Thanks for the PR. A few comments:

• Need to think of a proper consistent place for extension loading. E.g., loading from the main classpath happens inside JavaKernel constructor, this one happens in the builder (and likely in the wrong one... If someone extends JavaKernelBuilder, they'd lose this functionality). I guess let's keep it where it is, and I will think of it some more
• We need unit tests demonstrating the original problem (and the fix here). We must keep Improving the test coverage in general. Otherwise making any changes is very risky.

loading from the main classpath happens inside JavaKernel constructor, this one happens in the builder

Maybe we can even combine the two, and load everything once using a proper ClassLoader with the extra classpath

The PR is in. I really appreciate the kernel tests that don't require Docker and still allow to play with extensions.