shuts down an ExecutorService

• shuts down an ExecutorService in two phases, first by calling shutdown to reject incoming tasks, and then calling shutdownNow, if necessary, to cancel any lingering tasks: 

•  void shutdownAndAwaitTermination(ExecutorService pool) {
     pool.shutdown(); // Disable new tasks from being submitted
     try {
       // Wait a while for existing tasks to terminate
       if (!pool.awaitTermination(60, TimeUnit.SECONDS)) {
         pool.shutdownNow(); // Cancel currently executing tasks
         // Wait a while for tasks to respond to being cancelled
         if (!pool.awaitTermination(60, TimeUnit.SECONDS))
             System.err.println("Pool did not terminate");
       }
     } catch (InterruptedException ie) {
       // (Re-)Cancel if current thread also interrupted
       pool.shutdownNow();
       // Preserve interrupt status
       Thread.currentThread().interrupt();
     }
   }