I finally got a JNI connection to the netCDF-4 C library.
The idea is to test the Java and C libraries against each other. Since
they are independent implementations, agreement likely indicates correct
results.
Because I wanted to do the work on my Windows development machine, I
needed to wait until Ed could build a Windows DLL for netCDF-4. He had
to go through the learning hump for Visual Studio etc. I did that 8-9
years ago, and I was glad not to do it again.
I bought a commercial package
JNIWrapper,
a companion product for my IDE, IntelliJ. It wasn't much money for an
academic license ($150 ?), but still I expected that the documentation
would be more than minimum. Its OK, but not that good. It is supposed to
make it easy to write JNI to native code libraries.
Anyway, I couldn't get the damn thing to work on the netcdf.dll (it
worked on other dlls) so I mistakenly thought that there was something
wrong with the dll. In googling around to try to solve the problem, I
ran across another JNI helper library,
JNA,
an open source package on java.net that is actively being maintained
and supported. I switched over to it - still didnt work with netcdf.dll.
I finally downloaded a trial copy of
PE Explorer, which analyzed netcdf.dll and showed all the
netCDF-4 DLL dependencies.
So all I had to do is to make sure to explicitly load those libraries,
then it all worked. I liked PE Explorer, and would buy it if I had to
work much with DLLs.
So I immediately got things working with netCDF-3 files (I think I've
written this code 5 times before). The 2 libraries agreed on reading all
the test files I threw at it, so that's a nice feeling. There was a
moment of panic on one test file - then it turned out to be truncated,
and the C library silently fills in the missing parts with zeros. In the
Java library, Im returning garbage. I'm going to change the Java
library to detect truncation and reduce the record count to reflect the
actual number of complete records.
I like JNA, simple stuff is simple. There are some dark corners in
trying to write portable JNI. The netCDF library uses size_t a lot,
which is an integer whose size is system dependent, and possibly
compile-flag dependent. I'm mapping size_t to JNA's
NativeLongByReference which should do the right thing most of the time.
Mapping to structures and pointers to arrays of Structures etc gets
pretty complicated.
Im currently working on the netCDF-4 specific bindings, and learning the
C API (again). I should have done this when Ed was designing it, to
give him feedback, but oh well. There are a number of JNI mappings that
I'm not yet sure how to do.
If things get too hard in JNA, I will see if JNIWrapper does a better
job. At the moment I don't really have a comparison between them.
Oh yeah, am I glad I get to work in Java and not C. Thanks to Ed and
Russ and Dennis who use that lovely language so I don't have to!
