Why does the ExecutorService interface not implement AutoCloseable?

Question

Failing to call shutdown() on a thread executor will result in a never terminating application.

Best practice to shut down the ExecutorService is this:

ExecutorService service = null;
try {
  service = Executors.newSingleThreadExecutor();
  // add tasks to thread executor
  …
} finally {
  if (service != null) service.shutdown();
}

Since Java knows the try-with-resources concept, wouldn't it be nice if we could do this?

try (service = Executors.newSingleThreadExecutor())
{
  // add tasks to thread executor
  …
} 

Answer 1

That ExecutorService has actually two shutdown-related methods; based on the simple fact that both ways of shutting down a service make sense.

Thus: how would you auto-close a service then? In a consistent manner that works for everybody?!

So, the reasonable explanation in my eyes: you can't make an ExecutorService a AutoClosable because that service does not have a single "close" like operation; but two!

And if you think you could make good use of such an auto-closing service, writing up your own implementation using "delegation" would be a 5 minute thing! Or probably 10 minutes, because you would create one version calling shutdown() as close operation; and one that does shutdownNow() instead.

Answer 2

This is a mediocre workaround

ExecutorService service = Executors.newSingleThreadExecutor();
try (Closeable close = service::shutdown) {

}

Or, if the checked exception bothers you, you could write:

interface MyCloseable extends AutoCloseable {
    void close();
}

And then

ExecutorService service = Executors.newSingleThreadExecutor();
try (MyCloseable close = service::shutdown) {

}

Of course, you must never ever put anything between the assignment and the try statement, nor use the service local variable after the try statement.

Given the caveats, just use finally instead.