I've done a lot of webapp development in both Java and Rails. I would agree that the iteration cycle is shorter in Rails, but it's not as bad as your description makes it sound. If you have to redeploy a jar file as part of your development process, then that's a problem with your process, not your language.

When I do Java webapp development in Eclipse, it compiles and hot-swaps my code in the background without requiring any restart or redeployments. Works pretty well.

I do have to restart occasionally because the hot-swapping has limitations, so that does slow things down a bit, but when you consider the time saved by having good compiler integration, then I don't think there's a big difference in productivity.