DoubleCheck.java

/*
 * Copyright (C) 2016 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 Lazy} and {@link Provider} implementation that memoizes the value returned from a
 * 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);
                    }
                    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.
     */
    public static <T> Provider<T> provider(Provider<T> 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.
     */
    public static <T> Lazy<T> lazy(Provider<T> provider) {
        if (provider instanceof Lazy) {
            @SuppressWarnings("unchecked")
            final Lazy<T> lazy = (Lazy<T>) provider;
            // Avoids memoizing a value that is already memoized.
            // NOTE: There is a pathological case where Provider<P> may implement Lazy<L>, but P and L
            // are different types using covariant return on get(). Right now this is used with
            // DoubleCheck<T> exclusively, which is implemented such that P and L are always
            // the same, so it will be fine for that case.
            return lazy;
        }
        return new DoubleCheck<T>(checkNotNull(provider));
    }
}