205 lines
6.7 KiB
Java
205 lines
6.7 KiB
Java
/* java.lang.ref.Reference
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Copyright (C) 1999, 2002, 2003, 2006 Free Software Foundation, Inc.
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This file is part of GNU Classpath.
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GNU Classpath is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 2, or (at your option)
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any later version.
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GNU Classpath is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with GNU Classpath; see the file COPYING. If not, write to the
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Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
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02110-1301 USA.
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Linking this library statically or dynamically with other modules is
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making a combined work based on this library. Thus, the terms and
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conditions of the GNU General Public License cover the whole
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combination.
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As a special exception, the copyright holders of this library give you
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permission to link this library with independent modules to produce an
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executable, regardless of the license terms of these independent
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modules, and to copy and distribute the resulting executable under
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terms of your choice, provided that you also meet, for each linked
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independent module, the terms and conditions of the license of that
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module. An independent module is a module which is not derived from
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or based on this library. If you modify this library, you may extend
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this exception to your version of the library, but you are not
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obligated to do so. If you do not wish to do so, delete this
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exception statement from your version. */
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package java.lang.ref;
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/**
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* This is the base class of all references. A reference allows
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* refering to an object without preventing the garbage collector to
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* collect it. The only way to get the referred object is via the
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* <code>get()</code>-method. This method will return
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* <code>null</code> if the object was collected. <br>
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*
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* A reference may be registered with a queue. When a referred
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* element gets collected the reference will be put on the queue, so
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* that you will be notified. <br>
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*
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* There are currently three types of references: soft reference,
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* weak reference and phantom reference. <br>
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*
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* Soft references will be cleared if the garbage collector is told
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* to free some memory and there are no unreferenced or weakly referenced
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* objects. It is useful for caches. <br>
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*
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* Weak references will be cleared as soon as the garbage collector
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* determines that the refered object is only weakly reachable. They
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* are useful as keys in hashtables (see <code>WeakHashtable</code>) as
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* you get notified when nobody has the key anymore.
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*
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* Phantom references don't prevent finalization. If an object is only
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* phantom reachable, it will be finalized, and the reference will be
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* enqueued, but not cleared. Since you mustn't access an finalized
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* object, the <code>get</code> method of a phantom reference will never
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* work. It is useful to keep track, when an object is finalized.
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*
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* @author Jochen Hoenicke
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* @see java.util.WeakHashtable
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*/
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public abstract class Reference<T>
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{
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/**
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* The underlying object. This field is handled in a special way by
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* the garbage collector.
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* GCJ LOCAL:
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* This is a RawData because it must be disguised from the GC.
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* END GCJ LOCAL
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*/
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gnu.gcj.RawData referent;
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/**
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* This is like REFERENT but is not scanned by the GC. We keep a
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* copy around so that we can clean up our internal data structure
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* even after clear() is called.
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* GCJ LOCAL:
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* This field doesn't exist in Classpath.
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* END GCJ LOCAL
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*/
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gnu.gcj.RawData copy;
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/**
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* Set to true if {@link #clear()} is called.
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* GCJ LOCAL:
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* This field doesn't exist in Classpath. It is used internally in
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* natReference.cc, which enqueues the reference unless it is true
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* (has been cleared).
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* END GCJ LOCAL
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*/
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boolean cleared = false;
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/**
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* The queue this reference is registered on. This is null, if this
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* wasn't registered to any queue or reference was already enqueued.
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*/
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ReferenceQueue<? super T> queue;
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/**
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* Link to the next entry on the queue. If this is null, this
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* reference is not enqueued. Otherwise it points to the next
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* reference. The last reference on a queue will point to itself
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* (not to null, that value is used to mark a not enqueued
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* reference).
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*/
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Reference nextOnQueue;
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/**
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* This lock should be taken by the garbage collector, before
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* determining reachability. It will prevent the get()-method to
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* return the reference so that reachability doesn't change.
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*/
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static Object lock = new Object();
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/**
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* Creates a new reference that is not registered to any queue.
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* Since it is package private, it is not possible to overload this
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* class in a different package.
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* @param referent the object we refer to.
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*/
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Reference(T ref)
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{
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create (ref);
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}
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/**
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* Creates a reference that is registered to a queue. Since this is
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* package private, it is not possible to overload this class in a
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* different package.
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* @param referent the object we refer to.
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* @param q the reference queue to register on.
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* @exception NullPointerException if q is null.
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*/
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Reference(T ref, ReferenceQueue<? super T> q)
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{
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if (q == null)
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throw new NullPointerException();
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queue = q;
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create (ref);
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}
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/**
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* Notifies the VM that a new Reference has been created.
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*/
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private native void create (T o);
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/**
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* Returns the object, this reference refers to.
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* @return the object, this reference refers to, or null if the
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* reference was cleared.
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*/
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public native T get();
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/**
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* Clears the reference, so that it doesn't refer to its object
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* anymore. For soft and weak references this is called by the
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* garbage collector. For phantom references you should call
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* this when enqueuing the reference.
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*/
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public void clear()
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{
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// Must synchronize so changes are visible in finalizer thread.
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synchronized (lock)
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{
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referent = null;
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cleared = true;
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}
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}
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/**
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* Tells if the object is enqueued on a reference queue.
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* @return true if it is enqueued, false otherwise.
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*/
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public boolean isEnqueued()
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{
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return nextOnQueue != null;
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}
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/**
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* Enqueue an object on a reference queue. This is normally executed
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* by the garbage collector.
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*/
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public boolean enqueue()
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{
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if (queue != null && nextOnQueue == null)
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{
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queue.enqueue(this);
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queue = null;
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return true;
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}
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return false;
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}
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}
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