Reference

AI-generated Key Takeaways

• Reference is an abstract base class for reference objects providing common operations, and it cannot be directly subclassed due to its close interaction with the garbage collector.

• It offers methods to clear the reference, enqueue it to a registered queue, retrieve the referent object, and check if it has been enqueued.

• Three known direct subclasses are PhantomReference, SoftReference, and WeakReference, each with specific functionalities related to object reachability and garbage collection.

• The finalize method is invoked when the garbage collector detects the instance is unreachable, but it is recommended to avoid relying on finalizers for resource cleanup and instead provide explicit methods like close.

public abstract class Reference extends Object

Known Direct Subclasses PhantomReference<T>, SoftReference<T>, WeakReference<T> PhantomReference<T> Phantom reference objects, which are enqueued after the collector determines that their referents may otherwise be reclaimed.  SoftReference<T> Soft reference objects, which are cleared at the discretion of the garbage collector in response to memory demand.  WeakReference<T> Weak reference objects, which do not prevent their referents from being made finalizable, finalized, and then reclaimed.

Abstract base class for reference objects. This class defines the operations common to all reference objects. Because reference objects are implemented in close cooperation with the garbage collector, this class may not be subclassed directly.

Public Method Summary

void	clear() Clears this reference object.
boolean	enqueue() Adds this reference object to the queue with which it is registered, if any.
T	get() Returns this reference object's referent.
boolean	isEnqueued() Tells whether or not this reference object has been enqueued, either by the program or by the garbage collector.

Protected Method Summary

void	finalize() Invoked when the garbage collector has detected that this instance is no longer reachable.

Inherited Method Summary

From class java.lang.Object

Object	clone() Creates and returns a copy of this Object.
boolean	equals(Object obj) Compares this instance with the specified object and indicates if they are equal.
void	finalize() Invoked when the garbage collector has detected that this instance is no longer reachable.
final Class<?>	getClass() Returns the unique instance of Class that represents this object's class.
int	hashCode() Returns an integer hash code for this object.
final void	notify() Causes a thread which is waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
final void	notifyAll() Causes all threads which are waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
String	toString() Returns a string containing a concise, human-readable description of this object.
final void	wait(long timeout, int nanos) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait(long timeout) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait() Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object.

Public Methods

public void clear ()

Clears this reference object. Invoking this method will not cause this object to be enqueued.

This method is invoked only by Java code; when the garbage collector clears references it does so directly, without invoking this method.

public boolean enqueue ()

Adds this reference object to the queue with which it is registered, if any.

This method is invoked only by Java code; when the garbage collector enqueues references it does so directly, without invoking this method.

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

public T 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 returns null.

Returns

• The object to which this reference refers, or null if this reference object has been cleared

public boolean isEnqueued ()

Tells whether or not this reference object has been enqueued, either by the program or by the garbage collector. If this reference object was not registered with a queue when it was created, then this method will always return false.

Returns

• true if and only if this reference object has been enqueued

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.