java.util.concurrent.ScheduledExecutorService 是一种执行服务，它可以安排任务在延迟一定时间后运行，或者在每次执行之间以固定的时间间隔重复执行。而且此任务是由一个工作线程异步执行的，而不是由将任务交给预定执行程序服务的线程执行。

ScheduledExecutorService例子

首先创建一个带有5个线程的SechuleExecutorService。然后将Callbale接口的匿名实例类被传递给schedule()方法。最后两个参数指定了Callable将在5秒后执行。

public class ScheduledExecutorServiceDemo {
  public static void main(String[] args) throws ExecutionException, InterruptedException {
    ScheduledExecutorService executorService = Executors.newScheduledThreadPool(5);
    ScheduledFuture scheduledFuture = executorService.schedule(() -> {
      System.out.println("Executed!");
      return "Called!";
    }, 5, TimeUnit.SECONDS);
    Object result = scheduledFuture.get();
    System.out.println("result = " + result);
  }
}

官方接口实现

ScheduledExecutorService 是一个接口标记类，java.util.concurrent 包中有以下关于此接口的实现类：ScheduledThreadPoolExecutor

相关概念

Java Callable

The Java Callable interface, java.util.concurrent.Callable, represents an asynchronous task which can be executed by a separate thread. For instance, it is possible to submit a Callable object to a Java ExecutorService which will then execute it asynchronously.

java.util.concurrent.Callable 接口，表示可以用另外一个线程执行一个异步任务。比如，可以提交一个 Callable 对象给 Java ExecutorService 来异步执行。

Java Future

A Java Future, java.util.concurrent.Future, represents the result of an asynchronous computation. When the asynchronous task is created, a Java Future object is returned. This Future object functions as a handle to the result of the asynchronous task. Once the asynchronous task completes, the result can be accessed via the Future object returned when the task was started.

参考资料

• Java常用类源码——Thread源码解析
• Java并发包：ScheduledExecutorService
• Java Concurrency Utilities - ScheduledExecutorService
• Java concurrency (multi-threading) - Tutorial