--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/src/java.base/share/classes/java/lang/ref/Reference.java Tue Sep 12 19:03:39 2017 +0200
@@ -0,0 +1,433 @@
+/*
+ * Copyright (c) 1997, 2017, 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 jdk.internal.vm.annotation.DontInline;
+import jdk.internal.HotSpotIntrinsicCandidate;
+import jdk.internal.misc.JavaLangRefAccess;
+import jdk.internal.misc.SharedSecrets;
+import jdk.internal.ref.Cleaner;
+
+/**
+ * Abstract base class for reference objects. This class defines the
+ * operations common to all reference objects. Because reference objects are
+ * implemented in close cooperation with the garbage collector, this class may
+ * not be subclassed directly.
+ *
+ * @author Mark Reinhold
+ * @since 1.2
+ */
+
+public abstract class Reference<T> {
+
+ /* A Reference instance is in one of four possible internal states:
+ *
+ * Active: Subject to special treatment by the garbage collector. Some
+ * time after the collector detects that the reachability of the
+ * referent has changed to the appropriate state, it changes the
+ * instance's state to either Pending or Inactive, depending upon
+ * whether or not the instance was registered with a queue when it was
+ * created. In the former case it also adds the instance to the
+ * pending-Reference list. Newly-created instances are Active.
+ *
+ * Pending: An element of the pending-Reference list, waiting to be
+ * enqueued by the Reference-handler thread. Unregistered instances
+ * are never in this state.
+ *
+ * Enqueued: An element of the queue with which the instance was
+ * registered when it was created. When an instance is removed from
+ * its ReferenceQueue, it is made Inactive. Unregistered instances are
+ * never in this state.
+ *
+ * Inactive: Nothing more to do. Once an instance becomes Inactive its
+ * state will never change again.
+ *
+ * The state is encoded in the queue and next fields as follows:
+ *
+ * Active: queue = ReferenceQueue with which instance is registered, or
+ * ReferenceQueue.NULL if it was not registered with a queue; next =
+ * null.
+ *
+ * Pending: queue = ReferenceQueue with which instance is registered;
+ * next = this
+ *
+ * Enqueued: queue = ReferenceQueue.ENQUEUED; next = Following instance
+ * in queue, or this if at end of list.
+ *
+ * Inactive: queue = ReferenceQueue.NULL; next = this.
+ *
+ * With this scheme the collector need only examine the next field in order
+ * to determine whether a Reference instance requires special treatment: If
+ * the next field is null then the instance is active; if it is non-null,
+ * then the collector should treat the instance normally.
+ *
+ * To ensure that a concurrent collector can discover active Reference
+ * objects without interfering with application threads that may apply
+ * the enqueue() method to those objects, collectors should link
+ * discovered objects through the discovered field. The discovered
+ * field is also used for linking Reference objects in the pending list.
+ */
+
+ private T referent; /* Treated specially by GC */
+
+ volatile ReferenceQueue<? super T> queue;
+
+ /* When active: NULL
+ * pending: this
+ * Enqueued: next reference in queue (or this if last)
+ * Inactive: this
+ */
+ @SuppressWarnings("rawtypes")
+ volatile Reference next;
+
+ /* When active: next element in a discovered reference list maintained by GC (or this if last)
+ * pending: next element in the pending list (or null if last)
+ * otherwise: NULL
+ */
+ private transient Reference<T> discovered; /* used by VM */
+
+
+ /* High-priority thread to enqueue pending References
+ */
+ private static class ReferenceHandler extends Thread {
+
+ private static void ensureClassInitialized(Class<?> clazz) {
+ try {
+ Class.forName(clazz.getName(), true, clazz.getClassLoader());
+ } catch (ClassNotFoundException e) {
+ throw (Error) new NoClassDefFoundError(e.getMessage()).initCause(e);
+ }
+ }
+
+ static {
+ // pre-load and initialize Cleaner class so that we don't
+ // get into trouble later in the run loop if there's
+ // memory shortage while loading/initializing it lazily.
+ ensureClassInitialized(Cleaner.class);
+ }
+
+ ReferenceHandler(ThreadGroup g, String name) {
+ super(g, null, name, 0, false);
+ }
+
+ public void run() {
+ while (true) {
+ processPendingReferences();
+ }
+ }
+ }
+
+ /*
+ * system property to disable clearing before enqueuing.
+ */
+ private static final class ClearBeforeEnqueue {
+ static final boolean DISABLE =
+ Boolean.getBoolean("jdk.lang.ref.disableClearBeforeEnqueue");
+ }
+
+ /*
+ * Atomically get and clear (set to null) the VM's pending list.
+ */
+ private static native Reference<Object> getAndClearReferencePendingList();
+
+ /*
+ * Test whether the VM's pending list contains any entries.
+ */
+ private static native boolean hasReferencePendingList();
+
+ /*
+ * Wait until the VM's pending list may be non-null.
