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

public class DriverManagerConnectionPool  {

    static {
        freeDbConnections = new ArrayList<Connection>();
        try {
            Class.forName("com.mysql.cj.jdbc.Driver");
        } catch (ClassNotFoundException e) {
            e.getStackTrace();
        }
    }

    private static synchronized Connection createDBConnection() throws SQLException {

        String url = "jdbc:postgresql://localhost/test";
        Properties props = new Properties();
        props.setProperty("user","fred");
        props.setProperty("password","secret");
        props.setProperty("ssl","true");
        Connection conn = DriverManager.getConnection(url, props);



        newConnection.setAutoCommit(false);
        return newConnection;
    }

    public static synchronized Connection getConnection() throws SQLException {
        Connection connection;

        if (!freeDbConnections.isEmpty()) {
            connection = (Connection) freeDbConnections.get(0);
            freeDbConnections.remove(0);

            try {
                if (connection.isClosed())
                    connection = getConnection();
            } catch (SQLException e) {
                connection.close();
                connection = getConnection();
            }
        } else {
            connection = createDBConnection();
        }

        return connection;
    }

    public static synchronized void releaseConnection(Connection connection) throws SQLException {
        if(connection != null) freeDbConnections.add(connection);
    }


    private static List<Connection> freeDbConnections;
}