Module java.base
Package 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 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`.

  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

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

    - ### Methods declared in class java.lang.ref.Reference

      `clear, clone, enqueue, isEnqueued, reachabilityFence`

    - ### Methods declared in class java.lang.Object

      `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`