/*
 * %W% %E%
 *
 * Copyright (c) 2006, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

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
 * @version %I%, %E%
 */

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() {
            public Object run() {
            if (System.err != null)
                new Error("Cleaner terminated abnormally", x)
                .printStackTrace();
            System.exit(1);
            return null;
            }});
    }
    }

}