Reference - JRE Emulation | JRE Emulation
public abstract class

Reference

extends Object
java.lang.Object
   ↳ java.lang.ref.Reference<T>
Known Direct Subclasses

Class Overview

Provides an abstract class which describes behavior common to all reference objects. It is not possible to create immediate subclasses of Reference in addition to the ones provided by this package. It is also not desirable to do so, since references require very close cooperation with the system's garbage collector. The existing, specialized reference classes should be used instead.

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
  • There exists at least one path from the root set to the object that does not traverse any instance of a java.lang.ref.Reference subclass.
Softly reachable
  • The object is not strongly reachable.
  • There exists at least one path from the root set to the object that does traverse a java.lang.ref.SoftReference instance, but no java.lang.ref.WeakReference or java.lang.ref.PhantomReference instances.
Weakly reachable
  • The object is neither strongly nor softly reachable.
  • There exists at least one path from the root set to the object that does traverse a java.lang.ref.WeakReference instance, but no java.lang.ref.PhantomReference instances.
Phantom-reachable
  • The object is neither strongly, softly, nor weakly reachable.
  • The object is referenced by a java.lang.ref.PhantomReference instance.
  • The object has already been finalized.

Summary

Public Methods
void clear()
Makes the referent null.
boolean enqueue()
Forces the reference object to be enqueued if it has been associated with a queue.
T get()
Returns the referent of the reference object.
boolean isEnqueued()
Checks whether the reference object has been enqueued.
Protected Methods
void finalize()
Invoked when the garbage collector has detected that this instance is no longer reachable.
Inherited Methods
[Expand]
From class java.lang.Object

Public Methods

public void clear ()

Makes the referent null. This does not force the reference object to be enqueued.

public boolean enqueue ()

Forces the reference object to be enqueued if it has been associated with a queue.

Returns
  • true if this call has caused the Reference to become enqueued, or false otherwise

public T get ()

Returns the referent of the reference object.

Returns
  • the referent to which reference refers, or null if the object has been cleared.

public boolean isEnqueued ()

Checks whether the reference object has been enqueued.

Returns
  • true if the Reference has been enqueued, false otherwise

Protected Methods

protected void finalize ()

Invoked when the garbage collector has detected that this instance is no longer reachable. The default implementation does nothing, but this method can be overridden to free resources.

Note that objects that override finalize are significantly more expensive than objects that don't. Finalizers may be run a long time after the object is no longer reachable, depending on memory pressure, so it's a bad idea to rely on them for cleanup. Note also that finalizers are run on a single VM-wide finalizer thread, so doing blocking work in a finalizer is a bad idea. A finalizer is usually only necessary for a class that has a native peer and needs to call a native method to destroy that peer. Even then, it's better to provide an explicit close method (and implement Closeable), and insist that callers manually dispose of instances. This works well for something like files, but less well for something like a BigInteger where typical calling code would have to deal with lots of temporaries. Unfortunately, code that creates lots of temporaries is the worst kind of code from the point of view of the single finalizer thread.

If you must use finalizers, consider at least providing your own ReferenceQueue and having your own thread process that queue.

Unlike constructors, finalizers are not automatically chained. You are responsible for calling super.finalize() yourself.

Uncaught exceptions thrown by finalizers are ignored and do not terminate the finalizer thread. See Effective Java Item 7, "Avoid finalizers" for more.