JNI Demonstration

This application demonstrates basic usages of Java Native Interface (JNI).

There are three usages in this demo.

Basic hello world

The hello world application implementation is resides in the jni-hello module. It uses javac compiler and g++ compiler to compile the project and run. Main class of this module calls native library native.dll to print "hello world".

Generating header files

JDK includes tools to generate header files for the native method interface.

How to generate

in java 8 and prior using javah command and _java -h_in java 9 and higher.

Native library should be included in the java.library.path to link the library at the run time. Java path is defaulted to system PATH variable if not explicitly set.

This demo can be run using script-run.bat

Steps

1. Run cd /jni-hello
2. Run ./script-run.bat from cmd

Hash generator

MD5 hash code generate example using C native code to execute hashing algorithm. This example uses maven to compile the project. org.codehaus.mojo native-maven-plugin native maven plugin has been used to compile the native library. Since it internally uses the g++/gcc compiler g++/gcc installation is a requirement to this plugin. Native library implementation resides in the c-hash module and java code resides in the java-hash module.

javah goal in the maven plugin (above mentioned) has been used to generate the header files of the native code.

DLL Extractor

This project bundles native library into the jar file generated. DLLLoader class is used to extract native library from jar to directory jar is located since Java doesn't provide a mechanism to load a native library from the jar itself.

Note: Currently as the name implies this class supports dll extension in windows environment only. Can be modified to extract linux .so files with minimal set of changes.

Steps

1. Run mvn clean install from the project root
2. Run cd /java-hash
3. Run ./script-run.bat from cmd

Native cache

Cache implementation to store java objects in native memory. Like the hash generator this uses same maven plugin to build the project.

Cache is implemented in C++ using a map and can be stores the jobject as a global reference.

Native memory cache implementation is an important topic in the java ecosystem since on heap cache increases the heap size and can be a cause for long GC cycles. Since the native memory management is done manually this resolves that problem.

Steps

1. Run mvn clean install from the project root
2. Run cd java-cache-caller
3. Run ./script-run.bat from cmd

Requirements

1. JDK 8 or higher
2. maven installation
3. This project supports only dll libraries and windows environment, but easily can be modified to be used in the linux environment.
4. g++/gcc compiler

Install gcc/g++ compiler

In windows environment mingw can be used to compile c/c++ programs. mingw-w64 for 64bit envirnment or mingw for 32bit environment

In linux environment to install run sudo apt install gcc sudo apt install g++