/*
 * Copyright (c) 1998 by Gefion software.
 *
 * Permission to use, copy, and distribute this software for
 * NON-COMMERCIAL purposes and without fee is hereby granted
 * provided that this copyright notice appears in all copies.
 *
 */

import java.io.*;
import java.sql.*;
import java.util.*;
import java.util.Date;

/**
 * This class is a Singleton that provides access to one or many
 * connection pools defined in a Property file. A client gets
 * access to the single instance through the static getInstance()
 * method and can then check-out and check-in connections from a pool.
 * When the client shuts down it should call the release() method
 * to close all open connections and do other clean up.
 */
public class DBConnectionManager {
    static private DBConnectionManager instance;       // The single instance
    static private int clients;

    private Vector drivers = new Vector();
    private PrintWriter log;
    private Hashtable pools = new Hashtable();
    
    /**
     * Returns the single instance, creating one if it's the
     * first time this method is called.
     *
     * @return DBConnectionManager The single instance.
     */
    static synchronized public DBConnectionManager getInstance() {
        if (instance == null) {
            instance = new DBConnectionManager();
        }
        clients++;
        return instance;
    }
    
    /**
     * A private constructor since this is a Singleton
     */
    private DBConnectionManager() {
        init();
    }
    
    /**
     * Returns a connection to the named pool.
     *
     * @param name The pool name as defined in the properties file
     * @param con The Connection
     */
    public void freeConnection(String name, Connection con) {
        DBConnectionPool pool = (DBConnectionPool) pools.get(name);
        if (pool != null) {
            pool.freeConnection(con);
        }
    }
        
    /**
     * Returns an open connection. If no one is available, and the max
     * number of connections has not been reached, a new connection is
     * created.
     *
     * @param name The pool name as defined in the properties file
     * @return Connection The connection or null
     */
    public Connection getConnection(String name) {
        DBConnectionPool pool = (DBConnectionPool) pools.get(name);
        if (pool != null) {
            return pool.getConnection();
        }
        return null;
    }
    
    /**
     * Returns an open connection. If no one is available, and the max
     * number of connections has not been reached, a new connection is
     * created. If the max number has been reached, waits until one
     * is available or the specified time has elapsed.
     *
     * @param name The pool name as defined in the properties file
     * @param time The number of milliseconds to wait
     * @return Connection The connection or null
     */
    public Connection getConnection(String name, long time) {
        DBConnectionPool pool = (DBConnectionPool) pools.get(name);
        if (pool != null) {
            return pool.getConnection(time);
        }
        return null;
    }
    
    /**
     * Closes all open connections and deregisters all drivers.
     */
    public synchronized void release() {
        // Wait until called by the last client
        if (--clients != 0) {
            return;
        }
        
        Enumeration allPools = pools.elements();
        while (allPools.hasMoreElements()) {
            DBConnectionPool pool = (DBConnectionPool) allPools.nextElement();
            pool.release();
        }
        Enumeration allDrivers = drivers.elements();
        while (allDrivers.hasMoreElements()) {
            Driver driver = (Driver) allDrivers.nextElement();
            try {
                DriverManager.deregisterDriver(driver);
                log("Deregistered JDBC driver " + driver.getClass().getName());
            }
            catch (SQLException e) {
                log(e, "Can't deregister JDBC driver: " + driver.getClass().getName());
            }
        }
    }
    
    /**
     * Creates instances of DBConnectionPool based on the properties.
     * A DBConnectionPool can be defined with the following properties:
     * <PRE>
     * &lt;poolname&gt;.url         The JDBC URL for the database
     * &lt;poolname&gt;.user        A database user (optional)
     * &lt;poolname&gt;.password    A database user password (if user specified)
     * &lt;poolname&gt;.maxconn     The maximal number of connections (optional)
     * </PRE>
     *
     * @param props The connection pool properties
     */
    private void createPools(Properties props) {
        Enumeration propNames = props.propertyNames();
        while (propNames.hasMoreElements()) {
            String name = (String) propNames.nextElement();
            if (name.endsWith(".url")) {
                String poolName = name.substring(0, name.lastIndexOf("."));
                String url = props.getProperty(poolName + ".url");
                if (url == null) {
                    log("No URL specified for " + poolName);
                    continue;
                }
                String user = props.getProperty(poolName + ".user");
                String password = props.getProperty(poolName + ".password");
                String maxconn = props.getProperty(poolName + ".maxconn", "0");
                int max;
                try {
                    max = Integer.valueOf(maxconn).intValue();
                }
                catch (NumberFormatException e) {
                    log("Invalid maxconn value " + maxconn + " for " + poolName);
                    max = 0;
                }
                DBConnectionPool pool = 
                    new DBConnectionPool(poolName, url, user, password, max);
                pools.put(poolName, pool);
                log("Initialized pool " + poolName);
            }
        }
    }
    
    /**
     * Loads properties and initializes the instance with its values.
     */
    private void init() {
        InputStream is = getClass().getResourceAsStream("/db.properties");
        Properties dbProps = new Properties();
        try {
            dbProps.load(is);
        }
        catch (Exception e) {
            System.err.println("Can't read the properties file. " +
                "Make sure db.properties is in the CLASSPATH");
            return;
        }
        String logFile = dbProps.getProperty("logfile", "DBConnectionManager.log");
        try {
            log = new PrintWriter(new FileWriter(logFile, true), true);
        }
        catch (IOException e) {
            System.err.println("Can't open the log file: " + logFile);
            log = new PrintWriter(System.err);
        }
        loadDrivers(dbProps);
        createPools(dbProps);
    }
    
