1 /* 2 * Licensed to the Apache Software Foundation (ASF) under one 3 * or more contributor license agreements. See the NOTICE file 4 * distributed with this work for additional information 5 * regarding copyright ownership. The ASF licenses this file 6 * to you under the Apache License, Version 2.0 (the 7 * "License"); you may not use this file except in compliance 8 * with the License. You may obtain a copy of the License at 9 * 10 * http://www.apache.org/licenses/LICENSE-2.0 11 * 12 * Unless required by applicable law or agreed to in writing, software 13 * distributed under the License is distributed on an "AS IS" BASIS, 14 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 15 * See the License for the specific language governing permissions and 16 * limitations under the License. 17 */ 18 19 package org.apache.hadoop.hbase.zookeeper; 20 21 import java.util.concurrent.CountDownLatch; 22 23 /** 24 * Placeholder of an instance which will be accessed by other threads 25 * but is not yet created. Thread safe. 26 */ 27 class InstancePending<T> { 28 // Based on a subtle part of the Java Language Specification, 29 // in order to avoid a slight overhead of synchronization for each access. 30 31 private final CountDownLatch pendingLatch = new CountDownLatch(1); 32 33 /** Piggybacking on {@code pendingLatch}. */ 34 private InstanceHolder<T> instanceHolder; 35 36 private static class InstanceHolder<T> { 37 // The JLS ensures the visibility of a final field and its contents 38 // unless they are exposed to another thread while the construction. 39 final T instance; 40 41 InstanceHolder(T instance) { 42 this.instance = instance; 43 } 44 } 45 46 /** 47 * Returns the instance given by the method {@link #prepare}. 48 * This is an interruptible blocking method 49 * and the interruption flag will be set just before returning if any. 50 */ 51 T get() { 52 InstanceHolder<T> instanceHolder; 53 boolean interrupted = false; 54 55 while ((instanceHolder = this.instanceHolder) == null) { 56 try { 57 pendingLatch.await(); 58 } catch (InterruptedException e) { 59 interrupted = true; 60 } 61 } 62 63 if (interrupted) { 64 Thread.currentThread().interrupt(); 65 } 66 return instanceHolder.instance; 67 } 68 69 /** 70 * Associates the given instance for the method {@link #get}. 71 * This method should be called once, and {@code instance} should be non-null. 72 * This method is expected to call as soon as possible 73 * because the method {@code get} is uninterruptibly blocked until this method is called. 74 */ 75 void prepare(T instance) { 76 assert instance != null; 77 instanceHolder = new InstanceHolder<T>(instance); 78 pendingLatch.countDown(); 79 } 80 }