1 /* 2 * Copyright (C) 2014 The Dagger Authors. 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, 12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 * See the License for the specific language governing permissions and 14 * limitations under the License. 15 */ 16 17 package dagger.internal; 18 19 import static dagger.internal.Preconditions.checkNotNull; 20 import static dagger.internal.Providers.asDaggerProvider; 21 22 import org.jspecify.annotations.Nullable; 23 24 /** 25 * A {@link Provider} implementation that memoizes the result of another {@link Provider} using 26 * simple lazy initialization, not the double-checked lock pattern. 27 */ 28 public final class SingleCheck<T extends @Nullable Object> implements Provider<T> { 29 private static final Object UNINITIALIZED = new Object(); 30 31 private volatile @Nullable Provider<T> provider; 32 private volatile @Nullable Object instance = UNINITIALIZED; 33 SingleCheck(Provider<T> provider)34 private SingleCheck(Provider<T> provider) { 35 assert provider != null; 36 this.provider = provider; 37 } 38 39 @SuppressWarnings("unchecked") // cast only happens when result comes from the delegate provider 40 @Override get()41 public T get() { 42 @Nullable Object local = instance; 43 if (local == UNINITIALIZED) { 44 // provider is volatile and might become null after the check, so retrieve the provider first 45 @Nullable Provider<T> providerReference = provider; 46 if (providerReference == null) { 47 // The provider was null, so the instance must already be set 48 local = instance; 49 } else { 50 local = providerReference.get(); 51 instance = local; 52 53 // Null out the reference to the provider. We are never going to need it again, so we can 54 // make it eligible for GC. 55 provider = null; 56 } 57 } 58 return (T) local; 59 } 60 61 /** Returns a {@link Provider} that caches the value from the given delegate provider. */ provider(Provider<T> provider)62 public static <T> Provider<T> provider(Provider<T> provider) { 63 // If a scoped @Binds delegates to a scoped binding, don't cache the value again. 64 if (provider instanceof SingleCheck || provider instanceof DoubleCheck) { 65 return provider; 66 } 67 return new SingleCheck<T>(checkNotNull(provider)); 68 } 69 70 /** 71 * Legacy javax version of the method to support libraries compiled with an older version of 72 * Dagger. Do not use directly. 73 */ provider( P delegate)74 public static <P extends javax.inject.Provider<T>, T> javax.inject.Provider<T> provider( 75 P delegate) { 76 return provider(asDaggerProvider(delegate)); 77 } 78 } 79