Class PhantomReference<T>

java.lang.Object
java.lang.ref.Reference<T>
java.lang.ref.PhantomReference<T>
Type Parameters:
T - the type of the referent
public non-sealed class PhantomReference<T> extends Reference<T>
Phantom reference objects, which are enqueued after the collector determines that their referents may otherwise be reclaimed. Phantom references are most often used to schedule post-mortem cleanup actions.

Suppose the garbage collector determines at a certain point in time that an object is phantom reachable. At that time it will atomically clear all phantom references to that object and all phantom references to any other phantom-reachable objects from which that object is reachable. At the same time or at some later time it will enqueue those newly-cleared phantom references that are registered with reference queues.

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. The refersTo method can be used to test whether some object is the referent of a phantom reference.

Since:
1.2

  • Constructor Summary

    Constructors
    Constructor
    Description
    PhantomReference(T referent, ReferenceQueue<? super T> q)
    Creates a new phantom reference that refers to the given object and is registered with the given queue.

  • Method Summary

    Modifier and Type
    Method
    Description
    T
    get()
    Returns this reference object's referent.

    Methods inherited from class Reference

    clear, clone, enqueue, isEnqueued, reachabilityFence, refersTo
    Modifier and Type
    Method
    Description
    void
    clear()
    Clears this reference object.
    protected Object
    clone()
    Throws CloneNotSupportedException.
    boolean
    enqueue()
    Clears this reference object, then attempts to add it to the queue with which it is registered, if any.
    boolean
    isEnqueued()
    Deprecated.
    This method was originally specified to test if a reference object has been cleared and enqueued but was never implemented to do this test.
    static void
    reachabilityFence(Object ref)
    Ensures that the given object remains strongly reachable.
    final boolean
    refersTo(T obj)
    Tests if the referent of this reference object is obj.

    Methods inherited from class Object

    equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
    Modifier and Type
    Method
    Description
    boolean
    equals(Object obj)
    Indicates whether some other object is "equal to" this one.
    protected void
    finalize()
    Deprecated, for removal: This API element is subject to removal in a future version.
    Finalization is deprecated and subject to removal in a future release.
    final Class<?>
    getClass()
    Returns the runtime class of this Object.
    int
    hashCode()
    Returns a hash code value for this object.
    final void
    notify()
    Wakes up a single thread that is waiting on this object's monitor.
    final void
    notifyAll()
    Wakes up all threads that are waiting on this object's monitor.
    String
    toString()
    Returns a string representation of the object.
    final void
    wait()
    Causes the current thread to wait until it is awakened, typically by being notified or interrupted.
    final void
    wait(long timeoutMillis)
    Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.
    final void
    wait(long timeoutMillis, int nanos)
    Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.

  • Constructor Details

    • PhantomReference

      public PhantomReference(T referent, ReferenceQueue<? super T> q)
      Creates a new phantom reference that refers to the given object and is registered with the given queue.

      It is possible to create a phantom reference with a null queue. Such a reference will never be enqueued.

      Parameters:
      referent - the object the new phantom reference will refer to
      q - the queue with which the reference is to be registered, or null if registration is not required

  • Method Details

    • get

      public T get()
      Returns this reference object's referent. Because the referent of a phantom reference is always inaccessible, this method always returns null.
      Overrides:
      get in class Reference<T>
      Returns:
      null
      See Also: