为什么这个FutureTask会导致我的程序永远挂起？(Why does this FutureTask cause my program to hang forever?)

 我正在学习Java并试图了解FutureTasks是如何工作的。 在我的特定情况下，我正在尝试生成一个线程（FutureTask）来执行某些操作。 我其实并不关心它的回应; 我只是希望它异步执行。 我有很多与以下测试代码一起工作： 
 
public class Main {
    public static void main(String[] args) {
        ExecutorService exService = Executors.newSingleThreadInstance();
        FutureTask<Boolean> asynch = new FutureTask<>(new Callable<Boolean>() {
            @Override
            public Boolean call() {
                try {
                    //  wait for 5 seconds to simulate activity
                    Thread.sleep(5000);
                } catch (InterruptedException ex) {
                    Thread.currentThread().interrupt();
                }
                System.out.println("Complete");
                return true;
            }
        });

        System.out.println("Started");
        exService.execute(asynch);
    }
}
 
 当我运行这个代码时，它将“Started”打印到控制台，然后五秒钟后打印出“Complete”，就像我期望的那样。 然而......该计划在这一点上无限期地悬挂下来。 我假设，因为call（）方法已经完成，程序足够聪明，可以在那个时候退出，但显然它会让这个线程永远活着？ 我如何让它一次运行任务，然后退出？ 

I'm learning Java and trying to understand how FutureTasks work. In my particular case I'm trying to spawn a thread (FutureTask) to execute something. I don't actually care about its response; I just want it to execute asynchronously. I've got that much working with the following test code:
 
public class Main {
    public static void main(String[] args) {
        ExecutorService exService = Executors.newSingleThreadInstance();
        FutureTask<Boolean> asynch = new FutureTask<>(new Callable<Boolean>() {
            @Override
            public Boolean call() {
                try {
                    //  wait for 5 seconds to simulate activity
                    Thread.sleep(5000);
                } catch (InterruptedException ex) {
                    Thread.currentThread().interrupt();
                }
                System.out.println("Complete");
                return true;
            }
        });

        System.out.println("Started");
        exService.execute(asynch);
    }
}
 
When I run this code, it prints "Started" to the console, then five seconds later prints "Complete" like I would expect. However... the program just hangs indefinitely at that point. I was assuming that because the call() method had completed that the program would be smart enough to quit at that point, but apparently it keeps that thread alive forever? How do I get it to run the task once and then quit?

原文：https://stackoverflow.com/questions/27181196