简单的例子.txt

Java技术大合集,所有常用的技术在里面都有简单例子

        
		String logFile = "." + File.separator + "logs" + File.separator + "dbpool";        
        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);
        }
*/        
        log = new PrintWriter(System.err);
        loadDrivers(driver);
        createPools(poolName, url, user, passwd, maxconn);
    }
    
    /**
     * 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(String driverClassName) {
/*    	
        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);
            }
        }
*/        
        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;
        }
    }
}