SingleCheck.java

/*
 * Copyright (C) 2014 The Dagger Authors.
 *
 * 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 dagger.internal;

import dagger.Lazy;

import javax.inject.Provider;

import static dagger.internal.Preconditions.checkNotNull;

/**
 * A {@link Provider} implementation that memoizes the result of another {@link Provider} using
 * simple lazy initialization, not the double-checked lock pattern.
 */
public final class SingleCheck<T> implements Provider<T>, Lazy<T> {
    private static final Object UNINITIALIZED = new Object();

    private volatile Provider<T> provider;
    private volatile Object instance = UNINITIALIZED;

    private SingleCheck(Provider<T> provider) {
        assert provider != null;
        this.provider = provider;
    }

    @SuppressWarnings("unchecked") // cast only happens when result comes from the delegate provider
    @Override
    public T get() {
        // provider is volatile and might become null after the check to instance == UNINITIALIZED, so
        // retrieve the provider first, which should not be null if instance is UNINITIALIZED.
        // This relies upon instance also being volatile so that the reads and writes of both variables
        // cannot be reordered.
        Provider<T> providerReference = provider;
        if (instance == UNINITIALIZED) {
            instance = providerReference.get();
            // Null out the reference to the provider. We are never going to need it again, so we can make
            // it eligible for GC.
            provider = null;
        }
        return (T) instance;
    }

    /**
     * Returns a {@link Provider} that caches the value from the given delegate provider.
     */
    public static <T> Provider<T> provider(Provider<T> provider) {
        // If a scoped @Binds delegates to a scoped binding, don't cache the value again.
        if (provider instanceof SingleCheck || provider instanceof DoubleCheck) {
            return provider;
        }
        return new SingleCheck<T>(checkNotNull(provider));
    }
}