Java/Collections Data Structure/Weak List
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 usesjava.lang.ref.WeakReference
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. * <p/> * By default this list can contain 25 elements, where the first element is stored using a strong reference, which is * not garbage collected. * <p/> * Restoring the elements is not implemented, concrete implementations will have to override the *restoreChild(int)
method. ThegetMaxChildCount
method defines the maxmimum number of * children in the list. When more thanmaxChildCount
elements are contained in this list, add will always * return false to indicate that adding the element failed. * <p/> * To customize the list, override createReference to create a different kind of reference. * <p/> * 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 x, the expression:
** will be true, and that the expression:* x.clone() != x
** will be true, but these are not absolute requirements. While it is typically the case that:* x.clone().getClass() == x.getClass()
** x.clone().equals(x)
* will be true, this is not an absolute requirement. * <p/> * By convention, the returned object should be obtained by calling super.clone. If a class and all of its * superclasses (except Object) obey this convention, it will be the case that x.clone().getClass() == * x.getClass(). * <p/> * 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 * super.clone 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 super.clone need to be modified. * <p/> * The method clone for class Object performs a specific cloning operation. First, if the class of * this object does not implement the interface Cloneable, then a CloneNotSupportedException is * thrown. Note that all arrays are considered to implement the interface Cloneable. 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. * <p/> * The class Object does not itself implement the interface Cloneable, so calling the clone * method on an object whose class is Object 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 theCloneable
interface. * Subclasses that override theclone
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; }
}
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