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

class ConnectionPoolManager{

    String databaseUrl = "jdbc:mysql://localhost:3306/teste2?useUnicode=true&useJDBCCompliantTimezoneShift=true&useLegacyDatetimeCode=false&serverTimezone=UTC";
    String userName = "root";
    String password = "dropmusic";

    Vector connectionPool = new Vector();

    public ConnectionPoolManager(){
        initialize();
    }

    public ConnectionPoolManager(String databaseUrl, String userName, String password){
        this.databaseUrl = databaseUrl;
        this.userName = userName;
        this.password = password;
        initialize();
    }

    private void initialize() {
        //Here we can initialize all the information that we need
        initializeConnectionPool();
    }

    private void initializeConnectionPool() {
        while(!checkIfConnectionPoolIsFull()) {
            System.out.println("Connection Pool is NOT full. Proceeding with adding new connections");
            //Adding new connection instance until the pool is full
            connectionPool.addElement(createNewConnectionForPool());
        }
        System.out.println("Connection Pool is full.");
    }

    private synchronized boolean checkIfConnectionPoolIsFull() {
        final int MAX_POOL_SIZE = 5;

        //Check if the pool size
        if(connectionPool.size() < MAX_POOL_SIZE) {
            return false;
        }
        return true;
    }

    //Creating a connection
    private Connection createNewConnectionForPool() {
        Connection connection = null;

        try {
            Class.forName("com.mysql.cj.jdbc.Driver");
            connection = DriverManager.getConnection(databaseUrl, userName, password);
            connection.setTransactionIsolation(Connection.TRANSACTION_READ_COMMITTED);
            connection.setAutoCommit(false);

            System.out.println("Connection: "+connection);
        }
        catch(SQLException sqle) {
            System.err.println("SQLException: "+sqle);
            return null;
        }
        catch(ClassNotFoundException cnfe) {
            System.err.println("ClassNotFoundException: "+cnfe);
            return null;
        }

        return connection;
    }

    public synchronized Connection getConnectionFromPool() {
        Connection connection = null;

        //Check if there is a connection available. There are times when all the connections in the pool may be used up
        if(connectionPool.size() == 0){
            createNewConnectionForPool();
        }
        connection = (Connection) connectionPool.firstElement();
        connectionPool.removeElementAt(0);

        //Giving away the connection from the connection pool
        return connection;
    }

    public synchronized void returnConnectionToPool(Connection connection) {
        //Adding the connection from the client back to the connection pool
        connectionPool.addElement(connection);
    }
}