Java API for Merge Operator v2

[rhubner]

This is the first implementation of the merge operator API, where merge operators can be implemented in Java without any C++ code. It prefers simplicity and community feedback is appreciated.

[alanpaxton]

A few small things to tidy up. Looks good apart from those.

[alanpaxton]

The JNI manual says NewGlobalRef() can return nullptr WITHOUT an exception, e.g. if the system is out of memory. So we should throw a Java exception here. Can you also check the other JNI methods you call, to confirm that they always set an exception on nullptr, and if they don't we should set one in each case.

Then fix the comments to say "XYZException has been thrown", or whatever is appropriate.

[rhubner]

I check existing code and we don't throw exceptions when we can't create global references.

[alanpaxton]

Really ? That seems wrong, and may be an omission. If NewGlobalRef() is returning null, surely the right thing to do is to throw an exception, unless we somehow cope with j_merge_class as null ? So let us do it correctly here for now.

[rhubner]

@alanpaxton Fixed.

[alanpaxton]

Thanks. I'm happy.

[alanpaxton]

@rhubner LGTM now in respect of changes I requested on previous review.