libnji

libnji is a library that can be used to ease use of Java JNI in a native application. It consists of two parts:

1. njic - The code generation tool.
2. libnji - The runtime library for NJI.

njic

njic takes JSON based NJI files that describe Java classes and outputs C source code that provides a simple and conistsent interface to Java. The templates it uses to do this can be found in tools/njic/src/nji/templates. If you want use a custom template you can. Check the help for njic for details. Take care to implement all the required parts of the template.

Requirements

The following must be installed:

1. javap
2. Python 2.7 (or reasonably new Python 2.x at least - Python 3 support pending)

Installation

cd tools/njic
sudo python setup.py install

This will install njic and its dependencies (jinja2) system wide. If you want to use a virtualenv you can.

Usage

See the README.md in tools/njic for usage.

libnji

This is the runtime component. It takes care of getting a JavaVM, resolving classes and methods, and handling exceptions. It also provides an abstraction layer on top of JNI that handles the details on the Java environment.

You will need to statically link against this library to use the source files created by njic.

The best way to learn how to use this is through the examples.

Requirements

1. The Java Developement Kit (I used the openjdk-9-jdk-headless package)

This is required for jni.h and to link the test program against libjvm

NOTE

In libnji the jenv.c file is responisible for abstacting away the details of acquiring a JNIEnv* (Java Environment for each thread). The method of acquiring the JNIEnv* can vary by platform, as such, you may (will) find that modifying this code it necessary. The code provided here is targeting the a standard Java runtime (such as would be found in linux).

The Example

The code under test/ provides an example of NJI. It uses only classes from the Java standard library and has been tested on Ubuntu 18.04. It simply enumerates threads and prints some output about each thread and its ClassLoader if is has one.

Build System

Currently SCons is supported. I may looking at adding support for other build tools out of the box at some point, however, if you don't want to use SCons the library is pretty small and porting to another build tool is fairly straight forward.