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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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Here is a UML depiction of the Proxy pattern:
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[[Image:ProxyUML.gif|UML Diagram for the Proxy Design Pattern]]
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Source: Downloaded from http://www.developer.com/design/article.php/3309461
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[[Image:Proxy.gif|A Complete UML Diagram for the Proxy Design Pattern]]
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Source: Downloaded from http://www.dofactory.com/Patterns/PatternProxy.aspx
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[[Image:ProxyDCD.jpg|Design Class Diagram for the Proxy Design Pattern]]
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Source: Downloaded from http://home.earthlink.net/~huston2/dp/all_uml.html
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<pre>
# SUBSCRIBER: subscribes to certain events.
def __init__(self, ident=None):
Pyro.core.ObjBase.__init__(self)
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)
# 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())
# PUBLISHER: publishes events.
def __init__(self, ident=None):
Pyro.core.initClient()
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>
<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=+Proxy+Pattern+show:wc1r54UweykdgvZt53GNuE:YPuTRbFKH7c9VmFCU8eSwo:rOOtm_PT5xUnSqKvMM2F_M&sa=N&cd=13&ct=rcrl&cs_p=http://gentoo.osuosl.org/distfiles/Pyro-3.4.tar.gz&cs_f=Pyro-3.4/Pyro/EventService/Clients.py#a0 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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