Javeleon
From APIDesign
Javeleon is 100% pure Java implementation of Hotswap. On a base level I envision it as complementary to Hotswap by Thomas Würthinger, just not implemented directly inside HotSpot, but externally via java.lang.instrument package. However Javeleon value (from my point of view) is in providing additional hooks for integration with the NetBeans platform. For example one can (in contrast to JDK Hotswap), change Bundle.properties files.
Hotswap is More than Classes
My ode to true hotswap contained one important observation: Now it is easier to change any class in my program than to modify content of Bundle.properties! Here is where all the fun starts, as having the ability to update classes, is just first step, there is much more that needs to be done to make the hotswap real.