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2010-05-31T18:01:48Z

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== A bag of weakly referenced objects ==



<source lang="java">

/*
* Copyright (c) 1998 - 2005 Versant Corporation
* All rights reserved. This program and the accompanying materials
* are made available under the terms of the Eclipse Public License v1.0
* which accompanies this distribution, and is available at
* http://www.eclipse.org/legal/epl-v10.html
*
* Contributors:
* Versant Corporation - initial API and implementation
*/
import java.util.HashSet;
import java.util.List;
import java.util.ArrayList;
import java.util.Iterator;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.lang.ref.Reference;
/**
* This maintains a bag of weakly referenced objects. The clean method
* must be called from time to time to get rid of the objects that the
* garbage collector wants to nuke. This class is not synchronized.
*/
public final class WeakBag {
private HashSet set = new HashSet();
private ReferenceQueue refQueue = new ReferenceQueue();
public WeakBag() {
}
/**
* Add o to the bag and return the WeakReference that can be used to
* remove it.
*/
public WeakReference add(Object o) {
WeakReference ans = new WeakReference(o, refQueue);
set.add(ans);
return ans;
}
/**
* Remove o from the bag.
*/
public void remove(Reference o) {
set.remove(o);
}
/**
* Get the approximate number of objects in the bag.
*/
public int size() {
return set.size();
}
/**
* Get all the objects still in the bag.
*/
public List values() {
ArrayList a = new ArrayList(set.size());
for (Iterator i = set.iterator(); i.hasNext(); ) {
WeakReference r = (WeakReference)i.next();
Object o = r.get();
if (o != null) a.add(o);
}
return a;
}
/**
* Get rid of objects in the bag that the garbage collector wants to
* nuke. This does not block.
*/
public void clean() {
for (;;) {
Object r = refQueue.poll();
if (r == null) return;
set.remove(r);
}
}
}

</source>



== Weak List from objectweb jac ==



<source lang="java">

//Revised from objectweb jac
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.util.AbstractList;
import java.util.ArrayList;
import java.util.List;
public class WeakList extends AbstractList {
/**
* Reference queue for cleared weak references
*/
private final ReferenceQueue queue = new ReferenceQueue();

private final List list=new ArrayList();
public boolean add(Object o) {
expungeStaleEntries();
return list.add(new ListEntry(o,queue));
}
public Object get(int i) {
expungeStaleEntries();
return ((Reference)list.get(i)).get();
}
public int size() {
//new Exception().printStackTrace();
expungeStaleEntries();
return list.size();
}
public Object remove(int index) {
return ((ListEntry)list.remove(index)).get();
}
/**
* Expunge stale entries from the list.
*/
private void expungeStaleEntries() {
Object r;
while ( (r = queue.poll()) != null) {
ListEntry e = (ListEntry)r;
int i=list.indexOf(r);
if(i!=-1) {
list.remove(i);
}
}
}
private static class ListEntry extends WeakReference {
String objectString;
public ListEntry(Object o,ReferenceQueue queue) {
super(o,queue);
objectString=o.toString();
}
public boolean equals(Object o) {
if(o==null) {
return false;
} else {
if((o instanceof ListEntry)) {
return o.hashCode()==this.hashCode();
} else {
if(this.get()==null) return false;
return this.get().equals(o);
}
}
}
public String toString() {
if(this.get()==null) {
return ""entry "+objectString+" <GARBAGED>"";
} else {
return ""entry "+this.get()+""";
}
}

}
}

</source>



== WeakReference list uses java.lang.ref.WeakReferences to store its contents. ==



