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    
015    package org.apache.tapestry5.ioc.internal.services;
016    
017    import org.apache.tapestry5.ioc.ObjectCreator;
018    import 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     */
024    public 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    }