432 lines
18 KiB
Java
432 lines
18 KiB
Java
/*
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* Copyright (c) 1997, 2018, Oracle and/or its affiliates. All rights reserved.
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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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*
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* This code is free software; you can redistribute it and/or modify it
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* under the terms of the GNU General Public License version 2 only, as
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* published by the Free Software Foundation. Oracle designates this
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* particular file as subject to the "Classpath" exception as provided
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* by Oracle in the LICENSE file that accompanied this code.
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*
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* This code is distributed in the hope that it will be useful, but WITHOUT
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* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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* version 2 for more details (a copy is included in the LICENSE file that
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* accompanied this code).
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*
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* You should have received a copy of the GNU General Public License version
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* 2 along with this work; if not, write to the Free Software Foundation,
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* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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*
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* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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* or visit www.oracle.com if you need additional information or have any
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* questions.
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*/
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package java.lang.ref;
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import jdk.internal.vm.annotation.ForceInline;
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import jdk.internal.HotSpotIntrinsicCandidate;
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import jdk.internal.misc.JavaLangRefAccess;
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import jdk.internal.misc.SharedSecrets;
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import jdk.internal.ref.Cleaner;
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/**
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* Abstract base class for reference objects. This class defines the
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* operations common to all reference objects. Because reference objects are
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* implemented in close cooperation with the garbage collector, this class may
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* not be subclassed directly.
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*
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* @author Mark Reinhold
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* @since 1.2
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*/
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public abstract class Reference<T> {
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/* A Reference instance is in one of four possible internal states:
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*
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* Active: Subject to special treatment by the garbage collector. Some
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* time after the collector detects that the reachability of the
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* referent has changed to the appropriate state, it changes the
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* instance's state to either Pending or Inactive, depending upon
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* whether or not the instance was registered with a queue when it was
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* created. In the former case it also adds the instance to the
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* pending-Reference list. Newly-created instances are Active.
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*
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* Pending: An element of the pending-Reference list, waiting to be
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* enqueued by the Reference-handler thread. Unregistered instances
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* are never in this state.
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*
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* Enqueued: An element of the queue with which the instance was
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* registered when it was created. When an instance is removed from
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* its ReferenceQueue, it is made Inactive. Unregistered instances are
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* never in this state.
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*
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* Inactive: Nothing more to do. Once an instance becomes Inactive its
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* state will never change again.
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*
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* The state is encoded in the queue and next fields as follows:
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*
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* Active: queue = ReferenceQueue with which instance is registered, or
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* ReferenceQueue.NULL if it was not registered with a queue; next =
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* null.
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*
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* Pending: queue = ReferenceQueue with which instance is registered;
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* next = this
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*
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* Enqueued: queue = ReferenceQueue.ENQUEUED; next = Following instance
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* in queue, or this if at end of list.
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*
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* Inactive: queue = ReferenceQueue.NULL; next = this.
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*
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* With this scheme the collector need only examine the next field in order
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* to determine whether a Reference instance requires special treatment: If
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* the next field is null then the instance is active; if it is non-null,
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* then the collector should treat the instance normally.
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*
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* To ensure that a concurrent collector can discover active Reference
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* objects without interfering with application threads that may apply
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* the enqueue() method to those objects, collectors should link
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* discovered objects through the discovered field. The discovered
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* field is also used for linking Reference objects in the pending list.
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*/
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private T referent; /* Treated specially by GC */
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volatile ReferenceQueue<? super T> queue;
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/* When active: NULL
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* pending: this
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* Enqueued: next reference in queue (or this if last)
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* Inactive: this
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*/
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@SuppressWarnings("rawtypes")
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volatile Reference next;
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/* When active: next element in a discovered reference list maintained by GC (or this if last)
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* pending: next element in the pending list (or null if last)
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* otherwise: NULL
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*/
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private transient Reference<T> discovered; /* used by VM */
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/* High-priority thread to enqueue pending References
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*/
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private static class ReferenceHandler extends Thread {
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private static void ensureClassInitialized(Class<?> clazz) {
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try {
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Class.forName(clazz.getName(), true, clazz.getClassLoader());
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} catch (ClassNotFoundException e) {
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throw (Error) new NoClassDefFoundError(e.getMessage()).initCause(e);
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}
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}
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static {
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// pre-load and initialize Cleaner class so that we don't
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// get into trouble later in the run loop if there's
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// memory shortage while loading/initializing it lazily.
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ensureClassInitialized(Cleaner.class);
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}
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ReferenceHandler(ThreadGroup g, String name) {
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super(g, null, name, 0, false);
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}
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public void run() {
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while (true) {
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processPendingReferences();
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}
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}
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}
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/*
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* Atomically get and clear (set to null) the VM's pending list.
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*/
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private static native Reference<Object> getAndClearReferencePendingList();
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/*
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* Test whether the VM's pending list contains any entries.
