How to make Connection Pool from ThreadLocal?

Question

Java

I'm using ThreadLocal to manage transactions in pure JDBC. Like this

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

public class ConnectionFactory {
    public static ThreadLocal<Connection> connection = new ThreadLocal<>();

    public static Connection createConnection() throws SQLException {
        Connection connection = DriverManager.getConnection("jdbc:h2:mem:test");
        connection.setAutoCommit(false);
        ConnectionFactory.connection.set(connection);
        return connection;
    }

    public static void quietlyClose() {
        try {
            connection.get().close();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    public static void quietlyRollback() {
        try {
            connection.get().rollback();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}

And then in the DAO in the methods we take Connection like this

Connection connection = ConnectionFactory.connection.get();

And in the service we open like this in each method

Connection connection = ConnectionFactory.createConnection();

But how to make a pool under such conditions, I don’t understand at all.

PS Hibernate, Spring is forbidden to use.