<source lang="java">

/**
*
* JFreeReport : a free Java reporting library
*
*
* Project Info: http://reporting.pentaho.org/
*
* (C) Copyright 2001-2007, by Object Refinery Ltd, Pentaho Corporation and Contributors.
*
* This library is free software; you can redistribute it and/or modify it under the terms
* of the GNU Lesser General Public License as published by the Free Software Foundation;
* either version 2.1 of the License, or (at your option) any later version.
*
* This library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY;
* without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
* See the GNU Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public License along with this
* library; if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330,
* Boston, MA 02111-1307, USA.
*
* [Java is a trademark or registered trademark of Sun Microsystems, Inc.
* in the United States and other countries.]
*
* ------------
* WeakReferenceList.java
* ------------
* (C) Copyright 2001-2007, by Object Refinery Ltd, Pentaho Corporation and Contributors.
*/

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.Serializable;
import java.lang.ref.Reference;
import java.lang.ref.WeakReference;
/**
* The WeakReference list uses <code>java.lang.ref.WeakReference</code>s to store its contents. In contrast to the
* WeakHashtable, this list knows how to restore missing content, so that garbage collected elements can be restored
* when they are accessed.
* 
* By default this list can contain 25 elements, where the first element is stored using a strong reference, which is
* not garbage collected.
* 
* Restoring the elements is not implemented, concrete implementations will have to override the
* <code>restoreChild(int)</code> method. The <code>getMaxChildCount</code> method defines the maxmimum number of
* children in the list. When more than <code>maxChildCount</code> elements are contained in this list, add will always
* return false to indicate that adding the element failed.
* 
* To customize the list, override createReference to create a different kind of reference.
* 
* This list is able to add or replace elements, but inserting or removing of elements is not possible.
*
* @author Thomas Morgner
*/
public abstract class WeakReferenceList implements Serializable, Cloneable
{
/**
* The master element.
*/
private Object master;
/**
* Storage for the references.
*/
private transient Reference[] childs;
/**
* The current number of elements.
*/
private int size;
/**
* The maximum number of elements.
*/
private final int maxChilds;
/**
* Creates a new weak reference list. The storage of the list is limited to getMaxChildCount() elements.
*
* @param maxChildCount the maximum number of elements.
*/
protected WeakReferenceList(final int maxChildCount)
{
this.maxChilds = maxChildCount;
this.childs = new Reference[maxChildCount - 1];
}
/**
* Returns the maximum number of children in this list.
*
* @return the maximum number of elements in this list.
*/
protected final int getMaxChildCount()
{
return maxChilds;
}
/**
* Returns the master element of this list. The master element is the element stored by a strong reference and cannot
* be garbage collected.
*
* @return the master element
*/
protected Object getMaster()
{
return master;
}
/**
* Attempts to restore the child stored on the given index.
*
* @param index the index.
* @return null if the child could not be restored or the restored child.
*/
protected abstract Object restoreChild(int index);
/**
* Returns the child stored at the given index. If the child has been garbage collected, it gets restored using the
* restoreChild function.
*
* @param index the index.
* @return the object.
*/
public Object get(final int index)
{
if (isMaster(index))
{
return master;
}
else
{
final Reference ref = childs[getChildPos(index)];
if (ref == null)
{
throw new IllegalStateException("State: " + index);
}
Object ob = ref.get();
if (ob == null)
{
ob = restoreChild(index);
childs[getChildPos(index)] = createReference(ob);
}
return ob;
}
}
/**
* Replaces the child stored at the given index with the new child which can be null.
*
* @param report the object.
* @param index the index.
*/
public void set(final Object report, final int index)
{
if (isMaster(index))
{
master = report;
}
else
{
childs[getChildPos(index)] = createReference(report);
}
}
/**
* Creates a new reference for the given object.
*
* @param o the object.
* @return a WeakReference for the object o without any ReferenceQueue attached.
*/
private Reference createReference(final Object o)
{
return new WeakReference(o);
