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	/*
	* 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;
	}

	}

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