001 /*
002 * JBoss DNA (http://www.jboss.org/dna)
003 * See the COPYRIGHT.txt file distributed with this work for information
004 * regarding copyright ownership. Some portions may be licensed
005 * to Red Hat, Inc. under one or more contributor license agreements.
006 * See the AUTHORS.txt file in the distribution for a full listing of
007 * individual contributors.
008 *
009 * JBoss DNA is free software. Unless otherwise indicated, all code in JBoss DNA
010 * is licensed to you under the terms of the GNU Lesser General Public License as
011 * published by the Free Software Foundation; either version 2.1 of
012 * the License, or (at your option) any later version.
013 *
014 * JBoss DNA is distributed in the hope that it will be useful,
015 * but WITHOUT ANY WARRANTY; without even the implied warranty of
016 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
017 * Lesser General Public License for more details.
018 *
019 * You should have received a copy of the GNU Lesser General Public
020 * License along with this software; if not, write to the Free
021 * Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA
022 * 02110-1301 USA, or see the FSF site: http://www.fsf.org.
023 */
024 package org.jboss.dna.graph.observe;
025
026 import java.lang.ref.WeakReference;
027 import java.util.concurrent.CopyOnWriteArraySet;
028 import net.jcip.annotations.ThreadSafe;
029 import org.jboss.dna.common.util.Logger;
030
031 /**
032 * Abstract class that is used to signal that a change set has occurred. This class is typically subclassed by those that wish to
033 * observe changes in content, and {@link Observable#register(ChangeObserver) registered} with a {@link Observable}.
034 * <p>
035 * This class maintains a (weak) reference to the ChangeSource instances with which it is registered. Therefore, the observers
036 * will not keep a ChangeSource from being garbage collected. And, if a change source is garbage collected, calling
037 * {@link #unregister()} will clean up naturally.
038 * </p>
039 */
040 @ThreadSafe
041 public abstract class ChangeObserver implements Observer {
042
043 private final CopyOnWriteArraySet<ChangeSourceReference> sources = new CopyOnWriteArraySet<ChangeSourceReference>();
044
045 protected ChangeObserver() {
046 }
047
048 /**
049 * Records that this listener has successfully registered by the supplied {@link Observable}.
050 *
051 * @param source the source with which this listener was registered
052 */
053 final void registeredWith( Observable source ) {
054 sources.add(new ChangeSourceReference(source));
055 }
056
057 /**
058 * Records that this listener has successfully unregistered by the supplied {@link Observable}.
059 *
060 * @param source the source with which this listener was registered
061 */
062 final void unregisteredWith( Observable source ) {
063 sources.remove(new ChangeSourceReference(source));
064 }
065
066 /**
067 * Unregister this listener from all {@link Observable sources} that it was registered with. This is preferred over calling
068 * {@link Observable#unregister(ChangeObserver)} directly.
069 */
070 public void unregister() {
071 doUnregister();
072 }
073
074 /**
075 * Method called by {@link #unregister()} that actually does the unregistering. This method is final.
076 */
077 protected final void doUnregister() {
078 // Unregister this listener from each source ...
079 for (ChangeSourceReference sourceReference : sources) {
080 Observable source = sourceReference.get();
081 if (source != null) {
082 try {
083 source.unregister(this);
084 } catch (Throwable t) {
085 Logger.getLogger(getClass()).debug(t, "Error while unregistering {0} from {1}", this, source);
086 }
087 }
088 }
089 }
090
091 /**
092 * Determine whether this observer is currently registered with any {@link Observable} instances.
093 * <p>
094 * Although an observer might be registered with an {@link Observable}, if that Observable is garbage collected, then this
095 * observer will no longer be registered with it.
096 * </p>
097 *
098 * @return true if this observer is registered with at least one {@link Observable} instance, or false if this observer is not
099 * currently registered with any {@link Observable} instances.
100 */
101 public boolean isRegistered() {
102 for (ChangeSourceReference reference : sources) {
103 if (reference.get() != null) return true;
104 }
105 return false;
106 }
107
108 /**
109 * Method that is called for each {@link Changes} from the {@link Observable} instance(s) with which this listener is
110 * registered.
111 *
112 * @param changeSet the change set
113 */
114 public abstract void notify( Changes changeSet );
115
116 /**
117 * A {@link WeakReference} implementation that provides a valid
118 */
119 protected final class ChangeSourceReference extends WeakReference<Observable> {
120 final int hc;
121
122 protected ChangeSourceReference( Observable source ) {
123 super(source);
124 this.hc = source.hashCode();
125 }
126
127 /**
128 * {@inheritDoc}
129 *
130 * @see java.lang.Object#hashCode()
131 */
132 @Override
133 public int hashCode() {
134 return hc;
135 }
136
137 /**
138 * {@inheritDoc}
139 *
140 * @see java.lang.Object#equals(java.lang.Object)
141 */
142 @Override
143 public boolean equals( Object obj ) {
144 if (obj == this) return true;
145 if (obj instanceof ChangeSourceReference) {
146 ChangeSourceReference that = (ChangeSourceReference)obj;
147 Observable thisSource = this.get();
148 Observable thatSource = that.get();
149 return thisSource == thatSource; // reference equality, not object equality!
150 }
151 if (obj instanceof Observable) {
152 Observable that = (Observable)obj;
153 return this.get() == that; // reference equality, not object equality!
154 }
155 return false;
156 }
157 }
158 }