Touring Language APIs - Beginning Java 7 - page 335

reference to this object is a weak reference. When the weak references to a
weakly reachable object are cleared, the object becomes eligible for finaliza-
tion.(Apartfromthegarbagecollectorbeingmoreeagertocleanuptheweakly
reachable object, a weak reference is exactly like a soft reference.)
• An object is phantom reachable if it is neither strongly, softly, nor weakly
reachable,ithasbeenfinalized,anditisreferredtobysome phantom reference
(areferencetotheobjectwherethereferenceisstoredina PhantomRefer-
ence object). The strongest reference to this object is a phantom reference.
• Finally, an object is unreachable, and therefore eligible for removal from
memory during the next garbage collection cycle, when it is not reachable in
any of the above ways.
Theobjectwhosereferenceisstoredina SoftReference , WeakReference ,or
PhantomReference object is known as a referent .
Reference and ReferenceQueue
TheReference APIconsistsoffiveclasseslocatedinthe java.lang.ref package.
Central to this package are Reference and ReferenceQueue .
Reference istheabstractsuperclassofthispackage'sconcrete SoftReference ,
WeakReference , and PhantomReference subclasses.
ReferenceQueue is a concrete class whose instances describe queue data struc-
tures.Whenyouassociatea ReferenceQueue instancewitha Reference subclass
object ( Reference object, for short), the Reference object is added to the queue
when the referent to which its encapsulated reference refers becomes garbage.
Note You associate a ReferenceQueue object with a Reference object by
passing the ReferenceQueue object to an appropriate Reference subclass con-
structor.
Reference is declared as generic type Reference<T> , where T identifies the
referent's type. This class provides the following methods:
• void clear() assignsnulltothestoredreference;the Reference object
onwhichthismethodiscalledisnot enqueued (inserted)intoitsassociatedref-
erence queue (ifthere isanassociated reference queue). (The garbage collect-
orclearsreferencesdirectly;itdoesnotcall clear() .Instead,thismethodis
called by applications.)