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*/
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private static native boolean hasReferencePendingList();
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/*
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* Wait until the VM's pending list may be non-null.
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*/
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private static native void waitForReferencePendingList();
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private static final Object processPendingLock = new Object();
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private static boolean processPendingActive = false;
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private static void processPendingReferences() {
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// Only the singleton reference processing thread calls
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// waitForReferencePendingList() and getAndClearReferencePendingList().
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// These are separate operations to avoid a race with other threads
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// that are calling waitForReferenceProcessing().
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waitForReferencePendingList();
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Reference<Object> pendingList;
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synchronized (processPendingLock) {
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pendingList = getAndClearReferencePendingList();
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processPendingActive = true;
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}
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while (pendingList != null) {
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Reference<Object> ref = pendingList;
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pendingList = ref.discovered;
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ref.discovered = null;
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if (ref instanceof Cleaner) {
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((Cleaner)ref).clean();
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// Notify any waiters that progress has been made.
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// This improves latency for nio.Bits waiters, which
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// are the only important ones.
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synchronized (processPendingLock) {
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processPendingLock.notifyAll();
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}
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} else {
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ReferenceQueue<? super Object> q = ref.queue;
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if (q != ReferenceQueue.NULL) q.enqueue(ref);
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}
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}
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// Notify any waiters of completion of current round.
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synchronized (processPendingLock) {
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processPendingActive = false;
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processPendingLock.notifyAll();
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}
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}
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// Wait for progress in reference processing.
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//
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// Returns true after waiting (for notification from the reference
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// processing thread) if either (1) the VM has any pending
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// references, or (2) the reference processing thread is
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// processing references. Otherwise, returns false immediately.
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private static boolean waitForReferenceProcessing()
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throws InterruptedException
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{
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synchronized (processPendingLock) {
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if (processPendingActive || hasReferencePendingList()) {
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// Wait for progress, not necessarily completion.
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processPendingLock.wait();
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return true;
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} else {
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return false;
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}
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}
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}
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static {
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ThreadGroup tg = Thread.currentThread().getThreadGroup();
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for (ThreadGroup tgn = tg;
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tgn != null;
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tg = tgn, tgn = tg.getParent());
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Thread handler = new ReferenceHandler(tg, "Reference Handler");
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/* If there were a special system-only priority greater than
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* MAX_PRIORITY, it would be used here
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*/
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handler.setPriority(Thread.MAX_PRIORITY);
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handler.setDaemon(true);
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handler.start();
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// provide access in SharedSecrets
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SharedSecrets.setJavaLangRefAccess(new JavaLangRefAccess() {
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@Override
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public boolean waitForReferenceProcessing()
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throws InterruptedException
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{
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return Reference.waitForReferenceProcessing();
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}
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@Override
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public void runFinalization() {
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Finalizer.runFinalization();
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}
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});
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}
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/* -- Referent accessor and setters -- */
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/**
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* Returns this reference object's referent. If this reference object has
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* been cleared, either by the program or by the garbage collector, then
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* this method returns <code>null</code>.
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*
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* @return The object to which this reference refers, or
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* <code>null</code> if this reference object has been cleared
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*/
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@HotSpotIntrinsicCandidate
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public T get() {
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return this.referent;
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}
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/**
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* Clears this reference object. Invoking this method will not cause this
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* object to be enqueued.
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*
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* <p> This method is invoked only by Java code; when the garbage collector
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* clears references it does so directly, without invoking this method.
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*/
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public void clear() {
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this.referent = null;
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}
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/* -- Queue operations -- */
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/**
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* Tells whether or not this reference object has been enqueued, either by
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* the program or by the garbage collector. If this reference object was
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* not registered with a queue when it was created, then this method will
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* always return <code>false</code>.
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*
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* @return <code>true</code> if and only if this reference object has
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* been enqueued
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*/
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public boolean isEnqueued() {
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return (this.queue == ReferenceQueue.ENQUEUED);
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}
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/**
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* Clears this reference object and adds it to the queue with which
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* it is registered, if any.
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*
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* <p> This method is invoked only by Java code; when the garbage collector
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* enqueues references it does so directly, without invoking this method.
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*
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* @return <code>true</code> if this reference object was successfully
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* enqueued; <code>false</code> if it was already enqueued or if
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* it was not registered with a queue when it was created
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*/
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public boolean enqueue() {
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this.referent = null;
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return this.queue.enqueue(this);
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}
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/* -- Constructors -- */
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Reference(T referent) {
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this(referent, null);
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}
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Reference(T referent, ReferenceQueue<? super T> queue) {
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this.referent = referent;
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this.queue = (queue == null) ? ReferenceQueue.NULL : queue;
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}
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/**
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* Ensures that the object referenced by the given reference remains
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* <a href="package-summary.html#reachability"><em>strongly reachable</em></a>,
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* regardless of any prior actions of the program that might otherwise cause
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* the object to become unreachable; thus, the referenced object is not
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* reclaimable by garbage collection at least until after the invocation of
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* this method. Invocation of this method does not itself initiate garbage
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* collection or finalization.
