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Java > Open Source Codes > org > jboss > aspects > patterns > singleton > Singleton


1 /*
2   * JBoss, Home of Professional Open Source
3   * Copyright 2005, JBoss Inc., and individual contributors as indicated
4   * by the @authors tag. See the copyright.txt in the distribution for a
5   * full listing of individual contributors.
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7   * This is free software; you can redistribute it and/or modify it
8   * under the terms of the GNU Lesser General Public License as
9   * 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: 37406 $
43  */
44 public class Singleton
45 {
46    private Object JavaDoc singleton;
47
48    public synchronized Object JavaDoc constructorAdvice(ConstructorInvocation invocation) throws Throwable JavaDoc
49    {
50       if (singleton == null)
51          singleton = invocation.invokeNext();
52       return singleton;
53    }
54 }
55
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