core/src/main/java/org/apache/commons/proxy2/provider/SingletonProvider.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.commons.proxy2.provider;

import org.apache.commons.proxy2.ObjectProvider;

/**
 * Wraps another object provider, making sure to only call it once, returning the value returned from the wrapped
 * provider on all subsequent invocations.
 * 
 * @since 1.0
 */
public class SingletonProvider<T> extends ProviderDecorator<T>
{
    //******************************************************************************************************************
    // Fields
    //******************************************************************************************************************

    private static final long serialVersionUID = 1L;

    private T instance;

    //******************************************************************************************************************
    // Constructors
    //******************************************************************************************************************

    /**
     * Create a new SingletonProvider instance.
     * 
     * @param inner
     */
    public SingletonProvider(ObjectProvider<? extends T> inner)
    {
        super(inner);
    }

    //******************************************************************************************************************
    // ObjectProvider Implementation
    //******************************************************************************************************************

    /**
     * {@inheritDoc}
     */
    @Override
    public T getObject()
    {
        synchronized (this)
        {
            if (instance == null)
            {
                instance = super.getObject();
                // Garbage collection
                setInner(null);
            }
            return instance;
        }
    }
}