1/*
2 * Copyright (c) 2003, 2013, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.  Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
17 * You should have received a copy of the GNU General Public License version
18 * 2 along with this work; if not, write to the Free Software Foundation,
19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22 * or visit www.oracle.com if you need additional information or have any
23 * questions.
24 */
25
26package jdk.internal.ref;
27
28import java.lang.ref.*;
29import java.security.AccessController;
30import java.security.PrivilegedAction;
31
32
33/**
34 * General-purpose phantom-reference-based cleaners.
35 *
36 * <p> Cleaners are a lightweight and more robust alternative to finalization.
37 * They are lightweight because they are not created by the VM and thus do not
38 * require a JNI upcall to be created, and because their cleanup code is
39 * invoked directly by the reference-handler thread rather than by the
40 * finalizer thread.  They are more robust because they use phantom references,
41 * the weakest type of reference object, thereby avoiding the nasty ordering
42 * problems inherent to finalization.
43 *
44 * <p> A cleaner tracks a referent object and encapsulates a thunk of arbitrary
45 * cleanup code.  Some time after the GC detects that a cleaner's referent has
46 * become phantom-reachable, the reference-handler thread will run the cleaner.
47 * Cleaners may also be invoked directly; they are thread safe and ensure that
48 * they run their thunks at most once.
49 *
50 * <p> Cleaners are not a replacement for finalization.  They should be used
51 * only when the cleanup code is extremely simple and straightforward.
52 * Nontrivial cleaners are inadvisable since they risk blocking the
53 * reference-handler thread and delaying further cleanup and finalization.
54 *
55 *
56 * @author Mark Reinhold
57 */
58
59public class Cleaner
60    extends PhantomReference<Object>
61{
62
63    // Dummy reference queue, needed because the PhantomReference constructor
64    // insists that we pass a queue.  Nothing will ever be placed on this queue
65    // since the reference handler invokes cleaners explicitly.
66    //
67    private static final ReferenceQueue<Object> dummyQueue = new ReferenceQueue<>();
68
69    // Doubly-linked list of live cleaners, which prevents the cleaners
70    // themselves from being GC'd before their referents
71    //
72    private static Cleaner first = null;
73
74    private Cleaner
75        next = null,
76        prev = null;
77
78    private static synchronized Cleaner add(Cleaner cl) {
79        if (first != null) {
80            cl.next = first;
81            first.prev = cl;
82        }
83        first = cl;
84        return cl;
85    }
86
87    private static synchronized boolean remove(Cleaner cl) {
88
89        // If already removed, do nothing
90        if (cl.next == cl)
91            return false;
92
93        // Update list
94        if (first == cl) {
95            if (cl.next != null)
96                first = cl.next;
97            else
98                first = cl.prev;
99        }
100        if (cl.next != null)
101            cl.next.prev = cl.prev;
102        if (cl.prev != null)
103            cl.prev.next = cl.next;
104
105        // Indicate removal by pointing the cleaner to itself
106        cl.next = cl;
107        cl.prev = cl;
108        return true;
109
110    }
111
112    private final Runnable thunk;
113
114    private Cleaner(Object referent, Runnable thunk) {
115        super(referent, dummyQueue);
116        this.thunk = thunk;
117    }
118
119    /**
120     * Creates a new cleaner.
121     *
122     * @param  ob the referent object to be cleaned
123     * @param  thunk
124     *         The cleanup code to be run when the cleaner is invoked.  The
125     *         cleanup code is run directly from the reference-handler thread,
126     *         so it should be as simple and straightforward as possible.
127     *
128     * @return  The new cleaner
129     */
130    public static Cleaner create(Object ob, Runnable thunk) {
131        if (thunk == null)
132            return null;
133        return add(new Cleaner(ob, thunk));
134    }
135
136    /**
137     * Runs this cleaner, if it has not been run before.
138     */
139    public void clean() {
140        if (!remove(this))
141            return;
142        try {
143            thunk.run();
144        } catch (final Throwable x) {
145            AccessController.doPrivileged(new PrivilegedAction<>() {
146                    public Void run() {
147                        if (System.err != null)
148                            new Error("Cleaner terminated abnormally", x)
149                                .printStackTrace();
150                        System.exit(1);
151                        return null;
152                    }});
153        }
154    }
155}
156