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   * Copyright (C) 2014 The Android Open Source Project
   * 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
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   */
  
  package java.lang.ref;
  
  import dalvik.annotation.optimization.FastNative;
  
  
  /**
   * 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> {
      // BEGIN Android-changed: Reimplemented to accommodate a different GC and compiler.
      // ClassLinker knows about the fields of this class.
  
      /**
       * Forces JNI path.
       * If GC is not in progress (ie: not going through slow path), the referent
       * can be quickly returned through intrinsic without passing through JNI.
       * This flag forces the JNI path so that it can be tested and benchmarked.
       */
      private static boolean disableIntrinsic = false;
  
      /**
       * Slow path flag for the reference processor.
       * Used by the reference processor to determine whether or not the referent
       * can be immediately returned. Because the referent might get swept during
       * GC, the slow path, which passes through JNI, must be taken.
       */
      private static boolean slowPathEnabled = false;
  
      // Treated specially by GC. ART's ClassLinker::LinkFields() knows this is the
      // alphabetically last non-static field.
      volatile T referent;
  
      final ReferenceQueue<? super T> queue;
  
      /*
       * This field forms a singly-linked list of reference objects that have
       * been enqueued. The queueNext field is non-null if and only if this
       * reference has been enqueued. After this reference has been enqueued and
       * before it has been removed from its queue, the queueNext field points
       * to the next reference on the queue. The last reference on a queue
       * points to itself. Once this reference has been removed from the
       * reference queue, the queueNext field points to the
       * ReferenceQueue.sQueueNextUnenqueued sentinel reference object for the
       * rest of this reference's lifetime.
       * <p>
       * Access to the queueNext field is guarded by synchronization on a lock
       * internal to 'queue'.
       */
      Reference queueNext;
  
      /**
       * The pendingNext field is initially set by the GC. After the GC forms a
       * complete circularly linked list, the list is handed off to the
       * ReferenceQueueDaemon using the ReferenceQueue.class lock. The
       * ReferenceQueueDaemon can then read the pendingNext fields without
       * additional synchronization.
       */
      Reference<?> pendingNext;
  
      /* -- 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
       */
      public T get() {
          return getReferent();
      }
  
      @FastNative
      private final native T getReferent();
  
      /**
       * 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() {
          clearReferent();
      }
  
      // Direct access to the referent is prohibited, clearReferent blocks and set
      // the referent to null when it is safe to do so.
      @FastNative
      native void clearReferent();
  
      /* -- 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() {
          // Contrary to what the documentation says, this method returns false
          // after this reference object has been removed from its queue
          // (b/26647823). ReferenceQueue.isEnqueued preserves this historically
          // incorrect behavior.
          return queue != null && queue.isEnqueued(this);
      }
  
      /**
       * Adds this reference object 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() {
         return queue != null && queue.enqueue(this);
      }
  
      /* -- Constructors -- */
  
      Reference(T referent) {
          this(referent, null);
      }
  
      Reference(T referent, ReferenceQueue<? super T> queue) {
          this.referent = referent;
          this.queue = queue;
      }
      // END Android-changed: Reimplemented to accommodate a different GC and compiler.
  
      // BEGIN Android-added: reachabilityFence() from upstream OpenJDK9+181.
      // The actual implementation differs from OpenJDK9.
      /**
       * 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
       */
      // @DontInline
      public static void reachabilityFence(Object ref) {
          // This code is usually replaced by much faster intrinsic implementations.
          // It will be executed for tests run with the access checks interpreter in
          // ART, e.g. with --verify-soft-fail.  Since this is a volatile store, it
          // cannot easily be moved up past prior accesses, even if this method is
          // inlined.
          SinkHolder.sink = ref;
          // Leaving SinkHolder set to ref is unpleasant, since it keeps ref live
          // until the next reachabilityFence call. This causes e.g. 036-finalizer
          // to fail. Clear it again in a way that's unlikely to be optimizable.
          // The fact that finalize_count is volatile makes it hard to move the test up.
          if (SinkHolder.finalize_count == 0) {
              SinkHolder.sink = null;
          }
      }
  
      private static class SinkHolder {
          static volatile Object sink;
  
          // Ensure that sink looks live to even a reasonably clever compiler.
          private static volatile int finalize_count = 0;
  
          private static Object sinkUser = new Object() {
              protected void finalize() {
                  if (sink == null && finalize_count > 0) {
                      throw new AssertionError("Can't get here");
                  }
                  finalize_count++;
              }
          };
      }
      // END Android-added: reachabilityFence() from upstream OpenJDK9+181.
  }
  