This has been possible in Grails and Play for many years.

You can make a code change and refresh the browser and your changes are reflected.

TIL that that is possible. Just googled it apparently requires custom class-loader. Thanks for this.

Check out JRebel [1] which works with a plugin to your favorite IDE if you want to make it really easy.

[1] http://zeroturnaround.com/software/jrebel/

Or you can simply use OSGi.