001    /*
002     * Licensed to the Apache Software Foundation (ASF) under one or more
003     * contributor license agreements.  See the NOTICE file distributed with
004     * this work for additional information regarding copyright ownership.
005     * The ASF licenses this file to You under the Apache License, Version 2.0
006     * (the "License"); you may not use this file except in compliance with
007     * the License.  You may obtain a copy of the License at
008     *
009     *      http://www.apache.org/licenses/LICENSE-2.0
010     *
011     * Unless required by applicable law or agreed to in writing, software
012     * distributed under the License is distributed on an "AS IS" BASIS,
013     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014     * See the License for the specific language governing permissions and
015     * limitations under the License.
016     */
017    
018    package org.apache.commons.proxy.provider;
019    
020    import org.apache.commons.proxy.ObjectProvider;
021    
022    /**
023     * Wraps another object provider, making sure to only call it once, returning the value returned from the wrapped
024     * provider on all subsequent invocations.
025     *
026     * @author James Carman
027     * @since 1.0
028     */
029    public class SingletonProvider extends ProviderDecorator
030    {
031    //----------------------------------------------------------------------------------------------------------------------
032    // Fields
033    //----------------------------------------------------------------------------------------------------------------------
034    
035        private Object instance;
036    
037    //----------------------------------------------------------------------------------------------------------------------
038    // Constructors
039    //----------------------------------------------------------------------------------------------------------------------
040    
041        public SingletonProvider( ObjectProvider inner )
042        {
043            super( inner );
044        }
045    
046    //----------------------------------------------------------------------------------------------------------------------
047    // ObjectProvider Implementation
048    //----------------------------------------------------------------------------------------------------------------------
049    
050        public Object getObject()
051        {
052            synchronized( this )
053            {
054                if( instance == null )
055                {
056                    instance = super.getObject();
057                    inner = null; // Garbage collection
058                }
059            }
060            return instance;
061        }
062    }
063