PhantomReference (Java Platform SE 8 )
Keywords: java.lang.ref.PhantomReference class
Date: 2018-03-02

compact1, compact2, compact3
java.lang.ref

## Class PhantomReference<T>

- java.lang.Object
- - java.lang.ref.Reference<T>
  - - java.lang.ref.PhantomReference<T>

-     public 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 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.

  Since:
    1.2

- - ### Constructor Summary

    Constructors
    Constructor and 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

    All Methods Instance Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `T` | `get()` Returns this reference object's referent.

    - ### Methods inherited from class java.lang.ref.Reference

      `clear, enqueue, isEnqueued`

    - ### Methods inherited from class java.lang.Object

      `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- - ### Constructor Detail

    - #### 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, but such a reference is completely useless: Its `get` method will always return null and, since it does not have a queue, it 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 Detail

    - #### 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`

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