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Wed Mar 24 16:00:00 GMT 2004
SingletonProtocol
package ca.ubc.cs.spl.pattern.library; /* -*- Mode: Java; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- * * This file is part of the design patterns project at UBC * * 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 * either http://www.mozilla.org/MPL/ or http://aspectj.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. * * The Original Code is ca.ubc.cs.spl.patterns. * * Contributor(s): */ import java.util.Hashtable; /** * Defines the general behavior of the singleton design pattern. * * Each concrete sub-aspect of SingletonProtocol defines the Singleton * property for one class. * * The sub-aspect defines two things: <ol> * * <li> what type is a <i>Singleton</i> <br> * * <li> what classes can access the <i>Singleton</i>'s constructor (if any) * despite its property * </ol> * * for this implementation we choose to illustrate that it is not necessary * to provide a factory method for (like getSingleton()), but the regular * constructor may be used instead to access the Singleton instance * * @author Jan Hannemann * @author Gregor Kiczales * @version 1.0, 05/13/02 * */ public abstract aspect SingletonProtocol { private Hashtable singletons = new Hashtable(); public interface Singleton {} protected pointcut protectionExclusions(); // What if two aspects re-define protectionExclusions() ? Singleton around(): call((Singleton+).new(..)) && !protectionExclusions() { Class singleton = thisJoinPoint.getSignature().getDeclaringType(); if (singletons.get(singleton) == null) { // How to access the static instance variable here? singletons.put(singleton, proceed()); } return (Singleton) singletons.get(singleton); } // Alternative to protectionExclusions() ?: // // Define an interface NonSingleton and check in advice whether // we are asked to create a NonSingleton, then proceed }
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