java.lang.ref.PhantomReference Maven / Gradle / Ivy

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package java.lang.ref;

/** 
 * Phantom reference objects, which are enqueued after the collector
 * determines that their referents may otherwise be reclaimed.  Phantom
 * references are most often used for scheduling pre-mortem cleanup actions in
 * a more flexible way than is possible with the Java finalization mechanism.
 *
 *  If the garbage collector determines at a certain point in time that the
 * referent of a phantom reference is phantom reachable, then at that
 * time or at some later time it will enqueue the reference.
 *
 * 
 In order to ensure that a reclaimable object remains so, the referent of
 * a phantom reference may not be retrieved: The get method of a
 * phantom reference always returns null.
 *
 *  Unlike soft and weak references, phantom references are not
 * automatically cleared by the garbage collector as they are enqueued.  An
 * object that is reachable via phantom references will remain so until all
 * such references are cleared or themselves become unreachable.
 *
 * @version  1.12, 02/02/00
 * @author   Mark Reinhold
 * @since    1.2
 */
public class PhantomReference extends Reference
{

    /** 
     * Creates a new phantom reference that refers to the given object and
     * is registered with the given queue.
     *
     * @param referent the object the new phantom reference will refer to
     * @param q queue the phantom reference is registered with
     * @throws  NullPointerException  If the queue argument
     *                                is null
     */
    public PhantomReference(Object referent, ReferenceQueue q) { 
	super(referent, q);    
    }

    /** 
     * Returns this reference object's referent.  Because the referent of a
     * phantom reference is always inaccessible, this method always returns
     * null.
     *
     * @return  null
     */
    public Object get() {
        return null;
    }
}