/* * Copyright (c) 2003, 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.lang.ref.*;import java.security.AccessController;import java.security.PrivilegedAction;  /** * General-purpose phantom-reference-based cleaners. * * <p> Cleaners are a lightweight and more robust alternative to finalization. * They are lightweight because they are not created by the VM and thus do not * require a JNI upcall to be created, and because their cleanup code is * invoked directly by the reference-handler thread rather than by the * finalizer thread.  They are more robust because they use phantom references, * the weakest type of reference object, thereby avoiding the nasty ordering * problems inherent to finalization. * * <p> A cleaner tracks a referent object and encapsulates a thunk of arbitrary * cleanup code.  Some time after the GC detects that a cleaner's referent has * become phantom-reachable, the reference-handler thread will run the cleaner. * Cleaners may also be invoked directly; they are thread safe and ensure that * they run their thunks at most once. * * <p> Cleaners are not a replacement for finalization.  They should be used * only when the cleanup code is extremely simple and straightforward. * Nontrivial cleaners are inadvisable since they risk blocking the * reference-handler thread and delaying further cleanup and finalization. * * * @author Mark Reinhold */ public class Cleaner    extends PhantomReference{     // Dummy reference queue, needed because the PhantomReference constructor    // insists that we pass a queue.  Nothing will ever be placed on this queue    // since the reference handler invokes cleaners explicitly.    //    private static final ReferenceQueue dummyQueue = new ReferenceQueue();     // Doubly-linked list of live cleaners, which prevents the cleaners    // themselves from being GC'd before their referents    //    static private Cleaner first = null;     private Cleaner        next = null,        prev = null;     private static synchronized Cleaner add(Cleaner cl) {        if (first != null) {            cl.next = first;            first.prev = cl;        }        first = cl;        return cl;    }     private static synchronized boolean remove(Cleaner cl) {         // If already removed, do nothing        if (cl.next == cl)            return false;         // Update list        if (first == cl) {            if (cl.next != null)                first = cl.next;            else                first = cl.prev;        }        if (cl.next != null)            cl.next.prev = cl.prev;        if (cl.prev != null)            cl.prev.next = cl.next;         // Indicate removal by pointing the cleaner to itself        cl.next = cl;        cl.prev = cl;        return true;     }     private final Runnable thunk;     private Cleaner(Object referent, Runnable thunk) {        super(referent, dummyQueue);        this.thunk = thunk;    }     /**     * Creates a new cleaner.     *     * @param  thunk     *         The cleanup code to be run when the cleaner is invoked.  The     *         cleanup code is run directly from the reference-handler thread,     *         so it should be as simple and straightforward as possible.     *     * @return  The new cleaner     */    public static Cleaner create(Object ob, Runnable thunk) {        if (thunk == null)            return null;        return add(new Cleaner(ob, thunk));    }     /**     * Runs this cleaner, if it has not been run before.     */    public void clean() {        if (!remove(this))            return;        try {            thunk.run();        } catch (final Throwable x) {            AccessController.doPrivileged(new PrivilegedAction<Void>() {                    public Void run() {                        if (System.err != null)                            new Error("Cleaner terminated abnormally", x)                                .printStackTrace();                        System.exit(1);                        return null;                    }});        }    } }