如何使用ExecutorService递归调度任务

Question

final ExecutorService executor = Executors.newFixedThreadPool(4);
for (int i = 0; i < 8; i++) {
    executor.execute(() -> { // TASK 1
        Thread.sleep(100); // simulate some work

        executor.execute(() -> { // TASK2
            Thread.sleep(500); // simulate some longer work
        });

    });
}

# terminate when all tasks proceeded
executor.shutdown();
executor.awaitTermination(9999999, TimeUnit.DAYS);

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Answer 1

您的核心问题是某些任务在运行时可能会或不会产生更多的任务。因此，在关闭之前必须至少等待该任务完成。不能使用executor.shutdown()，否则肯定会过早关闭池。

您必须实现某种机制来编排哪些任务必须等待另一个任务完成，并且必须维护在关闭池之前必须完成的所有任务的列表。

下面是你需要做什么的初步演示。根据任务的相互关系要求，您可能需要更复杂的东西。

final ExecutorService executor = Executors.newFixedThreadPool(4);

class Task1 implements Callable<Void> {

    @Override
    public Void call() throws Exception {
        Thread.sleep(100); // simulate some work
        return null;
    }

}

class Task2 implements Callable<Void> {

    final Future<Void> waitFor;

    Task2(Future<Void> waitFor) {
        // This task must wait for a previous task to complete before commencement.
        this.waitFor = waitFor;
    }

    @Override
    public Void call() throws Exception {
        // Wait for the first task to complete.
        waitFor.get();
        Thread.sleep(100); // simulate some work
        return null;
    }

}

public void test() throws InterruptedException {
    // All of these tasks must complete before we close down the pool.
    List<Future<Void>> waitFor = new ArrayList<>();
    for (int i = 0; i < 8; i++) {
        Future<Void> f1 = executor.submit(new Task1());
        // We must wait for f1 to complete.
        waitFor.add(f1);
        // No need to wait for f2.
        executor.submit(new Task2(f1));
    }
    // Wait for all of the primary tasks to complete.
    for (Future<Void> wait : waitFor) {
        try {
            wait.get();
        } catch (InterruptedException | ExecutionException ex) {
            Logger.getLogger(Test.class.getName()).log(Level.SEVERE, null, ex);
        }
    }
    // Can now shut down - will wait for all sub-tasks to complete because they are all in the queue now.
    executor.shutdown();
    executor.awaitTermination(9999999, TimeUnit.DAYS);
}