| A Quick Review | |
public class HelloWorld {
public native void displayHelloWorld();
static{
System.loadLibrary("hello");
}
public static void main(String[] args) {
new HelloWorld().displayHelloWorld();
}
}
Actually getting this all working can be tricky. create header file: javah HelloWorld C program:
#include <jni.h>
#include "HelloWorld.h"
#include <stdio.h>
JNIEXPORT void JNICALL
Java_HelloWorld_displayHelloWorld(JNIEnv* env, jobject obj)
{
printf("Hello world!\n");
return;
}
Compile C program (include paths are for CS machines):
gcc -I/usr/local/pkg/jdk/current/include/ -I/usr/local/pkg/jdk/current/include/linux hello.c -c -o hello.o ld -G hello.o -o libhello.so Run java program: java -Djava.library.path=./ HelloWorld If you set your library path to include the local directory, you don't need the -D option: export LD_LIBRARY_PATH=./ |
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| Working with Arrays | |
Suppose we had an array of integers, and we wanted to sum them in native code:
native int sumArray(int arr[]);We would have this native code:
JNIEXPORT jint JNICALL
Java_IntArray_sumArray(JNIEnv* env, jobject obj, jintArray arr);
{
//Get Length of Array
jsize len = (*env)->GetArrayLength(env, arr);
int i, sum=0;
//Get pointer to the array of primitive Java objects
jint* body = (*env)->GetIntArrayElements(env,arr,0);
for(i=0; i<len, i++) {
sum += body[i];
}
//Tell Java we're done with the array. (This is important for the GC)
(*env)->ReleaseIntArrayElements(env,arr,body,0);
return sum;
}
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| Exercise |
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A common usage of JNI is to use native code for the CPU intensive part of the application. Write two program, one a Java program to multiply two 3x3 matrices, and a Java program that does the same thing with a native method. The signature of the Java method should be multiplyMatrices(int[] a, int[] b, int[] result). Here's a simple C program that multiplies two 3x3 matrices for your reference: det.c. Time the two programs. Which is faster? Is the overhead of transitioning to native code worth the speed-up of the multiply? What about for 4x4 matrices? |
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