/* java.lang.ref.Reference
Copyright (C) 1999, 2002, 2003 Free Software Foundation, Inc.
This file is part of GNU Classpath.
GNU Classpath is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
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Linking this library statically or dynamically with other modules is
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permission to link this library with independent modules to produce an
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package java.lang.ref;
/**
* This is the base class of all references. A reference allows
* refering to an object without preventing the garbage collector to
* collect it. The only way to get the referred object is via the
* <code>get()</code>-method. This method will return
* <code>null</code> if the object was collected. <br>
*
* A reference may be registered with a queue. When a referred
* element gets collected the reference will be put on the queue, so
* that you will be notified. <br>
*
* There are currently three types of references: soft reference,
* weak reference and phantom reference. <br>
*
* Soft references will be cleared if the garbage collector is told
* to free some memory and there are no unreferenced or weakly referenced
* objects. It is useful for caches. <br>
*
* Weak references will be cleared as soon as the garbage collector
* determines that the refered object is only weakly reachable. They
* are useful as keys in hashtables (see <code>WeakHashtable</code>) as
* you get notified when nobody has the key anymore.
*
* Phantom references don't prevent finalization. If an object is only
* phantom reachable, it will be finalized, and the reference will be
* enqueued, but not cleared. Since you mustn't access an finalized
* object, the <code>get</code> method of a phantom reference will never
* work. It is useful to keep track, when an object is finalized.
*
* @author Jochen Hoenicke
* @see java.util.WeakHashtable
*/
public abstract class Reference
{
/**
* The underlying object. This field is handled in a special way by
* the garbage collector.
*/
Object referent;
/**
* The queue this reference is registered on. This is null, if this
* wasn't registered to any queue or reference was already enqueued.
*/
ReferenceQueue queue;
/**
* Link to the next entry on the queue. If this is null, this
* reference is not enqueued. Otherwise it points to the next
* reference. The last reference on a queue will point to itself
* (not to null, that value is used to mark a not enqueued
* reference).
*/
Reference nextOnQueue;
/**
* This lock should be taken by the garbage collector, before
* determining reachability. It will prevent the get()-method to
* return the reference so that reachability doesn't change.
*/
static Object lock = new Object();
/**
* Creates a new reference that is not registered to any queue.
* Since it is package private, it is not possible to overload this
* class in a different package.
* @param referent the object we refer to.
*/
Reference(Object ref)
{
referent = ref;
}
/**
* Creates a reference that is registered to a queue. Since this is
* package private, it is not possible to overload this class in a
* different package.
* @param referent the object we refer to.
* @param q the reference queue to register on.
* @exception NullPointerException if q is null.
*/
Reference(Object ref, ReferenceQueue q)
{
if (q == null)
throw new NullPointerException();
referent = ref;
queue = q;
}
/**
* Returns the object, this reference refers to.
* @return the object, this reference refers to, or null if the
* reference was cleared.
*/
public Object get()
{
synchronized (lock)
{
return referent;
}
}
/**
* Clears the reference, so that it doesn't refer to its object
* anymore. For soft and weak references this is called by the
* garbage collector. For phantom references you should call
* this when enqueuing the reference.
*/
public void clear()
{
referent = null;
}
/**
* Tells if the object is enqueued on a reference queue.
* @return true if it is enqueued, false otherwise.
*/
public boolean isEnqueued()
{
return nextOnQueue != null;
}
/**
* Enqueue an object on a reference queue. This is normally executed
* by the garbage collector.
*/
public boolean enqueue()
{
if (queue != null && nextOnQueue == null)
{
queue.enqueue(this);
queue = null;
return true;
}
return false;
}
}