Waxing Cerebral

Came across this site that stocks some really fun looking hardware. Of particular note is stocking only opensource-capable routers, laptops without an OS (no M$ tax!) and single-board computers that use only 5-7 watts that you could really have some fun with. Oh and who could pass up a GSM modem with built-in GPS? Oh, the gadgetry and geekyness.

nicegear.co.nz

1. Create Java class containing native method(s) (static or instance) defining interface with C code.
   
   public class Processor {
   public static native double process(double x, double y);
   }


2. Compile to Java class file.


3. Run
   javah -jni [-o path/to/CProcessor.h] Processor
   to generate C header file from the Java. Eg. CProcessor.h:
   /* DO NOT EDIT THIS FILE - it is machine generated */
   #include <jni.h>
   /* Header for class Processor */
   
   #ifndef _Included_Processor
   #define _Included_Processor
   #ifdef __cplusplus
   extern "C" {
   #endif
   /*
   * Class: Processor
   * Method: process
   * Signature: (DD)D
   */
   JNIEXPORT jdouble JNICALL Java_Processor_process
   (JNIEnv *, jclass, jdouble, jdouble);
   
   #ifdef __cplusplus
   }
   #endif
   #endif


4. Create a .c C source file - this is the stub to call the target code. Include header and "Processor.h". The angle brackets mean it is registered as a library file for the compiler, whereas the speech marks denote a stand-alone header file. May need to include the path the Processor.h within the speech marks. Here is a simple example that just sums the two input numbers, rather than calling any other C:
   
   #include <jni.h>
   #include <math.h>
   #include "../headers/CProcessor.h"
   #include <stdio.h>
   JNIEXPORT jdouble JNICALL Java_Processor_process
   (JNIEnv *env , jclass obj, jdouble val1, jdouble val2) {
   return (val1+val2);
   }


5. Run (eg for linux)
   gcc -shared -I /usr/lib/jvm/java-1.5.0-sun-1.5.0.15/include/ -I
   /usr/lib/jvm/java-1.5.0-sun-1.5.0.15/include/linux/ -o CProcessor.so CProcessor.c
   to create the library file (indicated by 'shared' flag). Any compiler errors about missing .h headers should be solved by the inclusion (-I) of the jni and jni for linux paths.


6. Either in a method inside original Java class, or in a new calling class, create a static block to either load (System.load()) the .so library via its file path, or load (System.loadLibrary()) a registered library (e.g. dll on Windows) via system-specific addressing.


7. Once the static block has loaded the library, the methods are available either statically or from an object as determined in 1.
   public class Caller {
   static {
   System.load("/home/me/_projects/JNI/C/CProcessor.so");
   }
   public static void main(String[] args) {
   double result = Processor.process(2, 3);
   System.out.println(result);
   }
   }
   In this case our Processor in Java calls the CProcessor in C which adds the 2 doubles we passed it and returns a double. Here it is 2 + 3 with the output 5.