From loom-lab, given the code
var virtualThreadFactory = Thread.ofVirtual().factory();
try (var executorService = Executors.newThreadPerTaskExecutor(virtualThreadFactory)) {
IntStream.range(0, 15).forEach(item -> {
executorService.submit(() -> {
try {
var milliseconds = item * 1000;
System.out.println(Thread.currentThread() + " sleeping " + milliseconds + " milliseconds");
Thread.sleep(milliseconds);
System.out.println(Thread.currentThread() + " awake");
if (item == 8) throw new RuntimeException("task 8 is acting up");
} catch (InterruptedException e) {
System.out.println("Interrupted task = " + item + ", Thread ID = " + Thread.currentThread());
}
});
});
}
catch (RuntimeException e) {
System.err.println(e.getMessage());
}
My hope was that the code would catch the RuntimeException and print the message, but it does not.
Am I hoping for too much, or will this someday work as I hope?
In response to an amazing answer by Stephen C, which I can fully appreciate, upon further exploration I discovered via
static String spawn(
ExecutorService executorService,
Callable<String> callable,
Consumer<Future<String>> consumer
) throws Exception {
try {
var result = executorService.submit(callable);
consumer.accept(result);
return result.get(3, TimeUnit.SECONDS);
}
catch (TimeoutException e) {
// The timeout expired...
return callable.call() + " - TimeoutException";
}
catch (ExecutionException e) {
// Why doesn't malcontent get caught here?
return callable.call() + " - ExecutionException";
}
catch (CancellationException e) { // future.cancel(false);
// Exception was thrown
return callable.call() + " - CancellationException";
}
catch (InterruptedException e) { // future.cancel(true);
return callable.call() + "- InterruptedException ";
}
}
and
try (var executorService = Executors.newThreadPerTaskExecutor(threadFactory)) {
Callable<String> malcontent = () -> {
Thread.sleep(Duration.ofSeconds(2));
throw new IllegalStateException("malcontent acting up");
};
System.out.println("\n\nresult = " + spawn(executorService, malcontent, (future) -> {}));
} catch (Exception e) {
e.printStackTrace(); // malcontent gets caught here
}
I was expecting malcontent to get caught in spawn as an ExecutionException per the documentation, but it does not. Consequently, I have trouble reasoning about my expectations.
Much of my hope for Project Loom was that, unlike Functional Reactive Programming, I could once again rely on Exceptions to do the right thing, and reason about them such that I could predict what would happen without having to run experiments to validate what really happens.
As Steve Jobs (at NeXT) used to say: "It just works"
So far, my posting on loom-dev@openjdk.java.net has not been responded to... which is why I have used StackOverflow. I don't know the best way to engage the Project Loom developers.
