List<Runnable> returned from shutdownNow() can not be converted to submitted Runnable

Question

Below is the source code of the class.

I wanted to verify how does shutdownNow() works for not submitted task. Problem I am getting in below code is shutdownNow() return List<FutureTask> and not List<Runnable> which I have submitted List<Runnable> containing submitted instance of PrimeProducer .

In Below program I wanted to get the tasks which where not executed and their state so that I can reschedule them later. name() represents just state that I want to store.

So I am not able to convert to submitted Task.

class PrimeProducer implements Runnable {
private final SynchronousQueue<BigInteger> queue;

PrimeProducer(SynchronousQueue<BigInteger> queue) {
    this.queue = queue;
}

public void run() {
    try {
        BigInteger p = BigInteger.ONE;
        queue.put(p = p.nextProbablePrime());
    } catch (InterruptedException consumed) {
        System.out.println("Safe Exit");
        Thread.currentThread().interrupt();
    }

}

public String name() {
    return "PrimeProducer";
}

public static void main(String[] args) throws InterruptedException,
        ExecutionException {
    PrimeProducer primeProducer = new PrimeProducer(
            new SynchronousQueue<BigInteger>());//SynchronousQueue just to ensure it put is blocking
    ExecutorService executorService = Executors.newFixedThreadPool(1);
    executorService.submit(primeProducer);
    executorService.submit(primeProducer);
    List<Runnable> list = executorService.shutdownNow();
    //PrimeProducer producer = (PrimeProducer) list.get(0);// Class Cast
                                                            // Exception
    FutureTask<PrimeProducer> futureTask = (FutureTask<PrimeProducer>) list
            .get(0);
            System.out.println(futureTask.isDone());//Prints false
    futureTask.get().name();//futureTask-->PrimeProducer get() hangs.


}
}

Problematic Lines

//PrimeProducer producer = (PrimeProducer) list.get(0);// Class Cast
                                                            // Exception
 FutureTask<PrimeProducer> futureTask = (FutureTask<PrimeProducer>) list
            .get(0);
 futureTask.get().name();//futureTask-->PrimeProducer get() hangs.

Answer 1

Try "execute" instead of "submit".

Answer 2

This behavior happens because there is difference between how execute and submit handles task that are submitted.

execute method directly uses Runnable command which is passed to it where as submit create RunnableFuture and call execute

 RunnableFuture<Object> ftask = newTaskFor(task, null);