Class DoubleCheck

java/dagger/internal/DoubleCheck.java:28–96

A Lazy and Provider implementation that memoizes the value returned from a delegate using the double-check idiom described in Item 71 of Effective Java 2 .

 * delegate using the double-check idiom described in Item 71 of <i>Effective Java 2</i>.
 */
public final class DoubleCheck<T> implements Provider<T>, Lazy<T> {
  private static final Object UNINITIALIZED = new Object();

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

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

  @SuppressWarnings("unchecked") // cast only happens when result comes from the provider
  @Override
  public T get() {
    Object result = instance;
    if (result == UNINITIALIZED) {
      synchronized (this) {
        result = instance;
        if (result == UNINITIALIZED) {
          result = provider.get();
          /* Get the current instance and test to see if the call to provider.get() has resulted
           * in a recursive call.  If it returns the same instance, we'll allow it, but if the
           * instances differ, throw. */
          Object currentInstance = instance;
          if (currentInstance != UNINITIALIZED && currentInstance != result) {
            throw new IllegalStateException("Scoped provider was invoked recursively returning "
                + "different results: " + currentInstance + " & " + result + ". This is likely "
                + "due to a circular dependency.");
          }
          instance = result;
          /* 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) result;
  }

  /** 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> delegate)"
  // 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 delegate) {
    checkNotNull(delegate);
    if (delegate instanceof DoubleCheck) {
      /* This should be a rare case, but if we have a scoped @Binds that delegates to a scoped
       * binding, we shouldn't cache the value again. */
      return delegate;
    }
    return new DoubleCheck<T>(delegate);
  }

  /** Returns a {@link Lazy} that caches the value from the given provider. */
  // This method is declared this way instead of "<T> Lazy<T> lazy(Provider<T> delegate)"
  // to work around an Eclipse type inference bug: https://github.com/google/dagger/issues/949.
  public static <P extends Provider<T>, T> Lazy<T> lazy(P provider) {
    if (provider instanceof Lazy) {
      @SuppressWarnings("unchecked")