org.apache.hadoop.hbase.zookeeper.PendingWatcher Maven / Gradle / Ivy
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* 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,
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* limitations under the License.
*/
package org.apache.hadoop.hbase.zookeeper;
import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
/**
* Placeholder of a watcher which might be triggered before the instance is not yet created.
*
* {@code ZooKeeper} starts its event thread within its constructor (and that is an anti-pattern),
* and the watcher passed to the constructor might be called back by the event thread
* before you get the instance of {@code ZooKeeper} from the constructor.
* If your watcher calls methods of {@code ZooKeeper},
* pass this placeholder to the constructor of the {@code ZooKeeper},
* create your watcher using the instance of {@code ZooKeeper},
* and then call the method {@code PendingWatcher.prepare}.
*/
class PendingWatcher implements Watcher {
private final InstancePending pending = new InstancePending();
@Override
public void process(WatchedEvent event) {
pending.get().process(event);
}
/**
* Associates the substantial watcher of processing events.
* This method should be called once, and {@code watcher} should be non-null.
* This method is expected to call as soon as possible
* because the event processing, being invoked by the ZooKeeper event thread,
* is uninterruptibly blocked until this method is called.
*/
void prepare(Watcher watcher) {
pending.prepare(watcher);
}
}