1 /* 2 * Copyright (C) 2014 Google, Inc. 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 package dagger.internal; 17 18 import dagger.Lazy; 19 import javax.inject.Provider; 20 21 /** 22 * A {@link Provider} implementation that memoizes the result of a {@link Factory} instance. 23 * 24 * @author Gregory Kick 25 * @since 2.0 26 */ 27 public final class ScopedProvider<T> implements Provider<T>, Lazy<T> { 28 private static final Object UNINITIALIZED = new Object(); 29 30 private final Factory<T> factory; 31 private volatile Object instance = UNINITIALIZED; 32 33 private ScopedProvider(Factory<T> factory) { 34 assert factory != null; 35 this.factory = factory; 36 } 37 38 @SuppressWarnings("unchecked") // cast only happens when result comes from the factory 39 @Override 40 public T get() { 41 // double-check idiom from EJ2: Item 71 42 Object result = instance; 43 if (result == UNINITIALIZED) { 44 synchronized (this) { 45 result = instance; 46 if (result == UNINITIALIZED) { 47 instance = result = factory.get(); 48 } 49 } 50 } 51 return (T) result; 52 } 53 54 /** Returns a new scoped provider for the given factory. */ 55 public static <T> Provider<T> create(Factory<T> factory) { 56 if (factory == null) { 57 throw new NullPointerException(); 58 } 59 return new ScopedProvider<T>(factory); 60 } 61 } 62
