Java tutorial
/* * Copyright (c) 1997, 2019, 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.ForceInline; import jdk.internal.HotSpotIntrinsicCandidate; import jdk.internal.access.JavaLangRefAccess; import jdk.internal.access.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> { /* The state of a Reference object is characterized by two attributes. It * may be either "active", "pending", or "inactive". It may also be * either "registered", "enqueued", "dequeued", or "unregistered". * * 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, the collector * "notifies" the reference, changing the state to either "pending" or * "inactive". * referent != null; discovered = null, or in GC discovered list. * * Pending: An element of the pending-Reference list, waiting to be * processed by the ReferenceHandler thread. The pending-Reference * list is linked through the discovered fields of references in the * list. * referent = null; discovered = next element in pending-Reference list. * * Inactive: Neither Active nor Pending. * referent = null. * * Registered: Associated with a queue when created, and not yet added * to the queue. * queue = the associated queue. * * Enqueued: Added to the associated queue, and not yet removed. * queue = ReferenceQueue.ENQUEUE; next = next entry in list, or this to * indicate end of list. * * Dequeued: Added to the associated queue and then removed. * queue = ReferenceQueue.NULL; next = this. * * Unregistered: Not associated with a queue when created. * queue = ReferenceQueue.NULL. * * The collector only needs to examine the referent field and the * discovered field to determine whether a (non-FinalReference) Reference * object needs special treatment. If the referent is non-null and not * known to be live, then it may need to be discovered for possible later * notification. But if the discovered field is non-null, then it has * already been discovered. * * FinalReference (which exists to support finalization) differs from * other references, because a FinalReference is not cleared when * notified. The referent being null or not cannot be used to distinguish * between the active state and pending or inactive states. However, * FinalReferences do not support enqueue(). Instead, the next field of a * FinalReference object is set to "this" when it is added to the * pending-Reference list. The use of "this" as the value of next in the * enqueued and dequeued states maintains the non-active state. An * additional check that the next field is null is required to determine * that a FinalReference object is active. * * Initial states: * [active/registered] * [active/unregistered] [1] * * Transitions: * clear * [active/registered] -------> [inactive/registered] * | | * | | enqueue [2] * | GC enqueue [2] | * | -----------------| * | | * v | * [pending/registered] --- v * | | ReferenceHandler * | enqueue [2] |---> [inactive/enqueued] * v | | * [pending/enqueued] --- | * | | poll/remove * | poll/remove | * | | * v ReferenceHandler v * [pending/dequeued] ------> [inactive/dequeued] * * * clear/enqueue/GC [3] * [active/unregistered] ------ * | | * | GC | * | |--> [inactive/unregistered] * v | * [pending/unregistered] ------ * ReferenceHandler * * Terminal states: * [inactive/dequeued] * [inactive/unregistered] * * Unreachable states (because enqueue also clears): * [active/enqeued] * [active/dequeued] * * [1] Unregistered is not permitted for FinalReferences. * * [2] These transitions are not possible for FinalReferences, making * [pending/enqueued] and [pending/dequeued] unreachable, and * [inactive/registered] terminal. * * [3] The garbage collector may directly transition a Reference * from [active/unregistered] to [inactive/unregistered], * bypassing the pending-Reference list. */ private T referent; /* Treated specially by GC */ /* The queue this reference gets enqueued to by GC notification or by * calling enqueue(). * * When registered: the queue with which this reference is registered. * enqueued: ReferenceQueue.ENQUEUE * dequeued: ReferenceQueue.NULL * unregistered: ReferenceQueue.NULL */ volatile ReferenceQueue<? super T> queue; /* The link in a ReferenceQueue's list of Reference objects. * * When registered: null * enqueued: next element in queue (or this if last) * dequeued: this (marking FinalReferences as inactive) * unregistered: null */ @SuppressWarnings("rawtypes") volatile Reference next; /* Used by the garbage collector to accumulate Reference objects that need * to be revisited in order to decide whether they should be notified. * Also used as the link in the pending-Reference list. The discovered * field and the next field are distinct to allow the enqueue() method to * be applied to a Reference object while it is either in the * pending-Reference list or in the garbage collector's discovered set. * * When active: null or next element in a discovered reference list * maintained by the GC (or this if last) * pending: next element in the pending-Reference list (null if last) * inactive: null */ private transient Reference<T> discovered; /* 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(); } } } /* * Atomically get and clear (set to null) the VM's pending-Reference list. */ private static native Reference<Object> getAndClearReferencePendingList(); /* * Test whether the VM's pending-Reference list contains any entries. */ private static native boolean hasReferencePendingList(); /* * Wait until the VM's pending-Reference 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(); } @Override public void runFinalization() { Finalizer.runFinalization(); } }); } /* -- 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() { this.referent = null; return this.queue.enqueue(this); } /** * Throws {@link CloneNotSupportedException}. A {@code Reference} cannot be * meaningfully cloned. Construct a new {@code Reference} instead. * * @return never returns normally * @throws CloneNotSupportedException always * * @since 11 */ @Override protected Object clone() throws CloneNotSupportedException { throw new CloneNotSupportedException(); } /* -- 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 Section 12.6 * of <cite>The Java™ Language Specification</cite>) 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 * @jls 12.6 Finalization of Class Instances */ @ForceInline public static void reachabilityFence(Object ref) { // Does nothing. This method is annotated with @ForceInline to eliminate // most of the overhead that using @DontInline would cause with the // HotSpot JVM, when this fence is used in a wide variety of situations. // HotSpot JVM retains the ref and does not GC it before a call to // this method, because the JIT-compilers do not have GC-only safepoints. } }