There is little valid reason to use the methods of the ThreadGroup class. Some are deprecated (allowThreadSuspension(), resume(), stop(), and suspend()), some are obsolete, others aren't thread-safe, and still others are insecure (activeCount(), enumerate()) . For these reasons, any use of ThreadGroup is suspicious and should be avoided.

Compliant Solution

ThreadFactory threadFactory = Executors.defaultThreadFactory();
ThreadPoolExecutor executorPool = new ThreadPoolExecutor(3, 10, 5, TimeUnit.SECONDS, new ArrayBlockingQueue<Runnable>(2), threadFactory);

for (int i = 0; i < 10; i++) {
  executorPool.execute(new JobThread("Job: " + i));
}

System.out.println(executorPool.getActiveCount()); // Compliant
executorPool.shutdown();

See