[groestl]

AFAIK, Java's native HMR is not built on dynamic classloaders, but using a separate technology built into the JVMs debugger support, called HotSwap. You can only replace (certain) function bodies with it. To get around these restrictions, more elaborate HMR can be achieved with classloader magic, such as OSGi, but you subscribe to a world of ClassCastException-pain with it.

[kaba0]

It is still a very good system, and you can get quite a far away with only method-swaps, I love to use them in case of Spring apps, where changing a backend controller and doing another api call is a very fast REPL cycle basically.

But objects don’t give themselves very easily to hot reloading, there are a bunch of edge cases that are not well-defined in a semi-swapped state (e.g. this new field always gets a value in the constructor, but was no just added. Should you rerun the constructor, or have it be in an inconsistent state?)

Immutable/FP concepts are a much better fit, especially if the building blocks are as small as they are in case of Clojure.