This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
This chapter here (see 3.3) says about arrays in the JNI programming. If you want to return byte from a Java method (the array will internally have its length, so you don't need to pass it separately), try to compile it with javah and see what comes out. Then, using the methods described in the JNI Programmer's Guide you can convert the C array to a JNI-Java array and return it.