/*
* Copyright (c) 2003, 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
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*/
package sun.misc;
import java.lang.ref.*;
import java.security.AccessController;
import java.security.PrivilegedAction;
/**
* General-purpose phantom-reference-based cleaners.
*
* <p> Cleaners are a lightweight and more robust alternative to finalization.
* They are lightweight because they are not created by the VM and thus do not
* require a JNI upcall to be created, and because their cleanup code is
* invoked directly by the reference-handler thread rather than by the
* finalizer thread. They are more robust because they use phantom references,
* the weakest type of reference object, thereby avoiding the nasty ordering
* problems inherent to finalization.
*
* <p> A cleaner tracks a referent object and encapsulates a thunk of arbitrary
* cleanup code. Some time after the GC detects that a cleaner's referent has
* become phantom-reachable, the reference-handler thread will run the cleaner.
* Cleaners may also be invoked directly; they are thread safe and ensure that
* they run their thunks at most once.
*
* <p> Cleaners are not a replacement for finalization. They should be used
* only when the cleanup code is extremely simple and straightforward.
* Nontrivial cleaners are inadvisable since they risk blocking the
* reference-handler thread and delaying further cleanup and finalization.
*
*
* @author Mark Reinhold
*/
public class Cleaner
extends PhantomReference
{
// Dummy reference queue, needed because the PhantomReference constructor
// insists that we pass a queue. Nothing will ever be placed on this queue
// since the reference handler invokes cleaners explicitly.
//
private static final ReferenceQueue dummyQueue = new ReferenceQueue();
// Doubly-linked list of live cleaners, which prevents the cleaners
// themselves from being GC'd before their referents
//
static private Cleaner first = null;
private Cleaner
next = null,
prev = null;
private static synchronized Cleaner add(Cleaner cl) {
if (first != null) {
cl.next = first;
first.prev = cl;
}
first = cl;
return cl;
}
private static synchronized boolean remove(Cleaner cl) {
// If already removed, do nothing
if (cl.next == cl)
return false;
// Update list
if (first == cl) {
if (cl.next != null)
first = cl.next;
else
first = cl.prev;
}
if (cl.next != null)
cl.next.prev = cl.prev;
if (cl.prev != null)
cl.prev.next = cl.next;
// Indicate removal by pointing the cleaner to itself
cl.next = cl;
cl.prev = cl;
return true;
}
private final Runnable thunk;
private Cleaner(Object referent, Runnable thunk) {
super(referent, dummyQueue);
this.thunk = thunk;
}
/**
* Creates a new cleaner.
*
* @param thunk
* The cleanup code to be run when the cleaner is invoked. The
* cleanup code is run directly from the reference-handler thread,
* so it should be as simple and straightforward as possible.
*
* @return The new cleaner
*/
public static Cleaner create(Object ob, Runnable thunk) {
if (thunk == null)
return null;
return add(new Cleaner(ob, thunk));
}
/**
* Runs this cleaner, if it has not been run before.
*/
public void clean() {
if (!remove(this))
return;
try {
thunk.run();
} catch (final Throwable x) {
AccessController.doPrivileged(new PrivilegedAction() {
public Object run() {
if (System.err != null)
new Error("Cleaner terminated abnormally", x)
.printStackTrace();
System.exit(1);
return null;
}});
}
}
}