PhantomReference (Java SE 26 & JDK 26)
Description: declaration: module: java.base, package: java.lang.ref, class: PhantomReference
Keywords: java.lang.ref.PhantomReference class

# 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 declared in 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 declared in 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:
      - `Reference.refersTo(T)`