Class Reference<T>
- Type Parameters:
T
- the type of the referent
- Direct Known Subclasses:
PhantomReference
,SoftReference
,WeakReference
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Method Summary
Modifier and TypeMethodDescriptionvoid
clear()
Clears this reference object.protected Object
clone()
ThrowsCloneNotSupportedException
.boolean
enqueue()
Clears this reference object, then attempts to add 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
reachabilityFence
(Object ref) Ensures that the given object remains strongly reachable.final boolean
Tests if the referent of this reference object isobj
.
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Method Details
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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. The
refersTo
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, or
null
if this reference object has been cleared - See Also:
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refersTo
Tests if the referent of this reference object isobj
. Using anull
obj
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
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clear
public void clear()Clears this reference object. Invoking this method does not enqueue this object, and the garbage collector will not clear or enqueue this object.When the garbage collector or the
enqueue()
method clear references they do so directly, without invoking this method.- API Note:
- There is a potential race condition with the garbage collector. When this
method is called, the garbage collector may already be in the process of
(or already completed) clearing and/or enqueueing this reference.
Avoid this race by ensuring the referent remains strongly reachable until
after the call to clear(), using
reachabilityFence(Object)
if necessary.
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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
or
enqueue()
is called; and - this reference object is not yet removed from the queue.
false
. 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).
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enqueue
public boolean enqueue()Clears this reference object, then attempts to add it to the queue with which it is registered, if any.If this reference is registered with a queue but not yet enqueued, the reference is added to the queue; this method is successful and returns true. If this reference is not registered with a queue, or was already enqueued (by the garbage collector, or a previous call to
enqueue
), this method is unsuccessful and returns false.Memory consistency effects: Actions in a thread prior to a successful call to
enqueue
happen-before the reference is removed from the queue byReferenceQueue.poll()
orReferenceQueue.remove(long)
. Unsuccessful calls toenqueue
have no specified memory consistency effects.When this method clears references it does so directly, without invoking the
clear()
method. When the garbage collector clears and enqueues references it does so directly, without invoking theclear()
method or this method.- API Note:
- Use of this method allows the registered queue's
ReferenceQueue.poll()
andReferenceQueue.remove(long)
methods to return this reference even though the referent may still be strongly reachable. - 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
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clone
ThrowsCloneNotSupportedException
. AReference
cannot be meaningfully cloned. Construct a newReference
instead.- Overrides:
clone
in classObject
- Returns:
- never returns normally
- Throws:
CloneNotSupportedException
- always- See Also:
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reachabilityFence
Ensures that the given object remains strongly reachable. This reachability is assured regardless of any optimizing transformations the virtual machine may perform that might otherwise allow the object to become unreachable (see JLS 12.6.1). Thus, the given object is not reclaimable by garbage collection at least until after the invocation of this method. References to the given object will not be cleared (or enqueued, if applicable) by the garbage collector until after invocation of this method. Invocation of this method does not itself initiate reference processing, 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 applicable only when reclamation may have visible effects, such as for objects that use finalizers or
Cleaner
, or code that performs reference processing.Memory consistency effects: Actions in a thread prior to calling
reachabilityFence(x)
happen-before the garbage collector clears any reference tox
.- API Note:
- Reference processing or finalization can occur after an object becomes
unreachable. An object can become unreachable when the virtual machine
detects that there is no further need for the object (other than for
running a finalizer). In the course of optimization, the virtual machine
can reorder operations of an object's methods such that the object
becomes unneeded earlier than might naively be expected —
including while a method of the object is still running. For instance,
the VM can move the loading of values from the object's fields
to occur earlier. The object itself is then no longer needed and becomes
unreachable, and the method can continue running using the obtained values.
This may have surprising and undesirable effects when using a Cleaner or
finalizer for cleanup: there is a race between the
program thread running the method, and the cleanup thread running the
Cleaner or finalizer. The cleanup thread could free a
resource, followed by the program thread (still running the method)
attempting to access the now-already-freed resource.
Use of
reachabilityFence
can prevent this race by ensuring that the object remains strongly reachable.The following is an example in which the bookkeeping associated with a class is managed through array indices. Here, method
action
uses areachabilityFence
to ensure that theResource
object is not reclaimed before bookkeeping on an associatedExternalResource
has been performed; specifically, 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(...) { this.myIndex = ... externalResourceArray[myIndex] = ...; ... } protected void finalize() { externalResourceArray[this.myIndex] = null; ... } public void action() { try { // ... int i = this.myIndex; // last use of 'this' Resource in action() Resource.update(externalResourceArray[i]); } finally { Reference.reachabilityFence(this); } } private static void update(ExternalResource ext) { ext.status = ...; } }
reachabilityFence
is placed after 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
. ThereachabilityFence
call is placed in afinally
block to ensure that it is invoked across all paths in the method. A more complex method might need further precautions to ensure thatreachabilityFence
is encountered along all code paths.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 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 synchronization is not as efficient, desirable, or possible; for example because it would encounter deadlock. - Parameters:
ref
- the reference to the object to keep strongly reachable. Ifnull
, this method has no effect.- Since:
- 9
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