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

public class ConnPool {
    private static final int defaultMaxConnections = 3;

    private Vector<Connection> freeConnections;             //JDK1.5 Generic
    private Hashtable<Thread, Connection> boundConnections; //JDK1.5 Generic
    private String driverName;
    private String jdbcURL;
    private String username;
    private String password;
    private int maxConnections;

    public ConnPool() {
        this(defaultMaxConnections);
    }

    public ConnPool(int numConnections) {
        maxConnections = numConnections;
        boundConnections = null;
        freeConnections = null;
        driverName = "";
        jdbcURL = "";
        username = "";
        password = "";
    }

    public void setDriverName(String drvName) {
        driverName = drvName;
    }

    public void setJdbcURL(String url) {
        jdbcURL = url;
    }

    public void setUserName(String uname) {
        username = uname;
    }

    public void setPassword(String passwd) {
        password = passwd;
    }

    public void setMaxConnections(int numConnections) {
        maxConnections = numConnections;
    }

    public void setConnectionSwitch(String on_off) throws SQLException {
        if (on_off.equalsIgnoreCase("ON"))
            openDB(driverName, jdbcURL, username, password);
        else if (on_off.equalsIgnoreCase("OFF"))
            closeDB();
    }

    public void openDB(String drvName, String url, String uname, String passwd)
            throws SQLException {
        try {
            boundConnections = new Hashtable<Thread, Connection>();
            freeConnections = new Vector<Connection>();
            Class.forName(drvName);
            for (int i = 0; i < maxConnections; i++)
                freeConnections.add(DriverManager.getConnection(url, uname, passwd));
        } catch (Exception ex) {
            boundConnections = null;
            freeConnections = null;
            throw new SQLException(ex.toString());
        }
    }

    public void closeDB() throws SQLException {
        if (boundConnections != null) {
            for (Connection conn : boundConnections.values()) { //JDK1.5 for-each
                conn.close();
            }
            boundConnections.clear();
            boundConnections = null;
        }

        if (freeConnections != null) {
            for (Connection conn : freeConnections) { //JDK1.5 for-each
                conn.close();
            }
            freeConnections.removeAllElements();
            freeConnections = null;
        }
    }

    public synchronized Connection getConnection() throws SQLException {
        if (freeConnections == null)
            throw new SQLException(
                    "The conection pool has not been established yet.");
        if (boundConnections.get(Thread.currentThread()) != null)
            throw new SQLException(
                    "Cannot get connections over once for this current running thread.");
        try {
            if (freeConnections.size() == 0) {
                System.out
                        .println(Thread.currentThread().getName()
                                + " is waiting..............................................");
                wait();
            }
        } catch (InterruptedException ex) {
            throw new SQLException(ex.toString());
        }
        Connection conn = freeConnections.firstElement();
        freeConnections.removeElement(conn);
        boundConnections.put(Thread.currentThread(), conn);

        return conn;
    }

    public synchronized void returnConnection() throws SQLException {
        Connection conn = boundConnections.remove(Thread.currentThread());
        if (conn == null)
            throw new SQLException(
                    "The connection which this current running thread got is not found.");
        freeConnections.add(conn);
        notify();
    }

}