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/* * Copyright (c) 1997, 2017, Oracle and/or its affiliates. All rights reserved. * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. * * * * * * * * * * * * * * * * * * * * */ package java.lang.ref; import sun.misc.Cleaner; import sun.misc.JavaLangRefAccess; import sun.misc.SharedSecrets; /** * 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 { /* 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 */ transient private Reference discovered; /* used by VM */ /* Object used to synchronize with the garbage collector. The collector * must acquire this lock at the beginning of each collection cycle. It is * therefore critical that any code holding this lock complete as quickly * as possible, allocate no new objects, and avoid calling user code. */ static private class Lock { } private static Lock lock = new Lock(); /* List of References waiting to be enqueued. The collector adds * References to this list, while the Reference-handler thread removes * them. This list is protected by the above lock object. The * list uses the discovered field to link its elements. */ private static Reference pending = null; /* 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 InterruptedException and Cleaner classes // so that we don't get into trouble later in the run loop if there's // memory shortage while loading/initializing them lazily. ensureClassInitialized(InterruptedException.class); ensureClassInitialized(Cleaner.class); } ReferenceHandler(ThreadGroup g, String name) { super(g, name); } public void run() { while (true) { tryHandlePending(true); } } } /** * Try handle pending {@link Reference} if there is one.

* Return {@code true} as a hint that there might be another * {@link Reference} pending or {@code false} when there are no more pending * {@link Reference}s at the moment and the program can do some other * useful work instead of looping. * * @param waitForNotify if {@code true} and there was no pending * {@link Reference}, wait until notified from VM * or interrupted; if {@code false}, return immediately * when there is no pending {@link Reference}. * @return {@code true} if there was a {@link Reference} pending and it * was processed, or we waited for notification and either got it * or thread was interrupted before being notified; * {@code false} otherwise. */ static boolean tryHandlePending(boolean waitForNotify) { Reference r; Cleaner c; try { synchronized (lock) { if (pending != null) { r = pending; // 'instanceof' might throw OutOfMemoryError sometimes // so do this before un-linking 'r' from the 'pending' chain... c = r instanceof Cleaner ? (Cleaner) r : null; // unlink 'r' from 'pending' chain pending = r.discovered; r.discovered = null; } else { // The waiting on the lock may cause an OutOfMemoryError // because it may try to allocate exception objects. if (waitForNotify) { lock.wait(); } // retry if waited return waitForNotify; } } } catch (OutOfMemoryError x) { // Give other threads CPU time so they hopefully drop some live references // and GC reclaims some space. // Also prevent CPU intensive spinning in case 'r instanceof Cleaner' above // persistently throws OOME for some time... Thread.yield(); // retry return true; } catch (InterruptedException x) { // retry return true; } // Fast path for cleaners if (c != null) { c.clean(); return true; } ReferenceQueue super Object> q = r.queue; if (q != ReferenceQueue.NULL) q.enqueue(r); return true; } 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 tryHandlePendingReference() { return tryHandlePending(false); } }); } /* -- 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 null. * * @return The object to which this reference refers, or * null if this reference object has been cleared */ public T get() { return this.referent; } /** * Clears this reference object. Invoking this method will not cause this * object to be enqueued. * *

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 false. * * @return true if and only if this reference object has * been enqueued */ public boolean isEnqueued() { return (this.queue == ReferenceQueue.ENQUEUED); } /** * Adds this reference object to the queue with which it is registered, * if any. * *

This method is invoked only by Java code; when the garbage collector * enqueues references it does so directly, without invoking this method. * * @return true if this reference object was successfully * enqueued; false if it was already enqueued or if * it was not registered with a queue when it was created */ public boolean enqueue() { 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; } }