| /* |
| * Copyright (c) 2003, 2013, 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 jdk.internal.ref; |
| |
| 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<Object> |
| { |
| |
| // 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<Object> dummyQueue = new ReferenceQueue<>(); |
| |
| // Doubly-linked list of live cleaners, which prevents the cleaners |
| // themselves from being GC'd before their referents |
| // |
| private static 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 ob the referent object to be cleaned |
| * @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 Void run() { |
| if (System.err != null) |
| new Error("Cleaner terminated abnormally", x) |
| .printStackTrace(); |
| System.exit(1); |
| return null; |
| }}); |
| } |
| } |
| } |