    /**
     * Loads and registers all JDBC drivers. This is done by the
     * DBConnectionManager, as opposed to the DBConnectionPool,
     * since many pools may share the same driver.
     *
     * @param props The connection pool properties
     */
    private void loadDrivers(Properties props) {
        String driverClasses = props.getProperty("drivers");
        StringTokenizer st = new StringTokenizer(driverClasses);
        while (st.hasMoreElements()) {
            String driverClassName = st.nextToken().trim();
            try {
                Driver driver = (Driver) 
                    Class.forName(driverClassName).newInstance();
                DriverManager.registerDriver(driver);
                drivers.addElement(driver);
                log("Registered JDBC driver " + driverClassName);
            }
            catch (Exception e) {
                log("Can't register JDBC driver: " +
                    driverClassName + ", Exception: " + e);
            }
        }
    }
    
    /**
     * Writes a message to the log file.
     */
    private void log(String msg) {
        log.println(new Date() + ": " + msg);
    }
    
    /**
     * Writes a message with an Exception to the log file.
     */
    private void log(Throwable e, String msg) {
        log.println(new Date() + ": " + msg);
        e.printStackTrace(log);
    }
    
    /**
     * This inner class represents a connection pool. It creates new
     * connections on demand, up to a max number if specified.
     * It also makes sure a connection is still open before it is
     * returned to a client.
     */
    class DBConnectionPool {
        private int checkedOut;
        private Vector freeConnections = new Vector();
        private int maxConn;
        private String name;
        private String password;
        private String URL;
        private String user;
        
        /**
         * Creates new connection pool.
         *
         * @param name The pool name
         * @param URL The JDBC URL for the database
         * @param user The database user, or null
         * @param password The database user password, or null
         * @param maxConn The maximal number of connections, or 0
         *   for no limit
         */
        public DBConnectionPool(String name, String URL, String user, String password, 
                int maxConn) {
            this.name = name;
            this.URL = URL;
            this.user = user;
            this.password = password;
            this.maxConn = maxConn;
        }
        
        /**
         * Checks in a connection to the pool. Notify other Threads that
         * may be waiting for a connection.
         *
         * @param con The connection to check in
         */
        public synchronized void freeConnection(Connection con) {
            // Put the connection at the end of the Vector
            freeConnections.addElement(con);
            checkedOut--;
            notifyAll();
        }
        
        /**
         * Checks out a connection from the pool. If no free connection
         * is available, a new connection is created unless the max
         * number of connections has been reached. If a free connection
         * has been closed by the database, it's removed from the pool
         * and this method is called again recursively.
         */
        public synchronized Connection getConnection() {
            Connection con = null;
            if (freeConnections.size() > 0) {
                // Pick the first Connection in the Vector
                // to get round-robin usage
                con = (Connection) freeConnections.firstElement();
                freeConnections.removeElementAt(0);
                try {
                    if (con.isClosed()) {
                        log("Removed bad connection from " + name);
                        // Try again recursively
                        con = getConnection();
                    }
                }
                catch (SQLException e) {
                    log("Removed bad connection from " + name);
                    // Try again recursively
                    con = getConnection();
                }
            }
            else if (maxConn == 0 || checkedOut < maxConn) {
                con = newConnection();
            }
            if (con != null) {
                checkedOut++;
            }
            return con;
        }
        
        /**
         * Checks out a connection from the pool. If no free connection
         * is available, a new connection is created unless the max
         * number of connections has been reached. If a free connection
         * has been closed by the database, it's removed from the pool
         * and this method is called again recursively.
         * <P>
         * If no connection is available and the max number has been 
         * reached, this method waits the specified time for one to be
         * checked in.
         *
         * @param timeout The timeout value in milliseconds
         */
        public synchronized Connection getConnection(long timeout) {
            long startTime = new Date().getTime();
            Connection con;
            while ((con = getConnection()) == null) {
                try {
                    wait(timeout);
                }
                catch (InterruptedException e) {}
                if ((new Date().getTime() - startTime) >= timeout) {
                    // Timeout has expired
                    return null;
                }
            }
            return con;
        }
        
        /**
         * Closes all available connections.
         */
        public synchronized void release() {
            Enumeration allConnections = freeConnections.elements();
            while (allConnections.hasMoreElements()) {
                Connection con = (Connection) allConnections.nextElement();
                try {
                    con.close();
                    log("Closed connection for pool " + name);
                }
                catch (SQLException e) {
                    log(e, "Can't close connection for pool " + name);
                }
            }
            freeConnections.removeAllElements();
        }
        
        /**
         * Creates a new connection, using a userid and password
         * if specified.
         */
        private Connection newConnection() {
            Connection con = null;
            try {
                if (user == null) {
                    con = DriverManager.getConnection(URL);
                }
                else {
                    con = DriverManager.getConnection(URL, user, password);
                }
                log("Created a new connection in pool " + name);
            }
            catch (SQLException e) {
                log(e, "Can't create a new connection for " + URL);
                return null;
            }
            return con;
        }
    }
}