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
26 package jdk.internal.ref;
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<Object>
61 implements Runnable
62 {
63
64 // Dummy reference queue, needed because the PhantomReference constructor
65 // insists that we pass a queue. Nothing will ever be placed on this queue
66 // since the reference handler invokes cleaners explicitly.
67 //
68 private static final ReferenceQueue<Object> dummyQueue = new ReferenceQueue<>();
69
70 // Doubly-linked list of live cleaners, which prevents the cleaners
71 // themselves from being GC'd before their referents
72 //
73 private static Cleaner first = null;
74
75 private Cleaner
76 next = null,
77 prev = null;
78
79 private static synchronized Cleaner add(Cleaner cl) {
80 if (first != null) {
81 cl.next = first;
82 first.prev = cl;
83 }
84 first = cl;
85 return cl;
86 }
87
88 private static synchronized boolean remove(Cleaner cl) {
89
90 // If already removed, do nothing
91 if (cl.next == cl)
92 return false;
93
94 // Update list
95 if (first == cl) {
96 if (cl.next != null)
97 first = cl.next;
98 else
99 first = cl.prev;
100 }
101 if (cl.next != null)
102 cl.next.prev = cl.prev;
103 if (cl.prev != null)
104 cl.prev.next = cl.next;
105
106 // Indicate removal by pointing the cleaner to itself
107 cl.next = cl;
108 cl.prev = cl;
109 return true;
110
111 }
112
113 private final Runnable thunk;
114
115 private Cleaner(Object referent, Runnable thunk) {
116 super(referent, dummyQueue);
117 this.thunk = thunk;
118 }
119
120 /**
121 * Creates a new cleaner.
122 *
123 * @param ob the referent object to be cleaned
124 * @param thunk
125 * The cleanup code to be run when the cleaner is invoked. The
126 * cleanup code is run directly from the reference-handler thread,
127 * so it should be as simple and straightforward as possible.
128 *
129 * @return The new cleaner
130 */
131 public static Cleaner create(Object ob, Runnable thunk) {
132 if (thunk == null)
133 return null;
134 return add(new Cleaner(ob, thunk));
135 }
136
137 /**
138 * Runs this cleaner, if it has not been run before.
139 */
140 public void clean() {
141 if (!remove(this))
142 return;
143 try {
144 thunk.run();
145 } catch (final Throwable x) {
146 AccessController.doPrivileged(new PrivilegedAction<>() {
147 public Void run() {
148 if (System.err != null)
149 new Error("Cleaner terminated abnormally", x)
150 .printStackTrace();
151 System.exit(1);
152 return null;
153 }});
154 }
155 }
156
157 @Override public void run() {
158 SecurityManager security = System.getSecurityManager();
159 if (security != null)
160 security.checkPackageAccess("jdk.internal.ref");
161 this.clean();
162 }
163
164 }