FineGrainedLazySingletonScopeImpl.java

/*
 * Copyright 2013 Netflix, Inc.
 *
 *    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 com.netflix.governator.guice.lazy;

import com.google.common.collect.Maps;
import com.google.inject.Key;
import com.google.inject.Provider;
import com.google.inject.ProvisionException;
import com.google.inject.internal.CircularDependencyProxy;
import com.netflix.governator.internal.AbstractScope;

import java.util.Map;

/**
 * @deprecated Use javax.inject.Singleton instead.  FineGrainedLazySingleton is not needed 
 * as of Guice4 which fixes the global lock issue.
 */
@Deprecated
class FineGrainedLazySingletonScopeImpl extends AbstractScope
{
    private static final Object NULL = new Object();
    private static final Map<Key<?>, LockRecord> locks = Maps.newHashMap();

    private static class LockRecord
    {
        private final Object        lock = new Object();
        private int                 useCount = 0;
    }

    public <T> Provider<T> scope(final Key<T> key, final Provider<T> creator)
    {
        return new Provider<T>()
        {
            /*
             * The lazily initialized singleton instance. Once set, this will either have type T or will
             * be equal to NULL.
             */
            private volatile Object instance;

            // DCL on a volatile is safe as of Java 5, which we obviously require.
            @SuppressWarnings("DoubleCheckedLocking")
            public T get()
            {
                if ( instance == null )
                {
                    try
                    {
                        final Object      lock = getLock(key);
                        //noinspection SynchronizationOnLocalVariableOrMethodParameter
                        synchronized (lock)
                        {
                            if ( instance == null )
                            {
                                T provided = creator.get();

                                // don't remember proxies; these exist only to serve circular dependencies
                                if ( provided instanceof CircularDependencyProxy )
                                {
                                    return provided;
                                }

                                Object providedOrSentinel = (provided == null) ? NULL : provided;
                                if ( (instance != null) && (instance != providedOrSentinel) )
                                {
                                    throw new ProvisionException("Provider was reentrant while creating a singleton");
                                }

                                instance = providedOrSentinel;
                            }
                        }
                    }
                    finally
                    {
                        releaseLock(key);
                    }
                }

                Object localInstance = instance;
                // This is safe because instance has type T or is equal to NULL
                @SuppressWarnings({"unchecked", "UnnecessaryLocalVariable"})
                T returnedInstance = (localInstance != NULL) ? (T) localInstance : null;
                return returnedInstance;
            }

            public String toString()
            {
                return String.format("%s[%s]", creator, instance);
            }
        };
    }

    @SuppressWarnings("SynchronizationOnLocalVariableOrMethodParameter")
    private Object getLock(Key<?> key)
    {
        synchronized(locks)
        {
            LockRecord      lock = locks.get(key);
            if ( lock == null )
            {
                lock = new LockRecord();
                locks.put(key, lock);
            }
            ++lock.useCount;
            return lock.lock;
        }
    }

    private void releaseLock(Key<?> key)
    {
        synchronized(locks)
        {
            LockRecord      lock = locks.get(key);
            if ( lock != null )
            {
                if ( --lock.useCount <= 0 )
                {
                    locks.remove(key);
                }
            }
        }
    }

    public boolean isSingletonScope() {
        return true;
    }

    @Override
    public String toString()
    {
        return "FineGrainedLazySingletonScope.SCOPE";
    }
}