如何优雅的关闭线程池

        JUC中可以通过Executors框架来实现线程池，可以创建一个固定大小的线程池，如下所示：

ExecutorService service = Executors.newFixedThreadPool(15);

关闭线程池有两种方法：shutdown()和shutdownNow()，两种方法的区别何在？

1.shutdown()

当调用了shutdown()方法时，便进入关闭状态，此时意味着 ExecutorService不再接受新的任务，但它还在执行已经提交了的任务，当已经提交了的任务执行完后，便到达终止状态。如果不调用 shutdown()方法，ExecutorService 会一直处在运行状态，不断接收新的任务，执行新的任务，服务器端一般不需要关闭它，保持一直运行即可。

2.shutdownNow()

阻止等待任务启动并试图停止当前正在执行的任务，即停止当前执行的task，并返回尚未执行的task的list。

下面介绍一段代码来优雅的关闭线程池：

private void closeExecutorThreadPoolGraceFully(ExecutorService executor) {
    try {
        executor.shutdown();
        if(!executor.awaitTermination(SHUTDOWN_TIME, TimeUnit.SECONDS)) {
            logger.info("Executor did not terminate in the specified time.");
            List droppedTasks = executor.shutdownNow();
            logger.info("Executors was abruptly shut down." + droppedTasks.size() + " tasks will not be executed.");
        }

        logger.info("scheduled executor service closed normally.");
    }  catch (InterruptedException e) {
        logger.error("interrupted exception error...");
    }
}

当然也可以启动一个线程来操作，异步操作效率可能更高：

private void closeExecutorThreadPoolGraceFully(ExecutorService executor) {
    final ExecutorService executor;

    Runtime.getRuntime().addShutdownHook(new Thread() {
        public void run() {
            executor.shutdown();
            if (!executor.awaitTermination(SHUTDOWN_TIME)) { 
                Logger.log("Executor did not terminate in the specified time."); 
                List droppedTasks = executor.shutdownNow(); 
                Logger.log("Executor was abruptly shut down. " + droppedTasks.size() + " tasks will not be executed."); 
            }
        }
    }
}