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| 1 | /* | ||
| 2 | * Copyright (c) 2003, 2008, 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 | |||
| 26 | package sun.misc; | ||
| 27 | |||
| 28 | import java.lang.ref.*; | ||
| 29 | import java.security.AccessController; | ||
| 30 | import 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 | |||
| 59 | public class Cleaner | ||
| 60 | extends PhantomReference | ||
| 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 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 | static private 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 thunk | ||
| 123 | * The cleanup code to be run when the cleaner is invoked. The | ||
| 124 | * cleanup code is run directly from the reference-handler thread, | ||
| 125 | * so it should be as simple and straightforward as possible. | ||
| 126 | * | ||
| 127 | * @return The new cleaner | ||
| 128 | */ | ||
| 129 | public static Cleaner create(Object ob, Runnable thunk) { | ||
| 130 | if (thunk == null) | ||
| 131 | return null; | ||
| 132 | return add(new Cleaner(ob, thunk)); | ||
| 133 | } | ||
| 134 | |||
| 135 | /** | ||
| 136 | * Runs this cleaner, if it has not been run before. | ||
| 137 | */ | ||
| 138 | public void clean() { | ||
| 139 | if (!remove(this)) | ||
| 140 | return; | ||
| 141 | try { | ||
| 142 | thunk.run(); | ||
| 143 | } catch (final Throwable x) { | ||
| 144 | AccessController.doPrivileged(new PrivilegedAction<Void>() { | ||
| 145 | public Void run() { | ||
| 146 | if (System.err != null) | ||
| 147 | new Error("Cleaner terminated abnormally", x) | ||
| 148 | .printStackTrace(); | ||
| 149 | System.exit(1); | ||
| 150 | return null; | ||
| 151 | }}); | ||
| 152 | } | ||
| 153 | } | ||
| 154 | |||
| 155 | } | ||