1 /*
2 * Copyright (c) 1997, 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 java.lang.ref;
27
28 import java.security.PrivilegedAction;
29 import java.security.AccessController;
30 import java.util.concurrent.ThreadLocalRandom;
31 import sun.misc.ManagedLocalsThread;
32 import sun.misc.SharedSecrets;
33 import sun.misc.VM;
34
35 /* Package-private; must be in same package as the Reference class */
36 class Finalizer<T> extends FinalReference<T> implements Runnable {
37 /**
38 * Finalizers are registered in a doubly-linked list so that they are kept
39 * alive until discovered by VM, processed by ReferenceHandling pool and then
40 * unlinked. There are several lists to distribute Finalizers randomly into
41 * to reduce contention among concurrent threads trying to link/unlink them.
42 */
43 static final FinalizerList[] unfinalized;
44 static {
45 int cpus = Runtime.getRuntime().availableProcessors();
46 // smallest power of two equal or greater than 2 * # of CPUs
47 int lists = (cpus <= 1) ? 2 : Integer.highestOneBit(cpus - 1) << 2;
48 unfinalized = new FinalizerList[lists];
49 for (int i = 0; i < lists; i++) {
50 unfinalized[i] = new FinalizerList();
51 }
52 }
53
54 volatile Finalizer prev;
55 volatile Finalizer next;
56 private final int listIndex;
57
58 Finalizer(T finalizee, int listIndex) {
59 super(finalizee, ReferenceQueue.NULL);
60 this.listIndex = listIndex;
61 }
62
63 /** A constructor used for special Finalizer instances in FinalizerList */
64 Finalizer() {
65 super(null, ReferenceQueue.NULL);
66 listIndex = -1; // never registered in any list
67 }
68
69 /** Invoked by VM for objects overriding finalize() method */
70 static void register(Object finalizee) {
71 int rnd = nextSecondarySeed();
72 int index = (rnd >>> 1) & (unfinalized.length - 1);
73 unfinalized[index].link(new Finalizer<>(finalizee, index), (rnd & 1) == 0);
74 }
75
76 @Override
77 public void run() {
78 T finalizee = delete();
79 if (finalizee == null) {
80 return;
81 }
82 unfinalized[listIndex].unlink(this);
83 try {
84 if (!(finalizee instanceof java.lang.Enum)) {
85 invokeFinalizee(finalizee);
86
87 /* Clear stack slot containing this variable, to decrease
88 the chances of false retention with a conservative GC */
89 finalizee = null;
90 }
91 } catch (Throwable x) { }
92 }
93
94 /* Invoke the finalize() method on the finalizee (overridden by Finalizator) */
95 void invokeFinalizee(T finalizee) throws Throwable {
96 SharedSecrets.getJavaLangAccess().invokeFinalize(finalizee);
97 finalizee = null;
98 }
99
100 @Override
101 public void clear() {
102 T finalizee = delete();
103 if (finalizee == null) {
104 return;
105 }
106 unfinalized[listIndex].unlink(this);
107 /* Clear stack slot containing this variable, to decrease
108 the chances of false retention with a conservative GC */
109 finalizee = null;
110 }
111
112 /* Create a privileged secondary finalizer thread in the system thread
113 group for the given Runnable, and wait for it to complete.
114
115 This method is used by both runFinalization and runFinalizersOnExit.
116 The former method invokes all pending finalizers, while the latter
117 invokes all uninvoked finalizers if on-exit finalization has been
118 enabled.
119
120 These two methods could have been implemented by offloading their work
121 to the regular finalizer thread and waiting for that thread to finish.
122 The advantage of creating a fresh thread, however, is that it insulates
123 invokers of these methods from a stalled or deadlocked finalizer thread.
