| /* |
| * Copyright (c) 1997, 2018, 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.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> { |
| |
| /* 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. |
| * |
| * @returns 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 |
| * <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 |
| */ |
| @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. |
| } |
| } |
