple threads to prevent blocking code in the main thread. I read about connection pooling but I want to keep it as simple as it is.

This is my MySQL class:

package com.vanillage.bukkitutils.mysql;

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

public class MySQL {
    private final String host;
    private final int port;
    private final String database;
    private final String user;
    private final String password;
    private Connection connection;

    public MySQL(String host, int port, String database, String user, String password) {
        if (host == null) {
            //TODO
        }

        if (database == null) {
            //TODO
        }

        if (user == null) {
            //TODO
        }

        if (password == null) {
            //TODO
        }

        this.host = host;
        this.port = port;
        this.database = database;
        this.user = user;
        this.password = password;
    }

    public String getHost() {
        return host;
    }

    public int getPort() {
        return port;
    }

    public String getDatabase() {
        return database;
    }

    public String getUser() {
        return user;
    }

    public String getPassword() {
        return password;
    }

    public Connection getConnection() {
        return connection;
    }

    public synchronized void connect() throws SQLException {
        connection = DriverManager.getConnection("jdbc:mysql://" + host + ":" + port + "/" + database + "?autoReconnect=true", user, password);
    }

    public synchronized void checkConnection(int timeout) throws SQLException {
        if (connection == null) {
            connect();
        } else {
            boolean connectionValid = false;

            try {
                connectionValid = connection.isValid(timeout);
            } catch (SQLException e) {
                e.printStackTrace();
                connect();
                connectionValid = true;
            }

            if (!connectionValid) {
                connect();
            }
        }
    }

    public synchronized ResultSet query(String query) throws SQLException {
        return connection.prepareStatement(query).executeQuery();
    }

    public synchronized boolean update(String query) throws SQLException {
        return connection.prepareStatement(query).execute();
    }

    public synchronized void close() throws SQLException {
        if (connection != null) {
            connection.close();
            connection = null;
        }
    }

    public synchronized boolean hasConnection(boolean checkOpen, boolean checkValid, int timeout) throws SQLException {
        return connection != null && (!checkOpen || !connection.isClosed()) && (!checkValid || connection.isValid(timeout));
    }

    @Override
    public String toString() {
        return host + ":" + port + ", " + database + ", " + user + ", " + password;
    }
}

Is it possible to make my MySQL class Thread safe with the synchronized keyword, as I already used it in the code above?

I use this class like that from different threads:

try {
    mySQL.checkConnection(0);

    try {
        ResultSet resultSet = mySQL.query("SELECT * FROM Example");

        if (resultSet.next()) {
            System.out.println(resultSet.getString("Example"));
        }
    } catch (SQLException e) {
        System.out.println("Error while executing query: " + e.getMessage());
    }
} catch (SQLException e) {
    System.out.println("Could not create a valid connection: " + e.getMessage());
}

My question: Is it thread safe?

Edit:

package testprogramm;

import java.beans.PropertyVetoException;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;

import com.mchange.v2.c3p0.ComboPooledDataSource;

public class TestProgramm {
    private static final Scanner scanner = new Scanner(System.in);
    private static Map dataSources = new HashMap<>();

    public static void main(String[] args) {
        //setup connections
        ComboPooledDataSource testDataSource = new ComboPooledDataSource();

        try {
            testDataSource.setDriverClass("com.mysql.jdbc.Driver");
        } catch (PropertyVetoException e) {
            e.printStackTrace();
        }

        testDataSource.setJdbcUrl("jdbc:mysql://localhost:3306/database");
        testDataSource.setUser("user");
        testDataSource.setPassword("password");
        dataSources.put("test", testDataSource);

        while (true) {
            String line = scanner.nextLine();
            ComboPooledDataSource dataSource = dataSources.get(line);

            if (dataSource != null) {
                new Thread(new Runnable() {
                    ComboPooledDataSource dataSource = null;

                    public Runnable init(ComboPooledDataSource dataSource) {
                        this.dataSource = dataSource;
                        return this;
                    }

                    @Override
                    public void run() {
                        try {
                            Connection connection = dataSource.getConnection();
                            PreparedStatement preparedStatement = connection.prepareStatement("SELECT * FROM Example");
                            ResultSet resultSet = preparedStatement.executeQuery();
                            int i = 0;

                            while (resultSet.next()) {
                                i ++;
                            }

                            resultSet.close();//TODO: move to finally clause with null check and try/catch
                            preparedStatement.close();
                            connection.close();
                            System.out.println(i + " entries");
                        } catch (SQLException e) {
                            System.out.println("Error while executing statement: " + e.getMessage());
                        }
                    }
                }.init(dataSource)).start();
            } else {
                System.out.println("No such connection");
            }
        }
    }
}

Jan · Accepted Answer

Your question: Is it thread safe?

My answer: No, it's not.

Most simple way to break it: Have one of your threads call mySQL.getConnection().close();

Apart from that: Most connections don't like parallel statements at all. What should be the transaction scope of that?

You should seriously consider the use of a connection pool instead. My favorite choisce would be c3p0. See http://www.mchange.com/projects/c3p0/#quickstart for a quickstart example.

Making it safe

Instead of passing around an instance of MySQL, you create and configure a ComboPooledDataSource (or any other DataSource you want to use). Then inside your classes, obtain a Connection from that pool, perform your SQL statements and then close it. The most convenient way to do that would be with try-with-resource introduced with Java 7:

try(Connection con = pool.getConnection();
    PreparedStatement ps = con.prepareStatement("SELECT * FROM whatever");
    ResultSet rs = ps.executeQuery()) {

  while(rs.next()) {
    //handle resultset
  }
}

Some more info on your existing class

You fail to clean up a whole lot of Statements if you do

public synchronized ResultSet query(String query) throws SQLException {
    //Statement never closed
    return connection.prepareStatement(query).executeQuery();
}

public synchronized boolean update(String query) throws SQLException {
    //Statement never closed
    return connection.prepareStatement(query).execute();
}

Java async MySQL queries

Answers (2)

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