| /* |
| * Licensed to the Apache Software Foundation (ASF) under one or more |
| * contributor license agreements. See the NOTICE file distributed with |
| * this work for additional information regarding copyright ownership. |
| * The ASF licenses this file to You under the Apache License, Version 2.0 |
| * (the "License"); you may not use this file except in compliance with |
| * the License. You may obtain a copy of the License at |
| * |
| * http://www.apache.org/licenses/LICENSE-2.0 |
| * |
| * Unless required by applicable law or agreed to in writing, software |
| * distributed under the License is distributed on an "AS IS" BASIS, |
| * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| * See the License for the specific language governing permissions and |
| * limitations under the License. |
| */ |
| package org.apache.jackrabbit.oak.commons; |
| |
| import java.util.function.Supplier; |
| |
| /** |
| * An instances of this class represents a lazy value of type {@code T}. |
| * {@code LazyValue} implements an evaluate by need semantics: |
| * {@link #createValue()} is called exactly once when {@link #get()} |
| * is called for the first time. |
| * <p> |
| * {@code LazyValue} instances are thread safe. |
| */ |
| public abstract class LazyValue<T> implements Supplier<T> { |
| private volatile T value; |
| |
| /** |
| * Factory method called to create the value on an as need basis. |
| * @return a new instance for {@code T}. |
| */ |
| protected abstract T createValue(); |
| |
| /** |
| * @return {@code true} iff {@link #get()} has been called at least once. |
| */ |
| public boolean hasValue() { |
| return value != null; |
| } |
| |
| /** |
| * Get value. Calls {@link #createValue()} if called for the first time. |
| * @return the value |
| */ |
| @Override |
| public T get () { |
| // Double checked locking is fine since Java 5 as long as value is volatile. |
| // See http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html |
| if (value == null) { |
| synchronized (this) { |
| if (value == null) { |
| value = createValue(); |
| } |
| } |
| } |
| return value; |
| } |
| |
| } |