1 /* 2 * Licensed to the Apache Software Foundation (ASF) under one or more 3 * contributor license agreements. See the NOTICE file distributed with 4 * this work for additional information regarding copyright ownership. 5 * The ASF licenses this file to You under the Apache License, Version 2.0 6 * (the "License"); you may not use this file except in compliance with 7 * the License. You may obtain a copy of the License at 8 * 9 * http://www.apache.org/licenses/LICENSE-2.0 10 * 11 * Unless required by applicable law or agreed to in writing, software 12 * distributed under the License is distributed on an "AS IS" BASIS, 13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 14 * See the License for the specific language governing permissions and 15 * limitations under the License. 16 */ 17 18 package org.apache.commons.proxy2.provider; 19 20 import org.apache.commons.proxy2.ObjectProvider; 21 22 /** 23 * Wraps another object provider, making sure to only call it once, returning the value returned from the wrapped 24 * provider on all subsequent invocations. 25 * 26 * @since 1.0 27 */ 28 public class SingletonProvider<T> extends ProviderDecorator<T> 29 { 30 //****************************************************************************************************************** 31 // Fields 32 //****************************************************************************************************************** 33 34 private static final long serialVersionUID = 1L; 35 36 private T instance; 37 38 //****************************************************************************************************************** 39 // Constructors 40 //****************************************************************************************************************** 41 42 /** 43 * Create a new SingletonProvider instance. 44 * 45 * @param inner 46 */ 47 public SingletonProvider(ObjectProvider<? extends T> inner) 48 { 49 super(inner); 50 } 51 52 //****************************************************************************************************************** 53 // ObjectProvider Implementation 54 //****************************************************************************************************************** 55 56 /** 57 * {@inheritDoc} 58 */ 59 @Override 60 public T getObject() 61 { 62 synchronized (this) 63 { 64 if (instance == null) 65 { 66 instance = super.getObject(); 67 // Garbage collection 68 setInner(null); 69 } 70 return instance; 71 } 72 } 73 }