124 */
125 private static void forkSecondaryFinalizer(final Runnable proc) {
126 AccessController.doPrivileged(
127 new PrivilegedAction<>() {
128 public Void run() {
129 ThreadGroup tg = Thread.currentThread().getThreadGroup();
130 for (ThreadGroup tgn = tg;
131 tgn != null;
132 tg = tgn, tgn = tg.getParent());
133 Thread sft = new ManagedLocalsThread(tg, proc, "Secondary finalizer");
134 sft.start();
135 try {
136 sft.join();
137 } catch (InterruptedException x) {
138 Thread.currentThread().interrupt();
139 }
140 return null;
141 }});
142 }
143
144 /* Called by Runtime.runFinalization() */
145 static void runFinalization() {
146 if (!VM.isBooted()) {
147 return;
148 }
149
150 forkSecondaryFinalizer(new Runnable() {
151 private volatile boolean running;
152 public void run() {
153 // in case of recursive call to run()
154 if (running)
155 return;
156 running = true;
157 ReferenceHandling.runFinalization();
158 }
159 });
160 }
161
162 /* Invoked by java.lang.Shutdown */
163 static void runAllFinalizers() {
164 if (!VM.isBooted()) {
165 return;
166 }
167
168 forkSecondaryFinalizer(new Runnable() {
169 private volatile boolean running;
170 public void run() {
171 // in case of recursive call to run()
172 if (running)
173 return;
174 running = true;
175 for (FinalizerList uflist : unfinalized)
176 for (Finalizer<?> f = uflist.first(); f != null; f = uflist.succ(f)) {
177 f.run();
178 }}});
179 }
180
181 // Unsafe mechanics
182
183 /**
184 * Returns the pseudo-randomly initialized or updated secondary seed.
185 * Copied from ThreadLocalRandom due to package access restrictions.
186 */
187 static int nextSecondarySeed() {
188 int r;
189 Thread t = Thread.currentThread();
190 if ((r = UNSAFE.getInt(t, threadLocalRandomSecondarySeedOffset)) != 0) {
191 r ^= r << 13; // xorshift
192 r ^= r >>> 17;
193 r ^= r << 5;
194 }
195 else if ((r = ThreadLocalRandom.current().nextInt()) == 0)
196 r = 1; // avoid zero
197 UNSAFE.putInt(t, threadLocalRandomSecondarySeedOffset, r);
198 return r;
199 }
200
201 boolean isAlive() {
202 return getReferentVolatile() != null;
203 }
204
205 boolean isDeleted() {
206 return getReferentVolatile() == null;
207 }
208
209 private T delete() {
210 T referent = getReferentVolatile();
211 return (referent != null) && casReferent(referent, null)
212 ? referent : null;
213 }
214
215 void lazySetNext(Finalizer<?> val) {
216 UNSAFE.putOrderedObject(this, nextOffset, val);
217 }
218
219 boolean casNext(Finalizer<?> cmp, Finalizer<?> val) {
220 return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
221 }
222
223 void lazySetPrev(Finalizer<?> val) {
224 UNSAFE.putOrderedObject(this, prevOffset, val);
225 }
226
227 boolean casPrev(Finalizer<?> cmp, Finalizer<?> val) {
228 return UNSAFE.compareAndSwapObject(this, prevOffset, cmp, val);
229 }
230
231 private static final sun.misc.Unsafe UNSAFE;
232 private static final long prevOffset;
233 private static final long nextOffset;
234 private static final long threadLocalRandomSecondarySeedOffset;
235
236 static {
237 try {
238 UNSAFE = sun.misc.Unsafe.getUnsafe();
239 Class<Finalizer> fc = Finalizer.class;
240 prevOffset = UNSAFE.objectFieldOffset(fc.getDeclaredField("prev"));
241 nextOffset = UNSAFE.objectFieldOffset(fc.getDeclaredField("next"));
242 Class<Thread> tc = Thread.class;
243 threadLocalRandomSecondarySeedOffset = UNSAFE.objectFieldOffset
244 (tc.getDeclaredField("threadLocalRandomSecondarySeed"));
245 } catch (Exception e) {
246 throw new Error(e);
247 }
248 }
249 }