+ */
+ private static native void waitForReferencePendingList();
+
+ private static final Object processPendingLock = new Object();
+ private static boolean processPendingActive = false;
+
+ private static void processPendingReferences() {
+ // Only the singleton reference processing thread calls
+ // waitForReferencePendingList() and getAndClearReferencePendingList().
+ // These are separate operations to avoid a race with other threads
+ // that are calling waitForReferenceProcessing().
+ waitForReferencePendingList();
+ Reference<Object> pendingList;
+ synchronized (processPendingLock) {
+ pendingList = getAndClearReferencePendingList();
+ processPendingActive = true;
+ }
+ while (pendingList != null) {
+ Reference<Object> ref = pendingList;
+ pendingList = ref.discovered;
+ ref.discovered = null;
+
+ if (ref instanceof Cleaner) {
+ ((Cleaner)ref).clean();
+ // Notify any waiters that progress has been made.
+ // This improves latency for nio.Bits waiters, which
+ // are the only important ones.
+ synchronized (processPendingLock) {
+ processPendingLock.notifyAll();
+ }
+ } else {
+ ReferenceQueue<? super Object> q = ref.queue;
+ if (q != ReferenceQueue.NULL) q.enqueue(ref);
+ }
+ }
+ // Notify any waiters of completion of current round.
+ synchronized (processPendingLock) {
+ processPendingActive = false;
+ processPendingLock.notifyAll();
+ }
+ }
+
+ // Wait for progress in reference processing.
+ //
+ // Returns true after waiting (for notification from the reference
+ // processing thread) if either (1) the VM has any pending
+ // references, or (2) the reference processing thread is
+ // processing references. Otherwise, returns false immediately.
+ private static boolean waitForReferenceProcessing()
+ throws InterruptedException
+ {
+ synchronized (processPendingLock) {
+ if (processPendingActive || hasReferencePendingList()) {
+ // Wait for progress, not necessarily completion.
+ processPendingLock.wait();
+ return true;
+ } else {
+ return false;
+ }
+ }
+ }
+
+ static {
+ ThreadGroup tg = Thread.currentThread().getThreadGroup();
+ for (ThreadGroup tgn = tg;
+ tgn != null;
+ tg = tgn, tgn = tg.getParent());
+ Thread handler = new ReferenceHandler(tg, "Reference Handler");
+ /* If there were a special system-only priority greater than
+ * MAX_PRIORITY, it would be used here
+ */
+ handler.setPriority(Thread.MAX_PRIORITY);
+ handler.setDaemon(true);
+ handler.start();
+
+ // provide access in SharedSecrets
+ SharedSecrets.setJavaLangRefAccess(new JavaLangRefAccess() {
+ @Override
+ public boolean waitForReferenceProcessing()
+ throws InterruptedException
+ {
+ return Reference.waitForReferenceProcessing();
+ }
+ });
+ }
+
+ /* -- Referent accessor and setters -- */
+
+ /**
+ * Returns this reference object's referent. If this reference object has
+ * been cleared, either by the program or by the garbage collector, then
+ * this method returns <code>null</code>.
+ *
+ * @return The object to which this reference refers, or
+ * <code>null</code> if this reference object has been cleared
+ */
+ @HotSpotIntrinsicCandidate
+ public T get() {
+ return this.referent;
+ }
+
+ /**
+ * Clears this reference object. Invoking this method will not cause this
+ * object to be enqueued.
+ *
+ * <p> This method is invoked only by Java code; when the garbage collector
+ * clears references it does so directly, without invoking this method.
+ */
+ public void clear() {
+ this.referent = null;
+ }
+
+ /* -- Queue operations -- */
+
+ /**
+ * Tells whether or not this reference object has been enqueued, either by
+ * the program or by the garbage collector. If this reference object was
+ * not registered with a queue when it was created, then this method will
+ * always return <code>false</code>.
+ *
+ * @return <code>true</code> if and only if this reference object has
+ * been enqueued
+ */
+ public boolean isEnqueued() {
+ return (this.queue == ReferenceQueue.ENQUEUED);
+ }
+
+ /**
+ * Clears this reference object and adds it to the queue with which
+ * it is registered, if any.
+ *
+ * <p> This method is invoked only by Java code; when the garbage collector
+ * enqueues references it does so directly, without invoking this method.
+ *
+ * @return <code>true</code> if this reference object was successfully
+ * enqueued; <code>false</code> if it was already enqueued or if
+ * it was not registered with a queue when it was created
+ */
+ public boolean enqueue() {
+ if (!ClearBeforeEnqueue.DISABLE)
+ this.referent = null;
+ return this.queue.enqueue(this);
+ }
+
+ /* -- Constructors -- */
+
+ Reference(T referent) {
+ this(referent, null);
+ }
+
+ Reference(T referent, ReferenceQueue<? super T> queue) {
+ this.referent = referent;
+ this.queue = (queue == null) ? ReferenceQueue.NULL : queue;
+ }
+
+ /**
+ * Ensures that the object referenced by the given reference remains
+ * <a href="package-summary.html#reachability"><em>strongly reachable</em></a>,
+ * regardless of any prior actions of the program that might otherwise cause
+ * the object to become unreachable; thus, the referenced object is not
+ * reclaimable by garbage collection at least until after the invocation of
+ * this method. Invocation of this method does not itself initiate garbage
+ * collection or finalization.
