使用可调用类实例执行需要返回值的异步任务

通常需要执行长时间运行的任务，并在完成任务后使用该任务的结果。

在这个例子中，我们将创建两个类：一个实现 Callable <T>接口（其中 T 是我们希望返回的类型），另一个包含 main() 方法。

AsyncValueTypeTaskCompleter.java

import lombok.extern.java.Log;

import java.util.concurrent.Callable;
import java.util.concurrent.ThreadLocalRandom;
import java.util.concurrent.TimeUnit;

@Log
public class AsyncValueTypeTaskCompleter implements Callable<Integer> {
    private int taskNumber;

    public AsyncValueTypeTaskCompleter(int taskNumber) {
        this.taskNumber = taskNumber;
    }

    @Override
    public Integer call() throws Exception {
        int timeout = ThreadLocalRandom.current().nextInt(1, 20);
        try {
            log.info(String.format("Task %d is sleeping", taskNumber));
            TimeUnit.SECONDS.sleep(timeout);
            log.info(String.format("Task %d is done sleeping", taskNumber));

        } catch (InterruptedException e) {
            log.warning(e.getMessage());
        }
        return timeout;
    }
}

AsyncExample2.java

import lombok.extern.java.Log;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;

@Log
public class AsyncExample2 {
    public static void main(String[] args) {
        ExecutorService executorService = Executors.newCachedThreadPool();
        List<Future<Integer>> futures = new ArrayList<>();
        for (int i = 0; i < 10; i++){
            Future<Integer> submittedFuture = executorService.submit(new AsyncValueTypeTaskCompleter(i));
            futures.add(submittedFuture);
        }
        executorService.shutdown();
        while(!futures.isEmpty()){
            for(int j = 0; j < futures.size(); j++){
                Future<Integer> f = futures.get(j);
                if(f.isDone()){
                    try {
                        int timeout = f.get();
                        log.info(String.format("A task just completed after sleeping for %d seconds", timeout));
                        futures.remove(f);
                    } catch (InterruptedException | ExecutionException e) {
                        log.warning(e.getMessage());
                    }
                }
            }
        }
    }
}

运行 AsyncExample2.main() 导致以下输出：

Dec 28, 2016 3:07:15 PM AsyncValueTypeTaskCompleter call
INFO: Task 7 is sleeping
Dec 28, 2016 3:07:15 PM AsyncValueTypeTaskCompleter call
INFO: Task 8 is sleeping
Dec 28, 2016 3:07:15 PM AsyncValueTypeTaskCompleter call
INFO: Task 2 is sleeping
Dec 28, 2016 3:07:15 PM AsyncValueTypeTaskCompleter call
INFO: Task 1 is sleeping
Dec 28, 2016 3:07:15 PM AsyncValueTypeTaskCompleter call
INFO: Task 4 is sleeping
Dec 28, 2016 3:07:15 PM AsyncValueTypeTaskCompleter call
INFO: Task 9 is sleeping
Dec 28, 2016 3:07:15 PM AsyncValueTypeTaskCompleter call
INFO: Task 0 is sleeping
Dec 28, 2016 3:07:15 PM AsyncValueTypeTaskCompleter call
INFO: Task 6 is sleeping
Dec 28, 2016 3:07:15 PM AsyncValueTypeTaskCompleter call
INFO: Task 5 is sleeping
Dec 28, 2016 3:07:15 PM AsyncValueTypeTaskCompleter call
INFO: Task 3 is sleeping
Dec 28, 2016 3:07:16 PM AsyncValueTypeTaskCompleter call
INFO: Task 8 is done sleeping
Dec 28, 2016 3:07:16 PM AsyncExample2 main
INFO: A task just completed after sleeping for 1 seconds
Dec 28, 2016 3:07:17 PM AsyncValueTypeTaskCompleter call
INFO: Task 2 is done sleeping
Dec 28, 2016 3:07:17 PM AsyncExample2 main
INFO: A task just completed after sleeping for 2 seconds
Dec 28, 2016 3:07:17 PM AsyncValueTypeTaskCompleter call
INFO: Task 9 is done sleeping
Dec 28, 2016 3:07:17 PM AsyncExample2 main
INFO: A task just completed after sleeping for 2 seconds
Dec 28, 2016 3:07:19 PM AsyncValueTypeTaskCompleter call
INFO: Task 3 is done sleeping
Dec 28, 2016 3:07:19 PM AsyncExample2 main
INFO: A task just completed after sleeping for 4 seconds
Dec 28, 2016 3:07:20 PM AsyncValueTypeTaskCompleter call
INFO: Task 0 is done sleeping
Dec 28, 2016 3:07:20 PM AsyncExample2 main
INFO: A task just completed after sleeping for 5 seconds
Dec 28, 2016 3:07:21 PM AsyncValueTypeTaskCompleter call
INFO: Task 5 is done sleeping
Dec 28, 2016 3:07:21 PM AsyncExample2 main
INFO: A task just completed after sleeping for 6 seconds
Dec 28, 2016 3:07:25 PM AsyncValueTypeTaskCompleter call
INFO: Task 1 is done sleeping
Dec 28, 2016 3:07:25 PM AsyncExample2 main
INFO: A task just completed after sleeping for 10 seconds
Dec 28, 2016 3:07:27 PM AsyncValueTypeTaskCompleter call
INFO: Task 6 is done sleeping
Dec 28, 2016 3:07:27 PM AsyncExample2 main
INFO: A task just completed after sleeping for 12 seconds
Dec 28, 2016 3:07:29 PM AsyncValueTypeTaskCompleter call
INFO: Task 7 is done sleeping
Dec 28, 2016 3:07:29 PM AsyncExample2 main
INFO: A task just completed after sleeping for 14 seconds
Dec 28, 2016 3:07:31 PM AsyncValueTypeTaskCompleter call
INFO: Task 4 is done sleeping
Dec 28, 2016 3:07:31 PM AsyncExample2 main
INFO: A task just completed after sleeping for 16 seconds

注意事项：

在上面的输出中有几点需要注意，

1. 每次调用 ExecutorService.submit() 都会返回一个 Future 实例，它存储在一个列表中供以后使用
2. Future 包含一个名为 isDone() 的方法，可以在尝试检查其返回值之前检查我们的任务是否已完成。在尚未完成的 Future 上调用 Future.get() 方法将阻塞当前线程，直到任务完成，可能会否定从异步执行任务中获得的许多好处。
3. 在检查 Future 对象的返回值之前调用了 executorService.shutdown() 方法。这不是必需的，但是以这种方式完成以表明它是可能的。executorService.shutdown() 方法不会阻止已完成已提交给 ExecutorService 的任务，而是阻止将新任务添加到 Queue 中。