01: /*
02: * JBoss, Home of Professional Open Source.
03: * Copyright 2006, Red Hat Middleware LLC, and individual contributors
04: * as indicated by the @author tags. See the copyright.txt file in the
05: * distribution for a full listing of individual contributors.
06: *
07: * This is free software; you can redistribute it and/or modify it
08: * under the terms of the GNU Lesser General Public License as
09: * published by the Free Software Foundation; either version 2.1 of
10: * the License, or (at your option) any later version.
11: *
12: * This software is distributed in the hope that it will be useful,
13: * but WITHOUT ANY WARRANTY; without even the implied warranty of
14: * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
15: * Lesser General Public License for more details.
16: *
17: * You should have received a copy of the GNU Lesser General Public
18: * License along with this software; if not, write to the Free
19: * Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA
20: * 02110-1301 USA, or see the FSF site: http://www.fsf.org.
21: */
22: package org.jboss.aspects.patterns.singleton;
23:
24: import org.jboss.aop.joinpoint.ConstructorInvocation;
25:
26: /**
27: * A simple implementation of a singleton.<p>
28: *
29: * This advice should be PER_CLASS scoped<p>
30: *
31: * e.g.
32: * <pre>
33: * <aspect class="Singleton" scope="PER_CLASS"/>
34: *
35: * <bind pointcut="execution(POJO->new())">
36: * <advice name="constructorAdvice" aspect="Singleton"/>
37: * </bind>
38: * </pre>
39: *
40: *
41: * @author <a href="mailto:adrian@jboss.org">Adrian Brock</a>
42: * @version $Revision: 57186 $
43: */
44: public class Singleton {
45: private Object singleton;
46:
47: public synchronized Object constructorAdvice(
48: ConstructorInvocation invocation) throws Throwable {
49: if (singleton == null)
50: singleton = invocation.invokeNext();
51: return singleton;
52: }
53: }
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