Java tutorial
/* * 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.slider.core.main; import org.apache.hadoop.service.Service; import org.apache.hadoop.util.ShutdownHookManager; import org.slf4j.Logger; import org.slf4j.LoggerFactory; import java.lang.ref.WeakReference; /** * JVM Shutdown hook for Service which will stop the * Service gracefully in case of JVM shutdown. * This hook uses a weak reference to the service, so * does not cause services to be retained after they have * been stopped and deferenced elsewhere. */ public class ServiceShutdownHook implements Runnable { private static final Logger LOG = LoggerFactory.getLogger(ServiceShutdownHook.class); private final WeakReference<Service> serviceRef; private Runnable hook; public ServiceShutdownHook(Service service) { serviceRef = new WeakReference<Service>(service); } public void register(int priority) { unregister(); hook = this; ShutdownHookManager.get().addShutdownHook(hook, priority); } public synchronized void unregister() { if (hook != null) { try { ShutdownHookManager.get().removeShutdownHook(hook); } catch (IllegalStateException e) { LOG.info("Failed to unregister shutdown hook: {}", e, e); } hook = null; } } @Override public void run() { Service service; synchronized (this) { service = serviceRef.get(); serviceRef.clear(); } if (service == null) { return; } try { // Stop the Service service.stop(); } catch (Throwable t) { LOG.info("Error stopping {}: {}", service.getName(), t); } } }