package connectionPool;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.HashMap;

public class ConnectionPool {
    private static int maxConnectionCount = 3;
    private ArrayList<Connection> connectionPool;
    private HashMap<Thread, Connection> connectionInUse;
    private String url = "jdbc:oracle:thin:@localhost:1521:xe";
    private String account = "user08";
    private String password = "1111";
    static{
        try {
            Class.forName("oracle.jdbc.driver.OracleDriver");
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
    }
    public ConnectionPool(){}

    public void openDB(int aConnectionCount){
        connectionPool = new ArrayList<>();
        connectionInUse = new HashMap<>();
        addConnection(aConnectionCount);

    }

    public void closeDB(){
        if (connectionInUse != null) {
            for (Connection conn : connectionInUse.values()) { //JDK1.5 for-each
                try {
                    conn.close();
                } catch (SQLException e) {
                    e.printStackTrace();
                }
            }
            connectionInUse.clear();
            connectionInUse = null;
        }

        if (connectionPool != null) {
            for (Connection conn : connectionPool) { //JDK1.5 for-each
                try {
                    conn.close();
                    System.out.println("Connection in pool closed.");
                } catch (SQLException e) {
                    e.printStackTrace();
                }
            }
            connectionPool.clear();
            connectionPool = null;
        }
    }


    public synchronized Connection getConnection(){
        Connection con = null;
        int totalConnections = connectionPool.size() + connectionInUse.size();

        // dynamically create Connection object until it reaches its maximum limit
        if (connectionPool.size() == 0 && totalConnections < maxConnectionCount){
            addConnection(1);
            System.out.println("create new Connection.");
        }else if (connectionPool.size() == 0 && !(totalConnections < maxConnectionCount)){
            System.out.println("reach the maximum number of Connections. Can't create Connection anymore.");
        }

        // if no connection available now, tell the current Thread to wait
        if (connectionPool.size() == 0){
            System.out.println("This Thread is waiting**************************************************");
            try {
                wait(); // look for notifyAll() - to see when will this code keeps going down.
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            System.out.println("This Thread gets Connection.**************************************************");
        }

        // if connection available, give one to the current Thread
        if (connectionPool.size() > 0){
            con = connectionPool.remove(connectionPool.size()-1);
            connectionInUse.put(Thread.currentThread(), con);
        }
        return con;
    }

    public synchronized void returnConnection(Connection con){
        connectionPool.add(con);
        connectionInUse.remove(Thread.currentThread());
        //notifyAll();
        notify(); //一次叫一個Thread起來就好，不然會出事，因為若同時兩個Thread起來一起往下跑，會只有一個人拿到Connection，那另一個人就會NullPointerException
    }

    private synchronized void addConnection(int aConnectionCount){

        try {
            for(int i = 0; i < aConnectionCount; i++){
                Connection con =  DriverManager.getConnection(url, account, password);
                connectionPool.add(con);
            }
        } catch (SQLException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

    public int size(){
        return this.connectionPool.size();
    }


}