hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/service/ServiceStateChangeListener.java

/**
* 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.hadoop.service;

import org.apache.hadoop.classification.InterfaceAudience.Public;
import org.apache.hadoop.classification.InterfaceStability.Stable;

/**
 * Interface to notify state changes of a service.
 */
@Public
@Stable
public interface ServiceStateChangeListener {

  /**
   * Callback to notify of a state change. The service will already
   * have changed state before this callback is invoked.
   *
   * This operation is invoked on the thread that initiated the state change,
   * while the service itself in in a sychronized section.
   * <ol>
   *   <li>Any long-lived operation here will prevent the service state
   *   change from completing in a timely manner.</li>
   *   <li>If another thread is somehow invoked from the listener, and
   *   that thread invokes the methods of the service (including
   *   subclass-specific methods), there is a risk of a deadlock.</li>
   * </ol>
   *
   *
   * @param service the service that has changed.
   */
  void stateChanged(Service service);

}