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    
019    package org.apache.hadoop.service;
020    
021    import org.apache.hadoop.classification.InterfaceAudience.Public;
022    import org.apache.hadoop.classification.InterfaceStability.Stable;
023    
024    /**
025     * Interface to notify state changes of a service.
026     */
027    @Public
028    @Stable
029    public 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    }