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/*
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* Copyright 2003 Sun Microsystems, Inc. All Rights Reserved.
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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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*
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* This code is free software; you can redistribute it and/or modify it
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* under the terms of the GNU General Public License version 2 only, as
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* published by the Free Software Foundation. Sun designates this
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* particular file as subject to the "Classpath" exception as provided
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* by Sun in the LICENSE file that accompanied this code.
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*
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* This code is distributed in the hope that it will be useful, but WITHOUT
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* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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* version 2 for more details (a copy is included in the LICENSE file that
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* accompanied this code).
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*
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* You should have received a copy of the GNU General Public License version
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* 2 along with this work; if not, write to the Free Software Foundation,
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* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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*
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* Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
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* CA 95054 USA or visit www.sun.com if you need additional information or
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* have any questions.
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*/
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package sun.misc;
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import java.lang.ref.*;
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import java.security.AccessController;
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import java.security.PrivilegedAction;
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/**
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* General-purpose phantom-reference-based cleaners.
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*
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* <p> Cleaners are a lightweight and more robust alternative to finalization.
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* They are lightweight because they are not created by the VM and thus do not
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* require a JNI upcall to be created, and because their cleanup code is
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* invoked directly by the reference-handler thread rather than by the
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* finalizer thread. They are more robust because they use phantom references,
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* the weakest type of reference object, thereby avoiding the nasty ordering
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* problems inherent to finalization.
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*
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* <p> A cleaner tracks a referent object and encapsulates a thunk of arbitrary
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* cleanup code. Some time after the GC detects that a cleaner's referent has
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* become phantom-reachable, the reference-handler thread will run the cleaner.
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* Cleaners may also be invoked directly; they are thread safe and ensure that
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* they run their thunks at most once.
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*
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* <p> Cleaners are not a replacement for finalization. They should be used
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* only when the cleanup code is extremely simple and straightforward.
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* Nontrivial cleaners are inadvisable since they risk blocking the
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* reference-handler thread and delaying further cleanup and finalization.
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*
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*
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* @author Mark Reinhold
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*/
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public class Cleaner
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extends PhantomReference
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{
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// Dummy reference queue, needed because the PhantomReference constructor
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// insists that we pass a queue. Nothing will ever be placed on this queue
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// since the reference handler invokes cleaners explicitly.
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//
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private static final ReferenceQueue dummyQueue = new ReferenceQueue();
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// Doubly-linked list of live cleaners, which prevents the cleaners
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// themselves from being GC'd before their referents
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//
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static private Cleaner first = null;
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private Cleaner
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next = null,
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prev = null;
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private static synchronized Cleaner add(Cleaner cl) {
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if (first != null) {
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cl.next = first;
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first.prev = cl;
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}
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first = cl;
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return cl;
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}
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private static synchronized boolean remove(Cleaner cl) {
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// If already removed, do nothing
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if (cl.next == cl)
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return false;
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// Update list
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if (first == cl) {
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if (cl.next != null)
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first = cl.next;
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else
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first = cl.prev;
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}
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if (cl.next != null)
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cl.next.prev = cl.prev;
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if (cl.prev != null)
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cl.prev.next = cl.next;
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// Indicate removal by pointing the cleaner to itself
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cl.next = cl;
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cl.prev = cl;
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return true;
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}
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private final Runnable thunk;
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private Cleaner(Object referent, Runnable thunk) {
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super(referent, dummyQueue);
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this.thunk = thunk;
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}
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/**
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* Creates a new cleaner.
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*
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* @param thunk
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* The cleanup code to be run when the cleaner is invoked. The
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* cleanup code is run directly from the reference-handler thread,
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* so it should be as simple and straightforward as possible.
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*
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* @return The new cleaner
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*/
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public static Cleaner create(Object ob, Runnable thunk) {
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if (thunk == null)
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return null;
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return add(new Cleaner(ob, thunk));
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}
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/**
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* Runs this cleaner, if it has not been run before.
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*/
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public void clean() {
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if (!remove(this))
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return;
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try {
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thunk.run();
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} catch (final Throwable x) {
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AccessController.doPrivileged(new PrivilegedAction() {
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public Object run() {
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if (System.err != null)
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new Error("Cleaner terminated abnormally", x)
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.printStackTrace();
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System.exit(1);
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return null;
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}});
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}
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}
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}
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