001/* 002 * Licensed to the Apache Software Foundation (ASF) under one 003 * or more contributor license agreements. See the NOTICE file 004 * distributed with this work for additional information 005 * regarding copyright ownership. The ASF licenses this file 006 * to you under the Apache License, Version 2.0 (the 007 * "License"); you may not use this file except in compliance 008 * with the License. You may obtain a copy of the License at 009 * 010 * http://www.apache.org/licenses/LICENSE-2.0 011 * 012 * Unless required by applicable law or agreed to in writing, software 013 * distributed under the License is distributed on an "AS IS" BASIS, 014 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 015 * See the License for the specific language governing permissions and 016 * limitations under the License. 017 */ 018package org.apache.hadoop.hbase.zookeeper; 019 020import org.apache.zookeeper.WatchedEvent; 021import org.apache.zookeeper.Watcher; 022 023/** 024 * Placeholder of a watcher which might be triggered before the instance is not yet created. 025 * <p> 026 * {@code ZooKeeper} starts its event thread within its constructor (and that is an anti-pattern), 027 * and the watcher passed to the constructor might be called back by the event thread before you get 028 * the instance of {@code ZooKeeper} from the constructor. If your watcher calls methods of 029 * {@code ZooKeeper}, pass this placeholder to the constructor of the {@code ZooKeeper}, create your 030 * watcher using the instance of {@code ZooKeeper}, and then call the method 031 * {@code PendingWatcher.prepare}. 032 */ 033class PendingWatcher implements Watcher { 034 private final InstancePending<Watcher> pending = new InstancePending<>(); 035 036 @Override 037 public void process(WatchedEvent event) { 038 pending.get().process(event); 039 } 040 041 /** 042 * Associates the substantial watcher of processing events. This method should be called once, and 043 * {@code watcher} should be non-null. This method is expected to call as soon as possible because 044 * the event processing, being invoked by the ZooKeeper event thread, is uninterruptibly blocked 045 * until this method is called. 046 */ 047 void prepare(Watcher watcher) { 048 pending.prepare(watcher); 049 } 050}