That is, if this is the data structure you defined:

public class Image {
@Bound int width;
@Bound int height;
}

Then Preon will expect that the reflection API will also return the fields in this order.

I think I sort of knew this in the back of my head, but it never failed any of the tests, so I stopped worrying about it. So, the question is, should I start worrying now? It's clearly undesirable to depend on some coincidental properties of a Java VM, but are there actually VMs that will return a different field ordering? With IBMs VM being based on Sun's VM, it's unlikely they will differ. So that leaves us with JRockit.

In the end, I figured it would be wise to have an automated test that at least guarantees that the current setup also wors on JRockit. It doesn't solve the problem, but it does provide some degree of guarantee that the problem will usually not manifest itself. And it turns out, the test succeeds.