/*

 * Copyright 2007 Sun Microsystems, Inc.  All Rights Reserved.

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Sun designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Sun in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,

 * CA 95054 USA or visit www.sun.com if you need additional information or

 * have any questions.

 */

package com.sun.jmx.event;

import com.sun.jmx.remote.util.ClassLogger;

import java.util.concurrent.Executors;

import java.util.concurrent.FutureTask;

import java.util.concurrent.ScheduledExecutorService;

import java.util.concurrent.ScheduledFuture;

import java.util.concurrent.TimeUnit;

/**

 * <p>Manage a renewable lease.  The lease can be renewed indefinitely

 * but if the lease runs to its current expiry date without being renewed

 * then the expiry callback is invoked.  If the lease has already expired

 * when renewal is attempted then the lease method returns zero.</p>

 * @author sjiang

 * @author emcmanus

 */

// The synchronization logic of this class is tricky to deal correctly with the

// case where the lease expires at the same time as the |lease| or |stop| method

// is called.  If the lease is active then the field |scheduled| represents

// the expiry task; otherwise |scheduled| is null.  Renewing or stopping the

// lease involves canceling this task and setting |scheduled| either to a new

// task (to renew) or to null (to stop).

//

// Suppose the expiry task runs at the same time as the |lease| method is called.

// If the task enters its synchronized block before the method starts, then

// it will set |scheduled| to null and the method will return 0.  If the method

// starts before the task enters its synchronized block, then the method will

// cancel the task which will see that when it later enters the block.

// Similar reasoning applies to the |stop| method.  It is not expected that

// different threads will call |lease| or |stop| simultaneously, although the

// logic should be correct then too.

public class LeaseManager {

    public LeaseManager(Runnable callback) {

        this(callback, EventParams.getLeaseTimeout());

    }

    public LeaseManager(Runnable callback, long timeout) {

        if (logger.traceOn()) {

            logger.trace("LeaseManager", "new manager with lease: "+timeout);

        }

        if (callback == null) {

            throw new NullPointerException("Null callback.");

        }

        if (timeout <= 0)

            throw new IllegalArgumentException("Timeout must be positive: " + timeout);

        this.callback = callback;

        schedule(timeout);

    }

    /**

     * <p>Renew the lease for the given time.  The new time can be shorter

     * than the previous one, in which case the lease will expire earlier

     * than it would have.</p>

     *

     * <p>Calling this method after the lease has expired will return zero

     * immediately and have no other effect.</p>

     *

     * @param timeout the new lifetime.  If zero, the lease

     * will expire immediately.

     */

    public synchronized long lease(long timeout) {

        if (logger.traceOn()) {

            logger.trace("lease", "new lease to: "+timeout);

        }

        if (timeout < 0)

            throw new IllegalArgumentException("Negative lease: " + timeout);

        if (scheduled == null)

            return 0L;

        scheduled.cancel(false);

        if (logger.traceOn())

            logger.trace("lease", "start lease: "+timeout);

        schedule(timeout);

        return timeout;

    }

    private class Expire implements Runnable {

        ScheduledFuture<?> task;

        public void run() {

            synchronized (LeaseManager.this) {

                if (task.isCancelled())

                    return;

                scheduled = null;

            }

            callback.run();

        }

    }

    private synchronized void schedule(long timeout) {

        Expire expire = new Expire();

        scheduled = executor.schedule(expire, timeout, TimeUnit.MILLISECONDS);

        expire.task = scheduled;

    }

    /**

     * <p>Cancel the lease without calling the expiry callback.</p>

     */

    public synchronized void stop() {

        logger.trace("stop", "canceling lease");

        scheduled.cancel(false);

        scheduled = null;

    }

    private final Runnable callback;

    private ScheduledFuture scheduled;  // If null, the lease has expired.

    private final ScheduledExecutorService executor

            = Executors.newScheduledThreadPool(1,

            new DaemonThreadFactory("LeaseManager"));

    private static final ClassLogger logger =

            new ClassLogger("javax.management.event", "LeaseManager");

}