Module java.base
Package java.lang.ref

Class Reference<T>

java.lang.Object
java.lang.ref.Reference<T>
Direct Known Subclasses:
PhantomReference, SoftReference, WeakReference

public abstract class Reference<T> extends Object
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
    Clears this reference object.
    protected Object
    boolean
    Clears this reference object and adds it to the queue with which it is registered, if any.
    get()
    Returns this reference object's referent.
    boolean
    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
    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 declared in 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.
      API Note:
      This method returns a strong reference to the referent. This may cause the garbage collector to treat it as strongly reachable until some later collection cycle. The refersTo method can be used to avoid such strengthening when testing whether some object is the referent of a reference object; that is, use ref.refersTo(obj) rather than ref.get() == obj.
      Returns:
      The object to which this reference refers, or null if this reference object has been cleared
      See Also:
      refersTo(T)
    • 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:
      Cloneable
    • 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 12.6 of The Java Language Specification) that are implemented in ways that rely on ordering control for correctness.

      API Note:
      Finalization may occur whenever the virtual machine detects that no reference to an object will ever be stored in the heap: The garbage collector may reclaim an object even if the fields of that object are still in use, so long as the object has otherwise become unreachable. This may have surprising and undesirable effects in cases such as the following example in which the bookkeeping associated with a class is managed through array indices. Here, method action uses a reachabilityFence to ensure that the Resource object is not reclaimed before bookkeeping on an associated ExternalResource has been performed; in particular here, to ensure that the array slot holding the ExternalResource is not nulled out in method Object.finalize(), which may otherwise run concurrently.
       
       class Resource {
         private static ExternalResource[] externalResourceArray = ...
      
         int myIndex;
         Resource(...) {
           myIndex = ...
           externalResourceArray[myIndex] = ...;
           ...
         }
         protected void finalize() {
           externalResourceArray[myIndex] = null;
           ...
         }
         public void action() {
           try {
             // ...
             int i = myIndex;
             Resource.update(externalResourceArray[i]);
           } finally {
             Reference.reachabilityFence(this);
           }
         }
         private static void update(ExternalResource ext) {
           ext.status = ...;
         }
       }
      Here, the invocation of reachabilityFence is nonintuitively placed after the call to update, to ensure that the array slot is not nulled out by Object.finalize() before the update, even if the call to action was the last use of this object. This might be the case if, for example a usage in a user program had the form new Resource().action(); which retains no other reference to this Resource. While probably overkill here, reachabilityFence is placed in a finally block to ensure that it is invoked across all paths in the method. In a method with more complex control paths, you might need further precautions to ensure that reachabilityFence is encountered along all of them.

      It is sometimes possible to better encapsulate use of reachabilityFence. Continuing the above example, if it were acceptable for the call to method update to proceed even if the finalizer had already executed (nulling out slot), then you could localize use of reachabilityFence:

       
       public void action2() {
         // ...
         Resource.update(getExternalResource());
       }
       private ExternalResource getExternalResource() {
         ExternalResource ext = externalResourceArray[myIndex];
         Reference.reachabilityFence(this);
         return ext;
       }

      Method reachabilityFence is not required in constructions that themselves ensure reachability. For example, because objects that are locked cannot, in general, be reclaimed, it would suffice if all accesses of the object, in all methods of class Resource (including finalize) were enclosed in synchronized (this) blocks. (Further, such blocks must not include infinite loops, or themselves be unreachable, which fall into the corner case exceptions to the "in general" disclaimer.) However, method reachabilityFence remains a better option in cases where this approach is not as efficient, desirable, or possible; for example because it would encounter deadlock.

      Parameters:
      ref - the reference. If null, this method has no effect.
      See Java Language Specification:
      12.6 Finalization of Class Instances
      Since:
      9