Exceptions in Executor.submit

Question

Run the following test code:
public class Test {
    public static void main(String[] args) {
        ExecutorService executor = Executors.newCachedThreadPool();
        executor.submit(new Runnable() {
            public void run() {
                throw new RuntimeException("This is a runtime exception");
}Why doesn't the stack trace show up on System.out as it would normally do if I had executed the runnable in a new Thread? Is there anyway short of wrapping the whole run method in a try-catch block of getting that stack trace.
TIA.

Answer 1

When you submit your Runnable to the ExecutorService you get a Future back. Calling get() on that Future will result in an ExecutionException being thrown. The cause of that exception is your RuntimeException:
import java.util.concurrent.*;
public class Test {
    public static void main(String[] args) {
        ExecutorService executor = Executors.newCachedThreadPool();
        Future<?> f = executor.submit(new Runnable() {
            public void run() {
                throw new RuntimeException("This is a runtime exception");
        try {
            f.get();
        catch (Exception e) {
            e.printStackTrace();
}I've recently started to play around with these things myself, and here is my understanding of it:
An ExecutorService uses a FutureTask by default when you submit a Runnable (see the source code for AbstractExecutorService). That FutureTask basically wraps your Runnable inside atry { ... } catch (Throwable) {... }which explains why your exception doesn't slip through. Instead, the throwable is stored inside the FutureTask. When you call get() on the Future, the throwable is wrapped inside an ExecutionException, as described above.