nextapp.echo.webcontainer.util.IdTable Maven / Gradle / Ivy
/*
* This file is part of the Echo Web Application Framework (hereinafter "Echo").
* Copyright (C) 2002-2009 NextApp, Inc.
*
* Version: MPL 1.1/GPL 2.0/LGPL 2.1
*
* The contents of this file are subject to the Mozilla Public License Version
* 1.1 (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
* http://www.mozilla.org/MPL/
*
* Software distributed under the License is distributed on an "AS IS" basis,
* WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
* for the specific language governing rights and limitations under the
* License.
*
* Alternatively, the contents of this file may be used under the terms of
* either the GNU General Public License Version 2 or later (the "GPL"), or
* the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
* in which case the provisions of the GPL or the LGPL are applicable instead
* of those above. If you wish to allow use of your version of this file only
* under the terms of either the GPL or the LGPL, and not to allow others to
* use your version of this file under the terms of the MPL, indicate your
* decision by deleting the provisions above and replace them with the notice
* and other provisions required by the GPL or the LGPL. If you do not delete
* the provisions above, a recipient may use your version of this file under
* the terms of any one of the MPL, the GPL or the LGPL.
*/
package nextapp.echo.webcontainer.util;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;
import nextapp.echo.app.RenderIdSupport;
/**
* A table which provides an identifier-to-object mapping, with the objects
* being weakly referenced (i.e., the fact that they are held within this table
* will not prevent them from being garbage collected).
*
* When deserialized by Java serialization API, the references will be hard until
* purge() is invoked for the first time.
*/
public class IdTable
implements Serializable {
/** Serial Version UID. */
private static final long serialVersionUID = 20070101L;
/**
* Flag indicating whether hard references need to be converted to weak references (as a result of the object having been
* recently deserialized.
*/
private boolean hasHardReferences = false;
/** Mapping between identifiers and WeakReferences. */
private transient Map idToReferenceMap = new HashMap();
/** ReferenceQueue for garbage collected WeakReferences. */
private transient ReferenceQueue referenceQueue = new ReferenceQueue();
/**
* Registers an object with the IdTable
*
* @param object the object to identify
*/
public void register(RenderIdSupport object) {
purge();
String id = object.getRenderId();
WeakReference weakReference;
synchronized(idToReferenceMap) {
if (!idToReferenceMap.containsKey(id)) {
weakReference = new WeakReference(object, referenceQueue);
idToReferenceMap.put(id, weakReference);
}
}
}
/**
* Retrieves the object associated with the specified identifier.
*
* @param id the identifier
* @return the object (or null, if the object is not in the queue, perhaps
* due to having been dereferenced and garbage collected)
*/
public Object getObject(String id) {
purge();
WeakReference weakReference;
synchronized(idToReferenceMap) {
weakReference = (WeakReference) idToReferenceMap.get(id);
}
if (weakReference == null) {
return null;
}
Object object = weakReference.get();
return object;
}
/**
* Purges dereferenced/garbage collected entries from the
* IdTable.
*/
private void purge() {
// Convert any hard references to weak references.
if (hasHardReferences) {
synchronized (idToReferenceMap) {
Iterator idIt = idToReferenceMap.keySet().iterator();
while (idIt.hasNext()) {
String id = (String) idIt.next();
Object object = idToReferenceMap.get(id);
if (!(object instanceof WeakReference)) {
WeakReference weakReference = new WeakReference(object, referenceQueue);
idToReferenceMap.put(id, weakReference);
}
}
hasHardReferences = false;
}
}
// Purge weak references that are no longer hard referenced elsewhere.
Reference reference = referenceQueue.poll();
if (reference == null) {
// No such references exist.
return;
}
Set referenceSet = new HashSet();
while (reference != null) {
referenceSet.add(reference);
reference = referenceQueue.poll();
}
synchronized (idToReferenceMap) {
Iterator idIt = idToReferenceMap.keySet().iterator();
while (idIt.hasNext()) {
String id = (String) idIt.next();
if (referenceSet.contains(idToReferenceMap.get(id))) {
idIt.remove();
}
}
}
}
/**
* @see java.io.Serializable
*
* Writes objects directly into values of Map as straight references.
* The values will be changed to WeakReferences when
* purge() is called.
*/
private void readObject(ObjectInputStream in)
throws IOException, ClassNotFoundException {
in.defaultReadObject();
idToReferenceMap = new HashMap();
referenceQueue = new ReferenceQueue();
String id = (String) in.readObject();
if (id != null) {
// Hard references will be written.
hasHardReferences = true;
// Load map and store objects as hard references.
while (id != null) {
RenderIdSupport object = (RenderIdSupport) in.readObject();
idToReferenceMap.put(id, object);
id = (String) in.readObject();
}
}
}
/**
* @see java.io.Serializable
*/
private void writeObject(ObjectOutputStream out)
throws IOException {
out.defaultWriteObject();
Iterator it = idToReferenceMap.keySet().iterator();
while (it.hasNext()) {
String id = (String) it.next();
out.writeObject(id);
Object object = idToReferenceMap.get(id);
if (object instanceof WeakReference) {
object = ((WeakReference) object).get();
}
out.writeObject(object);
}
// Write null to specify end of object.
out.writeObject(null);
}
}