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Finalizer.java
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/
Finalizer.java
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/*
* Copyright (c) 1997, 2021, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation. Oracle designates this
* particular file as subject to the "Classpath" exception as provided
* by Oracle in the LICENSE file that accompanied this code.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*/
package java.lang.ref;
import java.security.PrivilegedAction;
import java.security.AccessController;
import jdk.internal.access.JavaLangAccess;
import jdk.internal.access.SharedSecrets;
import jdk.internal.misc.VM;
final class Finalizer extends FinalReference<Object> { /* Package-private; must be in
same package as the Reference
class */
private static ReferenceQueue<Object> queue = new ReferenceQueue<>();
/** Head of doubly linked list of Finalizers awaiting finalization. */
private static Finalizer unfinalized = null;
/** Lock guarding access to unfinalized list. */
private static final Object lock = new Object();
private Finalizer next, prev;
private Finalizer(Object finalizee) {
super(finalizee, queue);
// push onto unfinalized
synchronized (lock) {
if (unfinalized != null) {
this.next = unfinalized;
unfinalized.prev = this;
}
unfinalized = this;
}
}
static ReferenceQueue<Object> getQueue() {
return queue;
}
static final boolean ENABLED = isFinalizationEnabled();
private static native boolean isFinalizationEnabled();
/* Invoked by VM */
static void register(Object finalizee) {
if (ENABLED) {
new Finalizer(finalizee);
} else {
throw new InternalError("unexpected call to Finalizer::register when finalization is disabled");
}
}
private void runFinalizer(JavaLangAccess jla) {
synchronized (lock) {
if (this.next == this) // already finalized
return;
// unlink from unfinalized
if (unfinalized == this)
unfinalized = this.next;
else
this.prev.next = this.next;
if (this.next != null)
this.next.prev = this.prev;
this.prev = null;
this.next = this; // mark as finalized
}
try {
Object finalizee = this.get();
assert finalizee != null;
if (!(finalizee instanceof java.lang.Enum)) {
jla.invokeFinalize(finalizee);
reportComplete(finalizee);
// Clear stack slot containing this variable, to decrease
// the chances of false retention with a conservative GC
finalizee = null;
}
} catch (Throwable x) { }
super.clear();
}
private static native void reportComplete(Object finalizee);
/* Create a privileged secondary finalizer thread in the system thread
* group for the given Runnable, and wait for it to complete.
*
* This method is used by runFinalization.
*
* It could have been implemented by offloading the work to the
* regular finalizer thread and waiting for that thread to finish.
* The advantage of creating a fresh thread, however, is that it insulates
* invokers of that method from a stalled or deadlocked finalizer thread.
*/
@SuppressWarnings("removal")
private static void forkSecondaryFinalizer(final Runnable proc) {
AccessController.doPrivileged(
new PrivilegedAction<>() {
public Void run() {
ThreadGroup tg = Thread.currentThread().getThreadGroup();
for (ThreadGroup tgn = tg;
tgn != null;
tg = tgn, tgn = tg.getParent());
Thread sft = new Thread(tg, proc, "Secondary finalizer", 0, false);
sft.start();
try {
sft.join();
} catch (InterruptedException x) {
Thread.currentThread().interrupt();
}
return null;
}});
}
/* Called by Runtime.runFinalization() */
static void runFinalization() {
if (VM.initLevel() == 0 || ! ENABLED) {
return;
}
forkSecondaryFinalizer(new Runnable() {
private volatile boolean running;
public void run() {
// in case of recursive call to run()
if (running)
return;
final JavaLangAccess jla = SharedSecrets.getJavaLangAccess();
running = true;
for (Finalizer f; (f = (Finalizer)queue.poll()) != null; )
f.runFinalizer(jla);
}
});
}
private static class FinalizerThread extends Thread {
private volatile boolean running;
FinalizerThread(ThreadGroup g) {
super(g, null, "Finalizer", 0, false);
}
public void run() {
// in case of recursive call to run()
if (running)
return;
// Finalizer thread starts before System.initializeSystemClass
// is called. Wait until JavaLangAccess is available
while (VM.initLevel() == 0) {
// delay until VM completes initialization
try {
VM.awaitInitLevel(1);
} catch (InterruptedException x) {
// ignore and continue
}
}
final JavaLangAccess jla = SharedSecrets.getJavaLangAccess();
running = true;
for (;;) {
try {
Finalizer f = (Finalizer)queue.remove();
f.runFinalizer(jla);
} catch (InterruptedException x) {
// ignore and continue
}
}
}
}
static {
if (ENABLED) {
ThreadGroup tg = Thread.currentThread().getThreadGroup();
for (ThreadGroup tgn = tg;
tgn != null;
tg = tgn, tgn = tg.getParent());
Thread finalizer = new FinalizerThread(tg);
finalizer.setPriority(Thread.MAX_PRIORITY - 2);
finalizer.setDaemon(true);
finalizer.start();
}
}
}