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*
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* <p> This method establishes an ordering for
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* <a href="package-summary.html#reachability"><em>strong reachability</em></a>
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* with respect to garbage collection. It controls relations that are
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* otherwise only implicit in a program -- the reachability conditions
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* triggering garbage collection. This method is designed for use in
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* uncommon situations of premature finalization where using
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* {@code synchronized} blocks or methods, or using other synchronization
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* facilities are not possible or do not provide the desired control. This
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* method is applicable only when reclamation may have visible effects,
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* which is possible for objects with finalizers (See
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* <a href="https://docs.oracle.com/javase/specs/jls/se8/html/jls-12.html#jls-12.6">
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* Section 12.6 17 of <cite>The Java™ Language Specification</cite></a>)
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* that are implemented in ways that rely on ordering control for correctness.
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*
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* @apiNote
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* Finalization may occur whenever the virtual machine detects that no
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* reference to an object will ever be stored in the heap: The garbage
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* collector may reclaim an object even if the fields of that object are
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* still in use, so long as the object has otherwise become unreachable.
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* This may have surprising and undesirable effects in cases such as the
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* following example in which the bookkeeping associated with a class is
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* managed through array indices. Here, method {@code action} uses a
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* {@code reachabilityFence} to ensure that the {@code Resource} object is
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* not reclaimed before bookkeeping on an associated
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* {@code ExternalResource} has been performed; in particular here, to
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* ensure that the array slot holding the {@code ExternalResource} is not
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* nulled out in method {@link Object#finalize}, which may otherwise run
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* concurrently.
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*
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* <pre> {@code
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* class Resource {
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* private static ExternalResource[] externalResourceArray = ...
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*
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* int myIndex;
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* Resource(...) {
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* myIndex = ...
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* externalResourceArray[myIndex] = ...;
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* ...
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* }
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* protected void finalize() {
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* externalResourceArray[myIndex] = null;
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* ...
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* }
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* public void action() {
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* try {
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* // ...
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* int i = myIndex;
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* Resource.update(externalResourceArray[i]);
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* } finally {
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* Reference.reachabilityFence(this);
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* }
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* }
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* private static void update(ExternalResource ext) {
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* ext.status = ...;
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* }
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* }}</pre>
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*
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* Here, the invocation of {@code reachabilityFence} is nonintuitively
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* placed <em>after</em> the call to {@code update}, to ensure that the
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* array slot is not nulled out by {@link Object#finalize} before the
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* update, even if the call to {@code action} was the last use of this
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* object. This might be the case if, for example a usage in a user program
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* had the form {@code new Resource().action();} which retains no other
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* reference to this {@code Resource}. While probably overkill here,
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* {@code reachabilityFence} is placed in a {@code finally} block to ensure
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* that it is invoked across all paths in the method. In a method with more
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* complex control paths, you might need further precautions to ensure that
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* {@code reachabilityFence} is encountered along all of them.
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*
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* <p> It is sometimes possible to better encapsulate use of
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* {@code reachabilityFence}. Continuing the above example, if it were
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* acceptable for the call to method {@code update} to proceed even if the
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* finalizer had already executed (nulling out slot), then you could
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* localize use of {@code reachabilityFence}:
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*
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* <pre> {@code
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* public void action2() {
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* // ...
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* Resource.update(getExternalResource());
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* }
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* private ExternalResource getExternalResource() {
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* ExternalResource ext = externalResourceArray[myIndex];
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* Reference.reachabilityFence(this);
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* return ext;
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* }}</pre>
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*
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* <p> Method {@code reachabilityFence} is not required in constructions
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* that themselves ensure reachability. For example, because objects that
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* are locked cannot, in general, be reclaimed, it would suffice if all
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* accesses of the object, in all methods of class {@code Resource}
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* (including {@code finalize}) were enclosed in {@code synchronized (this)}
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* blocks. (Further, such blocks must not include infinite loops, or
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* themselves be unreachable, which fall into the corner case exceptions to
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* the "in general" disclaimer.) However, method {@code reachabilityFence}
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* remains a better option in cases where this approach is not as efficient,
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* desirable, or possible; for example because it would encounter deadlock.
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*
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* @param ref the reference. If {@code null}, this method has no effect.
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* @since 9
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*/
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@ForceInline
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public static void reachabilityFence(Object ref) {
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// Does nothing. This method is annotated with @ForceInline to eliminate
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// most of the overhead that using @DontInline would cause with the
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// HotSpot JVM, when this fence is used in a wide variety of situations.
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// HotSpot JVM retains the ref and does not GC it before a call to
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// this method, because the JIT-compilers do not have GC-only safepoints.
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}
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}
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