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 org.apache.zookeeper.WatchedEvent; 22 import org.apache.zookeeper.Watcher; 23 24 /** 25 * Placeholder of a watcher which might be triggered before the instance is not yet created. 26 * <p> 27 * {@code ZooKeeper} starts its event thread within its constructor (and that is an anti-pattern), 28 * and the watcher passed to the constructor might be called back by the event thread 29 * before you get the instance of {@code ZooKeeper} from the constructor. 30 * If your watcher calls methods of {@code ZooKeeper}, 31 * pass this placeholder to the constructor of the {@code ZooKeeper}, 32 * create your watcher using the instance of {@code ZooKeeper}, 33 * and then call the method {@code PendingWatcher.prepare}. 34 */ 35 class PendingWatcher implements Watcher { 36 private final InstancePending<Watcher> pending = new InstancePending<Watcher>(); 37 38 @Override 39 public void process(WatchedEvent event) { 40 pending.get().process(event); 41 } 42 43 /** 44 * Associates the substantial watcher of processing events. 45 * This method should be called once, and {@code watcher} should be non-null. 46 * This method is expected to call as soon as possible 47 * because the event processing, being invoked by the ZooKeeper event thread, 48 * is uninterruptibly blocked until this method is called. 49 */ 50 void prepare(Watcher watcher) { 51 pending.prepare(watcher); 52 } 53 }