PhantomReference (Java SE 9 & JDK 9 ) (original) (raw)

• java.lang.Object

• • java.lang.ref.Reference
  • • java.lang.ref.PhantomReference

• ---

public class PhantomReference
extends Reference
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 inherited from class java.lang.[Object](../../../java/lang/Object.html "class in java.lang")  
     `[clone](../../../java/lang/Object.html#clone--), [equals](../../../java/lang/Object.html#equals-java.lang.Object-), [finalize](../../../java/lang/Object.html#finalize--), [getClass](../../../java/lang/Object.html#getClass--), [hashCode](../../../java/lang/Object.html#hashCode--), [notify](../../../java/lang/Object.html#notify--), [notifyAll](../../../java/lang/Object.html#notifyAll--), [toString](../../../java/lang/Object.html#toString--), [wait](../../../java/lang/Object.html#wait--), [wait](../../../java/lang/Object.html#wait-long-), [wait](../../../java/lang/Object.html#wait-long-int-)`  
     * ### Methods inherited from class java.lang.ref.[Reference](../../../java/lang/ref/Reference.html "class in java.lang.ref")  
     `[clear](../../../java/lang/ref/Reference.html#clear--), [enqueue](../../../java/lang/ref/Reference.html#enqueue--), [isEnqueued](../../../java/lang/ref/Reference.html#isEnqueued--), [reachabilityFence](../../../java/lang/ref/Reference.html#reachabilityFence-java.lang.Object-)`

• • Constructor Detail

     * #### PhantomReference  
     public PhantomReference​([T](../../../java/lang/ref/PhantomReference.html "type parameter in PhantomReference") referent,  
                             [ReferenceQueue](../../../java/lang/ref/ReferenceQueue.html "class in java.lang.ref")<? super [T](../../../java/lang/ref/PhantomReference.html "type parameter in PhantomReference")> 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](../../../java/lang/ref/PhantomReference.html "type parameter in PhantomReference") get​()  
    Returns this reference object's referent. Because the referent of a phantom reference is always inaccessible, this method always returns`null`.  
    Overrides:  
    `[get](../../../java/lang/ref/Reference.html#get--)` in class `[Reference](../../../java/lang/ref/Reference.html "class in java.lang.ref")<[T](../../../java/lang/ref/PhantomReference.html "type parameter in PhantomReference")>`  
    Returns:  
    `null`

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