JavaRebel reloads changes to Java classes on-the-fly without redeploy
or restart including new methods and fields. It is a generic solution
that works for standalone Java applications as well as application
servers.

Ever since HotSwap support has been introduced in Java 1.4 people have
been craving for more. As witnessed by a bug (http://bugs.sun.com/
bugdatabase/view_bug.do?bug_id=4910812) in the Sun database and
numerous other discussions changing method bodies was just not enough.
Dynamic languages like Ruby were quick enough to capitalize on this
lacking touting Java EE development as slow and cumbersome.

As time passed by people have tried to work around the limitation by
reloading classes along with their classloader. However since this
requires also to reconstruct the object state it will only work in a
controlled and limited environment - e.g. that of a web framework like
RIFE, Tapestry 5 and so on.

Just as many others we were  working on a workaround. However instead
of limiting ourselves to the facilities provided by the Java platform
we devised a whole new approach. Using bytecode modification and a
little bit of magic JavaRebel extends the JVM with ability to reload
almost arbitrary changes to classes on-the-fly. This includes adding
and removing both methods and fields and the only forbidden changes
are to 'extends' and 'implements' clauses. Since all the existing
object instances are preserved the application and server can continue
running and code changes are immediately visible in the application.

Watch the demonstration screencast (~5 mins,
http://www.zeroturnaround.com/docs/javarebel-jpetstore-screencast),
read the feature list (http://www.zeroturnaround.com/javarebel/
features/) or just download JavaRebel from
http://www.zeroturnaround.com/download/
and give it a try. Disclaimer: JavaRebel is commercial software with a
free trial for 14 days and developer seat cost at 99$.

ZeroTurnaround is a spinoff of Webmedia, Ltd. that focuses on Java
developer productivity tools. Currently we are developing JavaRebel, a
generic Java class reloader, and JSP Weaver, an instant JSP
interpreter.