[WIP] Initial Java binding through JNI.

[mzhaom]

This is a draft implementation to JNI binding.

Major questions for the next step:

• Is going through the RiegeliReader/Writer the right starting point?
• Shall we provide a byte{reader,writer} implementation that can work with java.io.{Input,Out}Stream?

[QrczakMK]

Thank you!

I will go through this tomorrow.

I assume you mean Record{Reader,Writer}. Yes, it seems to be a good starting point.

Yes, there should be a riegeli::{Reader,Writer} adapting a java.io.{Input,Output}Stream (perhaps called riegeli::Java{Reader,Writer}), like riegeli::Python{Reader,Writer}. Given the API of Java streams (read/write functions which copy pieces of data to/from arrays, as opposed to e.g. providing pointers to buffers owned by the streams — which would not work well across Java/C++ anyway), they too should derive from riegeli::Buffered{Reader,Writer}.

I don’t know how random access should be supported. In Riegeli this is an optional functionality of a Reader (and also Writer, but that is rarely useful). I am not familiar enough with Java to know one typically expresses a random access input stream. There is java.io.RandomAccessFile, but it is file-specific. There is java.nio.channels.SeekableByteChannel, but channels are a separate class hierarchy from streams — are streams or channels recommended? Initially we can ignore random access (and skip wrap functions which require it, like riegeli::RecordReader::Seek()), but eventually something should be done with that.

The Python wrapper ignores the possibility of substituting a custom ChunkWriter (and a ChunkReader is not really substitutable even in C++, because it lacks a separate interface and implementation). This will be fine in Java as well.

[QrczakMK]

Instead of java.io.{Input,Output}Stream, I think Java{Reader,Writer} should take java.nio.{Readable,Writable}ByteChannel.

Benefits:

• An interface rather than a class makes it easier to be implemented without creating an adapter object.
• Random access can be queried with instanceof SeekableByteChannel.
• https://docs.oracle.com/javase/9/docs/specs/jni/functions.html#getdirectbufferaddress allows to reduce data copying. This means that riegeli::Java{Reader,Writer} would not be implemented using riegeli::Buffered{Reader,Writer} but using riegeli::{PullableReader,PushableWriter}.

[weixin-zhang-weride-ai]

Hi everyone, just curious how is the progress, I am also looking for Java support for riegeli, thanks!