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

public class ConnectionPool {
  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 {
 
    // Load the specified driver class
    Class.forName(driverClassName);

    this.dbURL = dbURL;
    this.user = user;
    this.password = password;
    this.increment = increment;

    connections = new Hashtable();
 
    // Put our pool of Connections in the Hashtable
    // The FALSE value indicates they're unused
    for(int i = 0; i < initialConnections; i++) {
      connections.put(DriverManager.getConnection(dbURL, user, password),
                      Boolean.FALSE);
    }
  }

  public Connection getConnection() throws SQLException {
    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) {
          // So we found an unused connection.
          // Test its integrity with a quick setAutoCommit(true) call.
          // For production use, more testing should be performed,
          // such as executing a simple query.
          try {
            con.setAutoCommit(true);
          }
          catch(SQLException e) {
            // Problem with the connection, replace it.
            con = DriverManager.getConnection(dbURL, user, password);
          }
          // Update the Hashtable to show this one's taken
          connections.put(con, Boolean.TRUE);
          // Return the connection
          return con;
        }
      }
    }
  
    // If we get here, there were no free connections.
    // We've got to make more.
    for(int i = 0; i < increment; i++) {
      connections.put(DriverManager.getConnection(dbURL, user, password),
                      Boolean.FALSE); 
    }	
 
    // Recurse to get one of the new connections.
    return getConnection();
  }

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