/*

 * Copyright (c) 1998, 2008, Oracle and/or its affiliates. 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.  Oracle designates this

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

 * by Oracle 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 Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

 * or visit www.oracle.com if you need additional information or have any

 * questions.

 */

package sun.misc;

import java.security.AccessController;

import java.security.PrivilegedAction;

import java.util.SortedSet;

import java.util.TreeSet;

/**

 * Support for garbage-collection latency requests.

 *

 * @author   Mark Reinhold

 * @since    1.2

 */

public class GC {

    private GC() { }            /* To prevent instantiation */

    /* Latency-target value indicating that there's no active target

     */

    private static final long NO_TARGET = Long.MAX_VALUE;

    /* The current latency target, or NO_TARGET if there is no target

     */

    private static long latencyTarget = NO_TARGET;

    /* The daemon thread that implements the latency-target mechanism,

     * or null if there is presently no daemon thread

     */

    private static Thread daemon = null;

    /* The lock object for the latencyTarget and daemon fields.  The daemon

     * thread, if it exists, waits on this lock for notification that the

     * latency target has changed.

     */

    private static class LatencyLock extends Object { };

    private static Object lock = new LatencyLock();

    /**

     * Returns the maximum <em>object-inspection age</em>, which is the number

     * of real-time milliseconds that have elapsed since the

     * least-recently-inspected heap object was last inspected by the garbage

     * collector.

     *

     * <p> For simple stop-the-world collectors this value is just the time

     * since the most recent collection.  For generational collectors it is the

     * time since the oldest generation was most recently collected.  Other

     * collectors are free to return a pessimistic estimate of the elapsed

     * time, or simply the time since the last full collection was performed.

     *

     * <p> Note that in the presence of reference objects, a given object that

     * is no longer strongly reachable may have to be inspected multiple times

     * before it can be reclaimed.

     */

    public static native long maxObjectInspectionAge();

    private static class Daemon extends Thread {

        public void run() {

            for (;;) {

                long l;

                synchronized (lock) {

                    l = latencyTarget;

                    if (l == NO_TARGET) {

                        /* No latency target, so exit */

                        GC.daemon = null;

                        return;

                    }

                    long d = maxObjectInspectionAge();

                    if (d >= l) {

                        /* Do a full collection.  There is a remote possibility

                         * that a full collection will occurr between the time

                         * we sample the inspection age and the time the GC

                         * actually starts, but this is sufficiently unlikely

                         * that it doesn't seem worth the more expensive JVM

                         * interface that would be required.

                         */

                        System.gc();

                        d = 0;

                    }

                    /* Wait for the latency period to expire,

                     * or for notification that the period has changed

                     */

                    try {

                        lock.wait(l - d);

                    } catch (InterruptedException x) {

                        continue;

                    }

                }

            }

        }

        private Daemon(ThreadGroup tg) {

            super(tg, "GC Daemon");

        }

        /* Create a new daemon thread in the root thread group */

        public static void create() {

            PrivilegedAction<Void> pa = new PrivilegedAction<Void>() {

                public Void run() {

                    ThreadGroup tg = Thread.currentThread().getThreadGroup();

                    for (ThreadGroup tgn = tg;

                         tgn != null;

                         tg = tgn, tgn = tg.getParent());

                    Daemon d = new Daemon(tg);

                    d.setDaemon(true);

                    d.setPriority(Thread.MIN_PRIORITY + 1);

                    d.start();

                    GC.daemon = d;

                    return null;

                }};

            AccessController.doPrivileged(pa);

        }

    }

    /* Sets the latency target to the given value.

     * Must be invoked while holding the lock.

     */

    private static void setLatencyTarget(long ms) {

        latencyTarget = ms;

        if (daemon == null) {

            /* Create a new daemon thread */

            Daemon.create();

        } else {

            /* Notify the existing daemon thread

             * that the lateency target has changed

             */

            lock.notify();

        }

    }

    /**

     * Represents an active garbage-collection latency request.  Instances of

     * this class are created by the <code>{@link #requestLatency}</code>

     * method.  Given a request, the only interesting operation is that of

     * cancellation.

     */

    public static class LatencyRequest

        implements Comparable<LatencyRequest> {

        /* Instance counter, used to generate unique identifers */

        private static long counter = 0;

        /* Sorted set of active latency requests */

        private static SortedSet<LatencyRequest> requests = null;

        /* Examine the request set and reset the latency target if necessary.

         * Must be invoked while holding the lock.

         */

        private static void adjustLatencyIfNeeded() {

            if ((requests == null) || requests.isEmpty()) {

                if (latencyTarget != NO_TARGET) {

                    setLatencyTarget(NO_TARGET);

                }

            } else {

                LatencyRequest r = requests.first();

                if (r.latency != latencyTarget) {

                    setLatencyTarget(r.latency);

                }

            }

        }

        /* The requested latency, or NO_TARGET

         * if this request has been cancelled

         */

        private long latency;

        /* Unique identifier for this request */

        private long id;

        private LatencyRequest(long ms) {

            if (ms <= 0) {

                throw new IllegalArgumentException("Non-positive latency: "

                                                   + ms);

            }

            this.latency = ms;

            synchronized (lock) {

                this.id = ++counter;

                if (requests == null) {

                    requests = new TreeSet<LatencyRequest>();

                }

                requests.add(this);

                adjustLatencyIfNeeded();

            }

        }

        /**

         * Cancels this latency request.

         *

         * @throws  IllegalStateException

         *          If this request has already been cancelled

         */

        public void cancel() {

            synchronized (lock) {

                if (this.latency == NO_TARGET) {

                    throw new IllegalStateException("Request already"

                                                    + " cancelled");

                }

                if (!requests.remove(this)) {

                    throw new InternalError("Latency request "

                                            + this + " not found");

                }

                if (requests.isEmpty()) requests = null;

                this.latency = NO_TARGET;

                adjustLatencyIfNeeded();

            }

        }

        public int compareTo(LatencyRequest r) {

            long d = this.latency - r.latency;

            if (d == 0) d = this.id - r.id;

            return (d < 0) ? -1 : ((d > 0) ? +1 : 0);

        }

        public String toString() {

            return (LatencyRequest.class.getName()

                    + "[" + latency + "," + id + "]");

        }

    }

    /**

     * Makes a new request for a garbage-collection latency of the given

     * number of real-time milliseconds.  A low-priority daemon thread makes a

     * best effort to ensure that the maximum object-inspection age never

     * exceeds the smallest of the currently active requests.

     *

     * @param   latency

     *          The requested latency

     *

     * @throws  IllegalArgumentException

     *          If the given <code>latency</code> is non-positive

     */

    public static LatencyRequest requestLatency(long latency) {

        return new LatencyRequest(latency);

    }

    /**

     * Returns the current smallest garbage-collection latency request, or zero

     * if there are no active requests.

     */

    public static long currentLatencyTarget() {

        long t = latencyTarget;

        return (t == NO_TARGET) ? 0 : t;

    }

}