| /* |
| * 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.lucene.util; |
| |
| import java.util.concurrent.atomic.AtomicReference; |
| |
| |
| /** |
| * A convenient class which offers a semi-immutable object wrapper |
| * implementation which allows one to set the value of an object exactly once, |
| * and retrieve it many times. If {@link #set(Object)} is called more than once, |
| * {@link AlreadySetException} is thrown and the operation |
| * will fail. |
| * |
| * @lucene.experimental |
| */ |
| public final class SetOnce<T> implements Cloneable { |
| |
| /** Thrown when {@link SetOnce#set(Object)} is called more than once. */ |
| public static final class AlreadySetException extends IllegalStateException { |
| public AlreadySetException() { |
| super("The object cannot be set twice!"); |
| } |
| } |
| |
| /** Holding object and marking that it was already set */ |
| private static final class Wrapper<T> { |
| private T object; |
| |
| private Wrapper(T object) { |
| this.object = object; |
| } |
| } |
| |
| private final AtomicReference<Wrapper<T>> set; |
| |
| /** |
| * A default constructor which does not set the internal object, and allows |
| * setting it by calling {@link #set(Object)}. |
| */ |
| public SetOnce() { |
| set = new AtomicReference<>(); |
| } |
| |
| /** |
| * Creates a new instance with the internal object set to the given object. |
| * Note that any calls to {@link #set(Object)} afterwards will result in |
| * {@link AlreadySetException} |
| * |
| * @throws AlreadySetException if called more than once |
| * @see #set(Object) |
| */ |
| public SetOnce(T obj) { |
| set = new AtomicReference<>(new Wrapper<>(obj)); |
| } |
| |
| /** Sets the given object. If the object has already been set, an exception is thrown. */ |
| public final void set(T obj) { |
| if (!trySet(obj)) { |
| throw new AlreadySetException(); |
| } |
| } |
| |
| /** Sets the given object if none was set before. |
| * |
| * @return true if object was set successfully, false otherwise |
| * */ |
| public final boolean trySet(T obj) { |
| return set.compareAndSet(null, new Wrapper<>(obj)); |
| } |
| |
| /** Returns the object set by {@link #set(Object)}. */ |
| public final T get() { |
| Wrapper<T> wrapper = set.get(); |
| return wrapper == null ? null : wrapper.object; |
| } |
| } |