+ *
+ * <p> This method establishes an ordering for
+ * <a href="package-summary.html#reachability"><em>strong reachability</em></a>
+ * with respect to garbage collection. It controls relations that are
+ * otherwise only implicit in a program -- the reachability conditions
+ * triggering garbage collection. This method is designed for use in
+ * uncommon situations of premature finalization where using
+ * {@code synchronized} blocks or methods, or using other synchronization
+ * facilities are not possible or do not provide the desired control. This
+ * method is applicable only when reclamation may have visible effects,
+ * which is possible for objects with finalizers (See
+ * <a href="https://docs.oracle.com/javase/specs/jls/se8/html/jls-12.html#jls-12.6">
+ * Section 12.6 17 of <cite>The Java™ Language Specification</cite></a>)
+ * that are implemented in ways that rely on ordering control for correctness.
+ *
+ * @apiNote
+ * Finalization may occur whenever the virtual machine detects that no
+ * reference to an object will ever be stored in the heap: The garbage
+ * collector may reclaim an object even if the fields of that object are
+ * still in use, so long as the object has otherwise become unreachable.
+ * This may have surprising and undesirable effects in cases such as the
+ * following example in which the bookkeeping associated with a class is
+ * managed through array indices. Here, method {@code action} uses a
+ * {@code reachabilityFence} to ensure that the {@code Resource} object is
+ * not reclaimed before bookkeeping on an associated
+ * {@code ExternalResource} has been performed; in particular here, to
+ * ensure that the array slot holding the {@code ExternalResource} is not
+ * nulled out in method {@link Object#finalize}, which may otherwise run
+ * concurrently.
+ *
+ * <pre> {@code
+ * class Resource {
+ * private static ExternalResource[] externalResourceArray = ...
+ *
+ * int myIndex;
+ * Resource(...) {
+ * myIndex = ...
+ * externalResourceArray[myIndex] = ...;
+ * ...
+ * }
+ * protected void finalize() {
+ * externalResourceArray[myIndex] = null;
+ * ...
+ * }
+ * public void action() {
+ * try {
+ * // ...
+ * int i = myIndex;
+ * Resource.update(externalResourceArray[i]);
+ * } finally {
+ * Reference.reachabilityFence(this);
+ * }
+ * }
+ * private static void update(ExternalResource ext) {
+ * ext.status = ...;
+ * }
+ * }}</pre>
+ *
+ * Here, the invocation of {@code reachabilityFence} is nonintuitively
+ * placed <em>after</em> the call to {@code update}, to ensure that the
+ * array slot is not nulled out by {@link Object#finalize} before the
+ * update, even if the call to {@code action} was the last use of this
+ * object. This might be the case if, for example a usage in a user program
+ * had the form {@code new Resource().action();} which retains no other
+ * reference to this {@code Resource}. While probably overkill here,
+ * {@code reachabilityFence} is placed in a {@code finally} block to ensure
+ * that it is invoked across all paths in the method. In a method with more
+ * complex control paths, you might need further precautions to ensure that
+ * {@code reachabilityFence} is encountered along all of them.
+ *
+ * <p> It is sometimes possible to better encapsulate use of
+ * {@code reachabilityFence}. Continuing the above example, if it were
+ * acceptable for the call to method {@code update} to proceed even if the
+ * finalizer had already executed (nulling out slot), then you could
+ * localize use of {@code reachabilityFence}:
+ *
+ * <pre> {@code
+ * public void action2() {
+ * // ...
+ * Resource.update(getExternalResource());
+ * }
+ * private ExternalResource getExternalResource() {
+ * ExternalResource ext = externalResourceArray[myIndex];
+ * Reference.reachabilityFence(this);
+ * return ext;
+ * }}</pre>
+ *
+ * <p> Method {@code reachabilityFence} is not required in constructions
+ * that themselves ensure reachability. For example, because objects that
+ * are locked cannot, in general, be reclaimed, it would suffice if all
+ * accesses of the object, in all methods of class {@code Resource}
+ * (including {@code finalize}) were enclosed in {@code synchronized (this)}
+ * blocks. (Further, such blocks must not include infinite loops, or
+ * themselves be unreachable, which fall into the corner case exceptions to
+ * the "in general" disclaimer.) However, method {@code reachabilityFence}
+ * remains a better option in cases where this approach is not as efficient,
+ * desirable, or possible; for example because it would encounter deadlock.
+ *
+ * @param ref the reference. If {@code null}, this method has no effect.
+ * @since 9
+ */
+ @DontInline
+ public static void reachabilityFence(Object ref) {
+ // Does nothing, because this method is annotated with @DontInline
+ // HotSpot needs to retain the ref and not GC it before a call to this
+ // method
+ }
+}