}
/**
* Adds the element to the list. If the maximum size of the list is exceeded, this function returns false to indicate
* that adding failed.
*
* @param rs the object.
* @return true, if the object was successfully added to the list, false otherwise
*/
public boolean add(final Object rs)
{
if (size == 0)
{
master = rs;
size = 1;
return true;
}
else
{
if (size < getMaxChildCount())
{
childs[size - 1] = createReference(rs);
size++;
return true;
}
else
{
// was not able to add this to this list, maximum number of entries reached.
return false;
}
}
}
/**
* Returns true, if the given index denotes a master index of this list.
*
* @param index the index.
* @return true if the index is a master index.
*/
protected boolean isMaster(final int index)
{
return index % getMaxChildCount() == 0;
}
/**
* Returns the internal storage position for the child.
*
* @param index the index.
* @return the internal storage index.
*/
protected int getChildPos(final int index)
{
return index % getMaxChildCount() - 1;
}
/**
* Returns the size of the list.
*
* @return the size.
*/
public int getSize()
{
return size;
}
/**
* Serialisation support. The transient child elements were not saved.
*
* @param in the input stream.
* @throws IOException if there is an I/O error.
* @throws ClassNotFoundException if a serialized class is not defined on this system.
*/
private void readObject(final ObjectInputStream in)
throws IOException, ClassNotFoundException
{
in.defaultReadObject();
childs = new Reference[getMaxChildCount() - 1];
for (int i = 0; i < childs.length; i++)
{
childs[i] = createReference(null);
}
}
/**
* Creates and returns a copy of this object. The precise meaning of "copy" may depend on the class of the object.
* The general intent is that, for any object <tt>x</tt>, the expression: <blockquote>
* <pre>
* x.clone() != x</pre></blockquote>
* will be true, and that the expression: <blockquote>
* <pre>
* x.clone().getClass() == x.getClass()</pre></blockquote>
* will be <tt>true</tt>, but these are not absolute requirements. While it is typically the case that: <blockquote>
* <pre>
* x.clone().equals(x)</pre></blockquote>
* will be <tt>true</tt>, this is not an absolute requirement.
* 
* By convention, the returned object should be obtained by calling <tt>super.clone</tt>. If a class and all of its
* superclasses (except <tt>Object</tt>) obey this convention, it will be the case that <tt>x.clone().getClass() ==
* x.getClass()</tt>.
* 
* By convention, the object returned by this method should be independent of this object (which is being cloned). To
* achieve this independence, it may be necessary to modify one or more fields of the object returned by
* <tt>super.clone</tt> before returning it. Typically, this means copying any mutable objects that comprise the
* internal "deep structure" of the object being cloned and replacing the references to these objects with references
* to the copies. If a class contains only primitive fields or references to immutable objects, then it is usually
* the case that no fields in the object returned by <tt>super.clone</tt> need to be modified.
* 
* The method <tt>clone</tt> for class <tt>Object</tt> performs a specific cloning operation. First, if the class of
* this object does not implement the interface <tt>Cloneable</tt>, then a <tt>CloneNotSupportedException</tt> is
* thrown. Note that all arrays are considered to implement the interface <tt>Cloneable</tt>. Otherwise, this method
* creates a new instance of the class of this object and initializes all its fields with exactly the contents of the
* corresponding fields of this object, as if by assignment; the contents of the fields are not themselves cloned.
* Thus, this method performs a "shallow copy" of this object, not a "deep copy" operation.
* 
* The class <tt>Object</tt> does not itself implement the interface <tt>Cloneable</tt>, so calling the <tt>clone</tt>
* method on an object whose class is <tt>Object</tt> will result in throwing an exception at run time.
*
* @return a clone of this instance.
* @throws CloneNotSupportedException if the object"s class does not support the <code>Cloneable</code> interface.
* Subclasses that override the <code>clone</code> method can also throw this
* exception to indicate that an instance cannot be cloned.
* @see Cloneable
*/
protected Object clone()
throws CloneNotSupportedException
{
final WeakReferenceList list = (WeakReferenceList) super.clone();
list.childs = (Reference[]) childs.clone();
return list;
}
}

</source>

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