The JNI is Javas compatibility layer with C. It follows a strict syntax to blindly call the (ABI-compatible) functions. It has however arcane syntax and conventions. Here are some useful notes every time I work with it I need to check.

Type conversions

When declaring a Java JNI function you need to declare the types with single characters that will be mapped to C types. The conversion is as follows:

Z = boolean B = byte C = char I = short J = long F = float D = double

Non scalar types need to be declared by their corresponding package:

// Will be passed to the C side as a JString
Ljava/lang/String;

Extern C

C++ code needs to be within a extern "C" to prevent name mangling by the compiler and allow the JNI to call the function blindly.

JNIEXPORT & JNICALL

JNIEXPORT ensures function is visible on the symbols table JNICALL ensures function uses the correct calling convention. On Android JNICALL has a different value based on the architecture where it is running.