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Enhanced hot swap using GraalVM has mostly caught up with capabilities https://www.graalvm.org/22.1/reference-manual/java-on-truffl...

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I’ve tried that and it didn’t work reliably for me. The jet brains runtime release works but it’s a bear to figure out which download to use.

Regardless, it’s ridiculous that at this point Java doesn’t have full hotswap. All the hooks are there (as is apparent from the error messages when a hotswap fails) and the dcevm is being maintained by jetbrains employees. It needs an internal champion at oracle/sun.

What should happen when you remove an existing field, remove a method already used, change its initial value, etc? You will quickly get some incorrect state by blind hot swapping, and it is not trivial to do in a mutable object graph.

Clojure (and other lisps) can do it well because their scope of changes can be really small. Nonetheless, method hot-swap is well-defined and is implemented by OpenJDK.

Dcevm handles all those reasonably well. The current version doesn’t support changing super classes but the old version did. This is in dev mode so it doesn’t need to be perfect, just right enough most of the time.

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