001// Copyright 2009, 2011, 2012 The Apache Software Foundation
002//
003// Licensed under the Apache License, Version 2.0 (the "License");
004// you may not use this file except in compliance with the License.
005// You may obtain a copy of the License at
006//
007// http://www.apache.org/licenses/LICENSE-2.0
008//
009// Unless required by applicable law or agreed to in writing, software
010// distributed under the License is distributed on an "AS IS" BASIS,
011// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
012// See the License for the specific language governing permissions and
013// limitations under the License.
014
015package org.apache.tapestry5.ioc.internal.services;
016
017import org.apache.tapestry5.ioc.ObjectCreator;
018import org.apache.tapestry5.ioc.internal.util.LockSupport;
019
020/**
021 * An {@link org.apache.tapestry5.ioc.ObjectCreator} that delegates to another
022 * {@link org.apache.tapestry5.ioc.ObjectCreator} and caches the result.
023 */
024public class CachingObjectCreator<T> extends LockSupport implements ObjectCreator<T>
025{
026    private boolean cached;
027
028    private T cachedValue;
029
030    private ObjectCreator<T> delegate;
031
032    public CachingObjectCreator(ObjectCreator<T> delegate)
033    {
034        this.delegate = delegate;
035    }
036
037    public T createObject()
038    {
039        try
040        {
041            acquireReadLock();
042
043            if (!cached)
044            {
045                cacheValueFromDelegate();
046            }
047
048            return cachedValue;
049        } finally
050        {
051            releaseReadLock();
052        }
053    }
054
055    private void cacheValueFromDelegate()
056    {
057        try
058        {
059            upgradeReadLockToWriteLock();
060
061            if (!cached)
062            {
063                cachedValue = delegate.createObject();
064                cached = true;
065                delegate = null;
066            }
067        } finally
068        {
069            downgradeWriteLockToReadLock();
070        }
071    }
072}