/*
    * Copyright (C) 2008-2016, Google Inc. and others
    *
    * This program and the accompanying materials are made available under the
    * terms of the Eclipse Distribution License v. 1.0 which is available at
    * https://www.eclipse.org/org/documents/edl-v10.php.
    *
    * SPDX-License-Identifier: BSD-3-Clause
    */
  
  package org.eclipse.jgit.lib.internal;
  
  import java.util.concurrent.Executors;
  import java.util.concurrent.ScheduledThreadPoolExecutor;
  import java.util.concurrent.ThreadFactory;
  
  /**
   * Simple work queue to run tasks in the background
   */
  public class WorkQueue {
  	private static final ScheduledThreadPoolExecutor executor;
  
  	static final Object executorKiller;
  
  	static {
  		// To support garbage collection, start our thread but
  		// swap out the thread factory. When our class is GC'd
  		// the executorKiller will finalize and ask the executor
  		// to shutdown, ending the worker.
  		//
  		int threads = 1;
  		executor = new ScheduledThreadPoolExecutor(threads,
  				new ThreadFactory() {
  					private final ThreadFactory baseFactory = Executors
  							.defaultThreadFactory();
  
  					@Override
  					public Thread newThread(Runnable taskBody) {
  						Thread thr = baseFactory.newThread(taskBody);
  						thr.setName("JGit-WorkQueue"); //$NON-NLS-1$
  						thr.setContextClassLoader(null);
  						thr.setDaemon(true);
  						return thr;
  					}
  				});
  		executor.setRemoveOnCancelPolicy(true);
  		executor.setContinueExistingPeriodicTasksAfterShutdownPolicy(false);
  		executor.setExecuteExistingDelayedTasksAfterShutdownPolicy(false);
  		executor.prestartAllCoreThreads();
  
  		// Now that the threads are running, its critical to swap out
  		// our own thread factory for one that isn't in the ClassLoader.
  		// This allows the class to GC.
  		//
  		executor.setThreadFactory(Executors.defaultThreadFactory());
  
  		executorKiller = new Object() {
  			@Override
  			protected void finalize() {
  				executor.shutdownNow();
  			}
  		};
  	}
  
  	/**
  	 * Get the WorkQueue's executor
  	 *
  	 * @return the WorkQueue's executor
  	 */
  	public static ScheduledThreadPoolExecutor getExecutor() {
  		return executor;
  	}
  }