FutureTask的使用示例

今天看书，有关于 FutureTask 的介绍，感觉还蛮有意思的，可以用它来做一些比较花时间的事情。下面打个通俗的比方来说明一下它的用处：

比如，早上一大早的去公交站台等公交，但那该死的公交20分钟才一班。如果一直死等公交，那么这20分钟无疑就被浪费了。我们可以利用这20分钟，去买个韭菜饼，再买一盒豆浆，然后一边吃一边等。这样就明显提高了时间的利用率。

下面给出一个段简单的代码来说明一下它的使用:

public class Preloader {

	private final FutureTask<Long> future = new FutureTask<Long>(new Callable<Long>() {
		@Override
		public Long call() throws Exception {
			Thread.currentThread().setName("Thread(3)");

			System.out.println(Thread.currentThread().getName() + ": simulate a latency ");
			try {
				Thread.sleep(5000);
			} catch (Exception e) {
				e.printStackTrace();
			}
			System.out.println(Thread.currentThread().getName() + ": the latency is over");
			return Math.round(Math.random() * 1000);
		}
	});

	private final Thread loader = new Thread(future);

	public void start() {
		System.out.println(Thread.currentThread().getName() + ": start the loader");
		loader.start();
		System.out.println(Thread.currentThread().getName() + ": loader started");
	}

	public Long get() {
		try {
			System.out.println(Thread.currentThread().getName() + ": begin to get");
			long start = System.currentTimeMillis();
			Long result = future.get();
			System.out.println(Thread.currentThread().getName() + ": got result: " + result);
			System.out.println(Thread.currentThread().getName() + ": spent time: "
					+ (System.currentTimeMillis() - start));
			return result;
		} catch (InterruptedException e) {
			e.printStackTrace();
		} catch (ExecutionException e) {
			e.printStackTrace();
		}
		System.out.println(Thread.currentThread().getName() + ": got nothing");
		return null;
	}

	public static void main(String[] args) {
		Thread.currentThread().setName("Thread(main)");
		final Preloader pl = new Preloader();
		pl.start();

		// try to get the result before the latency is over
		new Thread(new Runnable() {
			@Override
			public void run() {
				Thread.currentThread().setName("Thread(1)");
				pl.get();
			}
		}).start();

		// try to get the result after the latency is over
		new Thread(new Runnable() {
			@Override
			public void run() {
				Thread.currentThread().setName("Thread(2)");
				try {
					Thread.sleep(6000);
					pl.get();
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
			}
		}).start();
	}
}

 以上代码的运行结果为：

main: start the loader
main: loader started
Thread(3): simulate a latency 
Thread(1): begin to get
Thread(3): the latency is over
Thread(1): got result: 917
Thread(1): spent time: 5006
Thread(2): begin to get
Thread(2): got result: 917
Thread(2): spent time: 0  

 通过上面运行的结果，我们可以得到以下结论：

① 在FutureTask执行完了之前(即call执行完了之前)，去进行get的话，将被会一直阻塞至FutureTask执行完了才会返回。

② FutureTask执行完了之后，再调用get的话，会立刻返回相应的结果

③ FutureTask只会执行一次，所以，多次调用get方法，返回的值始终是一样的。