Provides the system's ReferenceQueue implementation as well as
different forms of reference objects which impose special behavior on the
garbage collector. The behavior depends on the type of Reference
being used. Three different type of references exist, each being weaker
than the preceding one: SoftReference, WeakReference, and
PhantomReference. "Weakness" here means that less restrictions are
being imposed on the garbage collector as to when it is allowed to
actually garbage-collect the referenced object.
In order to use reference objects properly it is important to understand the different types of reachability that trigger their clearing and enqueueing. The following table lists these, from strongest to weakest. For each row, an object is said to have the reachability on the left side if (and only if) it fulfills all of the requirements on the right side. In all rows, consider the root set to be a set of references that are "resistant" to garbage collection (that is, running threads, method parameters, local variables, static fields and the like).
| Strongly reachable |
|
| Softly reachable |
|
| Weakly reachable |
|
| Phantom-reachable |
|
| PhantomReference<T> | Implements a phantom reference, which is the weakest of the three types of references. |
| Reference<T> | Provides an abstract class which describes behavior common to all reference objects. |
| ReferenceQueue<T> | The ReferenceQueue is the container on which reference objects are
enqueued when the garbage collector detects the reachability type specified
for the referent. |
| SoftReference<T> | A reference that is cleared when its referent is not strongly reachable and there is memory pressure. |
| WeakReference<T> | Implements a weak reference, which is the middle of the three types of references. |