import java.sql.*;
import java.util.*;

public class ConnectionPool {
//  private static ConnectionPool instance = new ConnectionPool();
//  public static ConnectionPool getInstance(){
//    return instance;
//  }

  private Hashtable connections;
  private int increment;
  private String dbURL, user, password;

  public ConnectionPool(String dbURL, String user, String password,
                        String driverClassName, int initialConnections, int increment)
                        throws SQLException, ClassNotFoundException {
    Class.forName(driverClassName);
    this.dbURL = dbURL;
    this.user = user;
    this.password = password;
    this.increment = increment;

    connections = new Hashtable();
    for (int i = 0; i < initialConnections; i++){
      connections.put(DriverManager.getConnection(dbURL, user, password),Boolean.FALSE);
    }
  }

  public Connection getConnection() throws SQLException{
    System.out.println("Connection retrieved from the pool.");
    Connection con = null;
    Enumeration cons = connections.keys();

    synchronized(connections){
      while(cons.hasMoreElements()){
        con = (Connection)cons.nextElement();
        Boolean b = (Boolean)connections.get(con);
        if(b == Boolean.FALSE){
          try{
            con.setAutoCommit(true);
          }
          catch(SQLException e){
            con = DriverManager.getConnection(dbURL, user, password);
          }
          connections.put(con, Boolean.TRUE);
          return con;
        }
      }
    }

    for(int i = 0; i < increment; i++){
      connections.put(DriverManager.getConnection(dbURL, user, password), Boolean.FALSE);
    }
    return getConnection();
  }

  public void returnConnection(Connection returned){
    System.out.println("Connection returned to the pool.");  
    Connection con;
    Enumeration cons = connections.keys();

    while(cons.hasMoreElements()){
      con = (Connection)cons.nextElement();
      if(con == returned){
        connections.put(con, Boolean.FALSE);
        break;
      }
    }
  }
}