# CachingInjectionProviderDecorator.java

    /*
     * Copyright (c) 2007, Rickard Öberg. All Rights Reserved.
     *
     * Licensed 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.qi4j.runtime.injection.provider;

    import org.qi4j.runtime.injection.InjectionContext;
    import org.qi4j.runtime.injection.InjectionProvider;

    /**
     * If a dependency resolution should be a singleton, wrap it with this
     * to provide a single instance "cache".
     */
    public final class CachingInjectionProviderDecorator
        implements InjectionProvider
    {
        private final InjectionProvider decoratedProvider;
        private volatile Object singletonInstance;

        public CachingInjectionProviderDecorator( InjectionProvider injectionProvider )
        {
            this.decoratedProvider = injectionProvider;
        }

        public InjectionProvider decoratedProvider()
        {
            return decoratedProvider;
        }

        @Override
        public Object provideInjection( InjectionContext context )
            throws InjectionProviderException
        {
            if( singletonInstance == null )
            {
                synchronized( this )
                {
                    if( singletonInstance == null )
                    {
                        singletonInstance = decoratedProvider.provideInjection( context );
                    }
                }
            }

            return singletonInstance;
        }
    }