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"

10. issue "java HelloWorld" to see the "Hello world" text which is written by a C shared object called from Java

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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)