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 }
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