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A fast dependency injector for Android and Java.
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/*
 * 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 static dagger.internal.Preconditions.checkNotNull;

import dagger.Lazy;
import javax.inject.Provider;

/**
 * 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 implements Provider, Lazy {
  private static final Object UNINITIALIZED = new Object();

  private volatile Provider provider;
  private volatile Object instance = UNINITIALIZED;

  private SingleCheck(Provider 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 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. */
  // This method is declared this way instead of " Provider provider(Provider provider)" 
  // to work around an Eclipse type inference bug: https://github.com/google/dagger/issues/949.
  public static , T> Provider provider(P 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(checkNotNull(provider));
  }
}