Module java.base
Package java.lang.ref

Class Reference<T>

java.lang.Object
java.lang.ref.Reference<T>
Type Parameters:
T - the type of the referent
Direct Known Subclasses:
PhantomReference, SoftReference, WeakReference
public abstract sealed class Reference<T> extends Object permits PhantomReference<T>, SoftReference<T>, WeakReference<T> (not exhaustive)
Abstract base class for reference objects. This class defines the operations common to all reference objects. Because reference objects are implemented in close cooperation with the garbage collector, this class may not be subclassed directly.
Since:
1.2

  • Method Summary

    Modifier and Type
    Method
    Description
    void
    clear()
    Clears this reference object.
    protected Object
    clone()
    Throws CloneNotSupportedException.
    boolean
    enqueue()
    Clears this reference object and adds it to the queue with which it is registered, if any.
    T
    get()
    Returns this reference object's referent.
    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 object referenced by the given reference remains strongly reachable, regardless of any prior actions of the program that might otherwise cause the object to become unreachable; thus, the referenced object is not reclaimable by garbage collection at least until after the invocation of this method.
    final boolean
    refersTo(T obj)
    Tests if the referent of this reference object is obj.

    Methods inherited from class java.lang.Object

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

  • Method Details

    • get

      public T get()
      Returns this reference object's referent. If this reference object has been cleared, either by the program or by the garbage collector, then this method returns null.
      Returns:
      The object to which this reference refers, or null if this reference object has been cleared
      See Also:

    • refersTo

      public final boolean refersTo(T obj)
      Tests if the referent of this reference object is obj. Using a null obj returns true if the reference object has been cleared.
      Parameters:
      obj - the object to compare with this reference object's referent
      Returns:
      true if obj is the referent of this reference object
      Since:
      16

    • clear

      public void clear()
      Clears this reference object. Invoking this method will not cause this object to be enqueued.

      This method is invoked only by Java code; when the garbage collector clears references it does so directly, without invoking this method.

    • isEnqueued

      @Deprecated(since="16") public 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. This method could be misused due to the inherent race condition or without an associated ReferenceQueue. An application relying on this method to release critical resources could cause serious performance issue. An application should use ReferenceQueue to reliably determine what reference objects that have been enqueued or refersTo(null) to determine if this reference object has been cleared.
      Tests if this reference object is in its associated queue, if any. This method returns true only if all of the following conditions are met:
      • this reference object was registered with a queue when it was created; and
      • the garbage collector has added this reference object to the queue or enqueue() is called; and
      • this reference object is not yet removed from the queue.
      Otherwise, this method returns false. This method may return false if this reference object has been cleared but not enqueued due to the race condition.
      Returns:
      true if and only if this reference object is in its associated queue (if any).

    • enqueue

      public boolean enqueue()
      Clears this reference object and adds it to the queue with which it is registered, if any.

      This method is invoked only by Java code; when the garbage collector enqueues references it does so directly, without invoking this method.

      Returns:
      true if this reference object was successfully enqueued; false if it was already enqueued or if it was not registered with a queue when it was created

    • clone

      protected Object clone() throws CloneNotSupportedException
      Throws CloneNotSupportedException. A Reference cannot be meaningfully cloned. Construct a new Reference instead.
      Overrides:
      clone in class Object
      Returns:
      never returns normally
      Throws:
      CloneNotSupportedException - always
      Since:
      11
      See Also:

    • reachabilityFence

      public static void reachabilityFence(Object ref)
      Ensures that the object referenced by the given reference remains strongly reachable, regardless of any prior actions of the program that might otherwise cause the object to become unreachable; thus, the referenced object is not reclaimable by garbage collection at least until after the invocation of this method. Invocation of this method does not itself initiate garbage collection or finalization.

      This method establishes an ordering for strong reachability with respect to garbage collection. It controls relations that are otherwise only implicit in a program -- the reachability conditions triggering garbage collection. This method is designed for use in uncommon situations of premature finalization where using synchronized blocks or methods, or using other synchronization facilities are not possible or do not provide the desired control. This method is applicable only when reclamation may have visible effects, which is possible for objects with finalizers (See Section of The Java Language Specification) that are implemented in ways that rely on ordering control for correctness.

      Parameters:
      ref - the reference. If null, this method has no effect.
      Since:
      9