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The <i>Proxy</i> pattern is used in software development to create a placeholder for an object. The Proxies provide an interface to another object is not actually created until the information that the such as a network connection, a large object holds is required. This extra layer of abstraction saves time when in memory, a program must access a database file, or a disk for the information.<br /><br />If the information some other resource that is never required, the database/disk will never be queried and the system will run more efficiently with less slowdowns for unnecessary materializationsexpensive or impossible to duplicate.
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==Proxy Design Pattern==
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==UML Diagram==Here is a UML depiction of the Proxy pattern:<br />[[Image:ProxyUML.gif|UML Diagram for the Proxy Design Pattern]]<br />Source: Downloaded from http://www.developer.com/design/article.php/3309461<br /><br />A more complete UML diagram of the Proxy pattern:<br />[[Image:Proxy.gif|A Complete UML Diagram for the Proxy Design Pattern]]<br />Source: Downloaded from http://www.dofactory.com/Patterns/PatternProxy.aspx<br /><br />Here is a design class diagram using UML of how te Proxy pattern would be used in a system:<br />[[Image:ProxyDCD.jpg|Design Class Diagramfor the Proxy Design Pattern]]<br />Source: Downloaded from http://home.earthlink.net/~huston2/dp/all_uml.html<br /> ==Code Samples==These code samples will demonstrate real world uses of the Proxy Design Pattern. Each code sample is from a real working software system.<br />===Virtual Proxy - Python===This Sample code is from a Python Remote Method Invocation program. It is copyrighted by <b>Irmen de Jong</b> and licensed by <b>[http://www.google.com/codesearch?hl=en&q=show:dgvZt53GNuE:9VmFCU8eSwo:nSqKvMM2F_M&sa=N&cd=13&ct=rl&cs_p=http://gentoo.osuosl.org/distfiles/Pyro-3.4.tar.gz&cs_f=Pyro-3.4/Pyro/EventService/Clients.py MIT]</b>. The code in this example is located in the <i>Clients.py</i> file of the <b>Pyro</b> system. It allows a user to write a module containing a class to be accessed remotely. The class the user created is registered by the Pyro Name Server. The client creates proxies for the remote objects to be invoked when a call is made to the object. The Virtual Proxy pattern can be seen throughout this code file as highlighted:<br /><pre>import Pyro.core, Pyro.naming, Pyro.constantsimport Pyro.EventService.Serverfrom Pyro.errors import *
# SUBSCRIBER: subscribes to certain events.
class Subscriber(Pyro.core.CallbackObjBase):
def __init__(self, ident=None):
Pyro.core.ObjBase.__init__(self)
Pyro.core.initServer()
Pyro.core.initClient()
daemon = Pyro.core.Daemon()
locator = Pyro.naming.NameServerLocator(identification=ident)
self.NS = locator.getNS(host=Pyro.config.PYRO_NS_HOSTNAME)
daemon.useNameServer(self.NS)
daemon.connect(self) # will also set self.daemon...
uri = self.NS.resolve(Pyro.constants.EVENTSERVER_NAME)
# creates a proxy for the remote class to be called on later to access the
# remote object and saves making an expensive call to a DB/disk if it isn't
# required.
self.eventservice=Pyro.core.getProxyForURI(uri)
self.eventservice._setIdentification(ident)
self.abortListen=0
self.daemon=daemon # make sure daemon doesn't get garbage collected now
def subscribe(self,subjects):
# Subscribe to one or more subjects.
# It is safe to call this multiple times.
# gets a proxy for the service so that an expensive call isn't require until
# the actual service is needed.
self.eventservice.subscribe(subjects, self.getProxy())
def subscribeMatch(self,subjectPatterns):
# Subscribe to one or more subjects (by pattern)
# It is safe to call this multiple times.
# gets a proxy for the service so that an expensive call isn't require until
# the actual service is needed.
self.eventservice.subscribeMatch(subjectPatterns, self.getProxy())
def unsubscribe(self, subjects):
# Unsubscribe the subscriber for the given subject(s).
# gets a proxy for the service so that an expensive call isn't require until
# the actual service is needed.
self.eventservice.unsubscribe(subjects, self.getProxy())
def abort(self):
self.abortListen=1
def setThreading(self, threaded):
self.getDaemon().threaded=threaded
def listen(self):
self.getDaemon().requestLoop(lambda s=self: not s.abortListen)
def event(self, event): # callback, override this!
