001/*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements.  See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License.  You may obtain a copy of the License at
008 *
009 *      http://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017package org.apache.wicket.util;
018
019import java.util.function.Supplier;
020
021import org.apache.wicket.util.io.IClusterable;
022
023/**
024 * An abstraction for lazy-initializing values. Guarantees only a single instance of the value is
025 * created.
026 * 
027 * Initialized value <strong>WILL NOT</strong> be serialized, and will be recreated upon
028 * de-serialization.
029 * 
030 * @author igor
031 * @param <T>
032 *            type of value
033 */
034public abstract class LazyInitializer<T> implements Supplier<T>, IClusterable
035{
036        private static final long serialVersionUID = 1L;
037
038        private transient volatile T instance = null;
039
040        @Override
041        public T get()
042        {
043                if (instance == null)
044                {
045                        synchronized (this)
046                        {
047                                if (instance == null)
048                                {
049                                        instance = createInstance();
050                                }
051                        }
052                }
053                return instance;
054        }
055
056        /**
057         * Creates the lazy value
058         * 
059         * @return new instance of the value
060         */
061        protected abstract T createInstance();
062}