The java.lang.ref package defines classes that allow Java programs to interact with the Java garbage collector. A Reference represents an indirect reference to an arbitrary object, known as the referent. SoftReference, WeakReference, and PhantomReference are three concrete subclasses of Reference that interact with the garbage collector in different ways, as explained in the individual class descriptions that follow. ReferenceQueue represents a linked list of Reference objects. Any Reference object may have a ReferenceQueue associated with it. A Reference object is enqueued on its ReferenceQueue at some point after the garbage collector determines that the referent object has become appropriately unreachable. (The exact level of unreachability depends on the type of Reference being used.) An application can monitor a ReferenceQueue to determine when referent objects enter a new reachability status.

Using the mechanisms defined in this package, you can implement a cache that grows and shrinks in size according to the amount of available system memory. Or, you can implement a hashtable that associates auxiliary information with arbitrary objects, but does not prevent those objects from being garbage-collected if they are otherwise unused. The mechanisms provided by this package are low-level ones, however, and typical applications do not use java.lang.ref directly. Instead, they rely on higher-level utilities built on top of the package. See java.util.WeakHashMap for one example.

In Java 5.0, the classes in this package have all been made into generic types. The type variable T represents the type of the object that is referred to.

Classes

public abstract class Reference<T>;
   public class PhantomReference<T> extends Reference<T>;
   public class SoftReference<T> extends Reference<T>;
   public class WeakReference<T> extends Reference<T>;
public class ReferenceQueue<T>;