print event
# PUBLISHER: publishes events.
class Publisher:
def __init__(self, ident=None):
Pyro.core.initClient()
locator = Pyro.naming.NameServerLocator(identification=ident)
ns = locator.getNS(host=Pyro.config.PYRO_NS_HOSTNAME)
uri = ns.resolve(Pyro.constants.EVENTSERVER_NAME)
# creates a proxy for the remote class to be called on later to access the
# remote object and saves making an expensive call to a DB/disk if it isn't
# required.
self.eventservice=Pyro.core.getProxyForURI(uri)
self.eventservice._setIdentification(ident)
def publish(self, subjects, msg):
self.eventservice.publish(subjects,msg)
</pre>
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===Virtual Proxy - Java===
This sample code is from the Apache Geronimo server runtime framework written in J2EE. It is copyrighted by <b>IBM</b> and licensed by <b>[http://www.google.com/codesearch?hl=en&q=show:x-Vlj2phf84:hfeVps_3TKA:KxOJN1EfkwE&sa=N&ct=rd&cs_p=http://archive.apache.org/dist/geronimo/1.0-M5/geronimo-1.0-M5-src.tar.gz&cs_f=geronimo-1.0-M5/LICENSE.txt Apache]</b>. The code in this example is located in the <i>GBeanCollectionReference.java</i> of the <b>Geronimo</b> framework. It allows a user to create their own J2EE 1.4 certified application server. It can deploy servlets, XML, and other web applications. The Virtual Proxy pattern can be seen throughout this code file as highlighted:
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<pre>
/**
*
* Copyright 2003-2004 The Apache Software Foundation
*
* Licensed under the Apache License, Version 2.0 (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.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.geronimo.gbean.runtime;
import javax.management.ObjectName;
import org.apache.geronimo.gbean.GReferenceInfo;
import org.apache.geronimo.gbean.InvalidConfigurationException;
import org.apache.geronimo.kernel.DependencyManager;
import org.apache.geronimo.kernel.Kernel;
import org.apache.geronimo.kernel.lifecycle.LifecycleAdapter;
import org.apache.geronimo.kernel.lifecycle.LifecycleListener;
/**
* @version $Rev: 71492 $ $Date: 2004-11-14 21:31:50 -0800 (Sun, 14 Nov 2004) $
*/
public class GBeanCollectionReference extends AbstractGBeanReference {
public GBeanCollectionReference(GBeanInstance gbeanInstance, GReferenceInfo referenceInfo,
Kernel kernel, DependencyManager dependencyManager) throws InvalidConfigurationException {
super(gbeanInstance, referenceInfo, kernel, dependencyManager);
}
public synchronized boolean start() {
// We only need to start if there are patterns and we don't already have a proxy
if (!getPatterns().isEmpty() && getProxy() == null) {
// add a dependency on our target and create the proxy
/* sets a collection of proxies for each object instead of making an expensive
call to a DB/disk to retrieve all of the object information when it isn't
required. */
setProxy(new ProxyCollection(getName(), getReferenceType(),
getKernel().getProxyManager(), getTargets()));
}
return true;
}
/* Destroys all of the proxies for the objects as the system is deconstructing. */
public synchronized void stop() {
ProxyCollection proxy = (ProxyCollection) getProxy();
if (proxy != null) {
proxy.destroy();
setProxy(null);
}
}
/* Adds a target object to the proxy to be instantiated if or when it is
required. */
protected synchronized void targetAdded(ObjectName target) {
ProxyCollection proxy = (ProxyCollection) getProxy();
if (proxy != null) {
proxy.addTarget(target);
}
}
/* Removes a target object from the proxy to to open it's use up to another
object. */
protected synchronized void targetRemoved(ObjectName target) {
ProxyCollection proxy = (ProxyCollection) getProxy();
if (proxy != null) {
proxy.removeTarget(target);
}
}
protected LifecycleListener createLifecycleListener() {
return new LifecycleAdapter() {
public void running(ObjectName objectName) {
addTarget(objectName);
}
public void stopping(ObjectName objectName) {
removeTarget(objectName);
}
public void stopped(ObjectName objectName) {
removeTarget(objectName);
}
public void failed(ObjectName objectName) {
removeTarget(objectName);
}
public void unloaded(ObjectName objectName) {
removeTarget(objectName);
}
};
}
}
</pre>
==References==
<ul>
<li>[http://en.wikipedia.org/wiki/Proxy_pattern Wikipedia article on the Proxy Pattern]</li>
<li>[http://www.developer.com/design/article.php/3309461 Design Patterns - Proxy UML Diagram]
</li>
<li>Gamma, E., Helm, R., Johnson, R., Vlissides, J. (1995). <i>Design Patterns: Elements of
Reusable Object-Oriented Software.</i> Addison-Wesley. ISBN 0-201-63361-2</li>
<li>[http://home.earthlink.net/~huston2/dp/all_uml.html Design Class Diagrams - Proxy UML Diagram]</li>
<li>[http://www.dofactory.com/Patterns/PatternProxy.aspx Information on the Proxy Pattern with a UML Diagram]</li>
<li>[http://www.google.com/codesearch?hl=en&q=show:dgvZt53GNuE:9VmFCU8eSwo:nSqKvMM2F_M&sa=N&cd=13&ct=rl&cs_p=http://gentoo.osuosl.org/distfiles/Pyro-3.4.tar.gz&cs_f=Pyro-3.4/Pyro/EventService/Clients.py Python Remote Method Invocation]</li>
<li>[http://www.google.com/codesearch?hl=en&q=show:hfeVps_3TKA:OPU6OVZro5s&sa=N&ct=rdp&cs_p=http://archive.apache.org/dist/geronimo/1.0-M5/geronimo-1.0-M5-src.tar.gz&cs_f=geronimo-1.0-M5 Geronimo Server Runtime Framework]</li>
</ul>
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==Links==
[[BTP600]]
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