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*/
018
019package org.apache.hadoop.service;
020
021import org.apache.hadoop.classification.InterfaceAudience.Public;
022import org.apache.hadoop.classification.InterfaceStability.Stable;
023
024/**
025 * Interface to notify state changes of a service.
026 */
027@Public
028@Stable
029public interface ServiceStateChangeListener {
030
031  /**
032   * Callback to notify of a state change. The service will already
033   * have changed state before this callback is invoked.
034   *
035   * This operation is invoked on the thread that initiated the state change,
036   * while the service itself in in a sychronized section.
037   * <ol>
038   *   <li>Any long-lived operation here will prevent the service state
039   *   change from completing in a timely manner.</li>
040   *   <li>If another thread is somehow invoked from the listener, and
041   *   that thread invokes the methods of the service (including
042   *   subclass-specific methods), there is a risk of a deadlock.</li>
043   * </ol>
044   *
045   *
046   * @param service the service that has changed.
047   */
048  void stateChanged(Service service);
049
050}