Class SingleCheck

java/dagger/internal/SingleCheck.java:28–66

A Provider implementation that memoizes the result of another Provider using simple lazy initialization, not the double-checked lock pattern.

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