Reference.java

/* java.lang.ref.Reference
   Copyright (C) 1999, 2002, 2003, 2004, 2006 Free Software Foundation, Inc.

This file is part of GNU Classpath.

GNU Classpath is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
General Public License for more details.

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02110-1301 USA.

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permission to link this library with independent modules to produce an
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this exception to your version of the library, but you are not
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exception statement from your version. */


package java.lang.ref;

/**
 * This is the base class of all references.  A reference allows
 * refering to an object without preventing the garbage collector to
 * collect it.  The only way to get the referred object is via the
 * <code>get()</code>-method.  This method will return
 * <code>null</code> if the object was collected. <br>
 *
 * A reference may be registered with a queue.  When a referred
 * element gets collected the reference will be put on the queue, so
 * that you will be notified. <br>
 *
 * There are currently three types of references:  soft reference,
 * weak reference and phantom reference. <br>
 *
 * Soft references will be cleared if the garbage collector is told
 * to free some memory and there are no unreferenced or weakly referenced
 * objects.  It is useful for caches. <br>
 *
 * Weak references will be cleared as soon as the garbage collector
 * determines that the refered object is only weakly reachable.  They
 * are useful as keys in hashtables (see <code>WeakHashtable</code>) as
 * you get notified when nobody has the key anymore.
 *
 * Phantom references don't prevent finalization.  If an object is only
 * phantom reachable, it will be finalized, and the reference will be
 * enqueued, but not cleared.  Since you mustn't access an finalized
 * object, the <code>get</code> method of a phantom reference will never
 * work.  It is useful to keep track, when an object is finalized.
 *
 * @author Jochen Hoenicke
 * @see java.util.WeakHashMap
 */
public abstract class Reference<T>
{
  /**
   * The underlying object.  This field is handled in a special way by
   * the garbage collector.
   */
  T referent;

  /**
   * The queue this reference is registered on. This is null, if this
   * wasn't registered to any queue or reference was already enqueued.
   */
  volatile ReferenceQueue<? super T> queue;

  /**
   * Link to the next entry on the queue.  If this is null, this
   * reference is not enqueued.  Otherwise it points to the next
   * reference.  The last reference on a queue will point to itself
   * (not to null, that value is used to mark a not enqueued
   * reference).
   */
  volatile Reference nextOnQueue;

  /**
   * This lock should be taken by the garbage collector, before
   * determining reachability.  It will prevent the get()-method to
   * return the reference so that reachability doesn't change.
   */
  static Object lock = new Object();

  /**
   * Creates a new reference that is not registered to any queue.
   * Since it is package private, it is not possible to overload this
   * class in a different package.
   * @param ref the object we refer to.
   */
  Reference(T ref)
  {
    referent = ref;
  }

  /**
   * Creates a reference that is registered to a queue.  Since this is
   * package private, it is not possible to overload this class in a
   * different package.
   * @param ref the object we refer to.
   * @param q the reference queue to register on.
   * @exception NullPointerException if q is null.
   */
  Reference(T ref, ReferenceQueue<? super T> q)
  {
    if (q == null)
      throw new NullPointerException();
    referent = ref;
    queue = q;
  }

  /**
   * Returns the object, this reference refers to.
   * @return the object, this reference refers to, or null if the
   * reference was cleared.
   */
  public T get()
  {
    synchronized (lock)
      {
        return referent;
      }
  }

  /**
   * Clears the reference, so that it doesn't refer to its object
   * anymore.  For soft and weak references this is called by the
   * garbage collector.  For phantom references you should call
   * this when enqueuing the reference.
   */
  public void clear()
  {
    referent = null;
  }

  /**
   * Tells if the object is enqueued on a reference queue.
   * @return true if it is enqueued, false otherwise.
   */
  public boolean isEnqueued()
  {
    return nextOnQueue != null;
  }

  /**
   * Enqueue an object on a reference queue.  This is normally executed
   * by the garbage collector.
   */
  public boolean enqueue()
  {
    ReferenceQueue q = queue;
    if (q != null)
      {
        return q.enqueue(this);
      }
    return false;
  }
}