Method Summary

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 returnsnull.

  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. TherefersTo method can be used to avoid such strengthening when testing whether some object is the referent of a reference object; that is, useref.refersTo(obj) rather thanref.get() == obj.

  Returns:

  The object to which this reference refers, ornull 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 isobj. Using anullobj returnstrue if the reference object has been cleared.

  Parameters:

  obj - the object to compare with this reference object's referent

  Returns:

  true ifobj 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 associatedReferenceQueue. An application relying on this method to release critical resources could cause serious performance issue. An application should useReferenceQueue to reliably determine what reference objects that have been enqueued orrefersTo(null) to determine if this reference object has been cleared.

  Tests if this reference object is in its associated queue, if any. This method returnstrue 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 orenqueue() is called; and
  • this reference object is not yet removed from the queue.

  Otherwise, this method returnsfalse. This method may returnfalse 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() throwsCloneNotSupportedException

  ThrowsCloneNotSupportedException. AReference cannot be meaningfully cloned. Construct a newReference 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 remainsstrongly 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 forstrong 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 usingsynchronized 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 Section12.6 ofThe 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, methodaction uses areachabilityFence to ensure that theResource object is not reclaimed before bookkeeping on an associatedExternalResource has been performed; in particular here, to ensure that the array slot holding theExternalResource is not nulled out in methodObject.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 ofreachabilityFence is nonintuitively placedafter the call toupdate, to ensure that the array slot is not nulled out byObject.finalize() before the update, even if the call toaction was the last use of this object. This might be the case if, for example a usage in a user program had the formnew Resource().action(); which retains no other reference to thisResource. While probably overkill here,reachabilityFence is placed in afinally 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 thatreachabilityFence is encountered along all of them.

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

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

  MethodreachabilityFence 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 classResource (includingfinalize) were enclosed insynchronized (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, methodreachabilityFence 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. Ifnull, this method has no effect.

  SeeJava Language Specification:

  12.6 Finalization of Class Instances
  

  Since:

  9