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java-jni

Overview

This is a very simple "Hello World" example of how to use Java Native Interface (JNI) to call a C function from Java.

Prerequisites

  • JDK
  • Git

To compile and run this sample in linux

  1. change directories (cd) to a development directory

  2. issue "git clone https://github.com/dkelosky/java-jni.git"

  3. issue "cd java-jni" to change to the java-jni directory

  4. issue "javac HelloWorld.java" to compile the Java source

  5. issue "javah -jni HelloWorld" to generate a HelloWorld.h C header file (see Note1 )

  6. create the HelloWorld.c source including the jni.h and HelloWorld.h C header files in addition to required C runtime library headers (see Note1 )

  7. edit the make file so that the include paths contain the location of the generated HelloWorld.h C header file, your_JDK_install_path/include, and your_JDK_install_path/include/linux (see Note2 and Makefile)

  8. issue "make" to generate the libHelloWorld.so (so = Shared Object which is like a Windows DLL)

  9. set the LD_LIBRARY_PATH variable to the libHelloWorld.so directory and export so that the JVM can locate the shared object

  • (e.g. "LD_LIBRARY_PATH=./" to set to the current directory and then "export LD_LIBRARY_PATH"
  1. issue "java HelloWorld" to see the "Hello world" text which is written by a C shared object called from Java

To compile and run this sample in mac osx

  • git clone https://github.com/dkelosky/java-jni.git
  • cd java-jni
  • javac HelloWorld.java
  • javah -jni HelloWorld
  • create jni dynamic library for mac osx
  • g++ "-I/System/Library/Frameworks/JavaVM.framework/Versions/Current/Headers" -x c -c HelloWorld.c
  • g++ -dynamiclib -o libHelloWorld.jnilib HelloWorld.o
  • you will see a libHelloWorld.jnilib in current dir
  • excute HelloWolrd.class and get C print result Hello world!
  • java HelloWorld

Misc Notes

  • Note0
    • be sure to consitently use the same version of Java for all steps (i.e. same release number and only 31 OR 64-bit) .
  • Note1
    • these steps are optional since the HelloWorld.h and HelloWorld.c source are contained in this sample.
  • Note2
    • you could alternatively use the C_INCLUDE_PATH variable to specifiy JDK or other C include headers instead of using the "gcc -I" include options
  • Makefile
    • the makefile uses -Wall to turn on most warnings and -Werror to make warnings errors (and fail the compilation)