At this years JavaOne, we will present an open source project that aims at bringing unlimited hotswapping capabilities to the Java HotSpot(TM) virtual machine. Our modified version of the virtual machine is capable of adding and removing fields and methods of Java classes at runtime. Additionally, it is possible to change the super type (e.g., the set of implemented interfaces or the super class). There is no performance penalty or indirection introduced in contrast to existing bytecode rewriting techniques. You can use the VM without additional tools from within the debugger of your favorite Java IDE.
The enhanced virtual machine is freely available from http://ssw.jku.at/dcevm/. You can download an installer to patch an existing Java 6 or Java 7 installation. If you are interested in technical details about our approach, you are welcome to attend my JavaOne presentation on Wednesday, September 22, 13:00 at the Hilton San Francisco, Yosemite C. A recent version of the source code is also available from the MLVM wiki pages at http://wikis.sun.com/display/mlvm/HotSwap. The project is currently sponsored by Oracle and Guidewire.