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 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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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&trade; 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.
 }
}