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
018package org.apache.commons.proxy2.provider;
019
020import org.apache.commons.proxy2.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 * @since 1.0
027 */
028public class SingletonProvider<T> extends ProviderDecorator<T>
029{
030    //******************************************************************************************************************
031    // Fields
032    //******************************************************************************************************************
033
034    private static final long serialVersionUID = 1L;
035
036    private T instance;
037
038    //******************************************************************************************************************
039    // Constructors
040    //******************************************************************************************************************
041
042    /**
043     * Create a new SingletonProvider instance.
044     * 
045     * @param inner
046     */
047    public SingletonProvider(ObjectProvider<? extends T> inner)
048    {
049        super(inner);
050    }
051
052    //******************************************************************************************************************
053    // ObjectProvider Implementation
054    //******************************************************************************************************************
055
056    /**
057     * {@inheritDoc}
058     */
059    @Override
060    public T getObject()
061    {
062        synchronized (this)
063        {
064            if (instance == null)
065            {
066                instance = super.getObject();
067                // Garbage collection
068                setInner(null);
069            }
070            return instance;
071        }
072    }
073}