Submitting to an Executor from a running task

Question

Is it safe for a task (a Runnable) being run by an Executor to submit (execute()) a task? Can it result in deadlock, if using any of the standard Java executors

Answer 1

If you call get on the future then you can surely deadlock, in simple example as below this might be obvious to spot but if you have some class hierarchy that hides executors usage then someone by mistake can introduce such bug.

class TestApp {
    public static class MyCallable
            implements Callable {
        public Integer call() throws ExecutionException, InterruptedException {
            Future future = pool.submit(new MyCallable());
            System.out.println("MyCallable: before get 2");
            future.get(); // deadlocks here
            System.out.println("MyCallable: after get 2");
            return 0;
        }
    }

    static ExecutorService pool = Executors.newSingleThreadExecutor();
    public static void main(String [] args) throws ExecutionException, InterruptedException {
        Future future = pool.submit(new MyCallable());
        System.out.println("MyCallable: before get 1");
        future.get();
        System.out.println("MyCallable: after get 1");
    }
}

prints

MyCallable: before get 1
MyCallable: before get 2
MyCallable: before get 2
MyCallable: before get 2