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 /** 23 * A {@link Provider} implementation that memoizes the result of another {@link Provider} using 24 * simple lazy initialization, not the double-checked lock pattern. 25 */ 26 public final class SingleCheck<T> implements Provider<T> { 27 private static final Object UNINITIALIZED = new Object(); 28 29 private volatile Provider<T> provider; 30 private volatile Object instance = UNINITIALIZED; 31 SingleCheck(Provider<T> provider)32 private SingleCheck(Provider<T> provider) { 33 assert provider != null; 34 this.provider = provider; 35 } 36 37 @SuppressWarnings("unchecked") // cast only happens when result comes from the delegate provider 38 @Override get()39 public T get() { 40 Object local = instance; 41 if (local == UNINITIALIZED) { 42 // provider is volatile and might become null after the check, so retrieve the provider first 43 Provider<T> providerReference = provider; 44 if (providerReference == null) { 45 // The provider was null, so the instance must already be set 46 local = instance; 47 } else { 48 local = providerReference.get(); 49 instance = local; 50 51 // Null out the reference to the provider. We are never going to need it again, so we can 52 // make it eligible for GC. 53 provider = null; 54 } 55 } 56 return (T) local; 57 } 58 59 /** Returns a {@link Provider} that caches the value from the given delegate provider. */ 60 // This method is declared this way instead of "<T> Provider<T> provider(Provider<T> provider)" 61 // to work around an Eclipse type inference bug: https://github.com/google/dagger/issues/949. provider(P provider)62 public static <P extends Provider<T>, T> Provider<T> provider(P 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