Calling Java Class Methods from C with JNI

helloWorld.java

public class helloWorld{
	public static void main(String[] args){
		System.out.println("Hello, World");
	}
	public static int square(int input){
		int output = input * input;
		return output;
	}
	public static int power(int input, int exponent){
		int output,i;
		output=1;
		for(i=0;i<exponent;i++){
			output *= input;
		}
		return output;
	}
}

hello_world.c

/*
These three pages were instrumental while trying to get this working.
I started with existing examples, got them working, and updated
them to make sure I understood how things worked. If you are trying this
for the first time, I suggest adding a new method with different arguments
to the Java class and then calling it from this C program.
Use 'javap -s -p helloWorld.class' to get the new method signature.
After that is working, package the class in a jar file, update the class
path in this C source file, and get that working.
http://www.inonit.com/cygwin/jni/invocationApi/c.html
http://www.codeproject.com/Articles/22881/How-to-Call-Java-Functions-from-C-Using-JNI
http://java.sun.com/developer/onlineTraining/Programming/JDCBook/jniexamp.html
*/

#include <stdio.h>
#include <jni.h>

JNIEnv* create_vm(JavaVM **jvm)
{
    JNIEnv* env;
    JavaVMInitArgs args;
    JavaVMOption options;
    args.version = JNI_VERSION_1_6;
    args.nOptions = 1;
    options.optionString = "-Djava.class.path=./";
    args.options = &options;
    args.ignoreUnrecognized = 0;
    int rv;
    rv = JNI_CreateJavaVM(jvm, (void**)&env, &args);
    if (rv < 0 || !env)
        printf("Unable to Launch JVM %d\n",rv);
    else
        printf("Launched JVM! :)\n");
    return env;
}

void invoke_class(JNIEnv* env)
{
    jclass hello_world_class;
    jmethodID main_method;
    jmethodID square_method;
    jmethodID power_method;
    jint number=20;
    jint exponent=3;
    hello_world_class = (*env)->FindClass(env, "helloWorld");
    main_method = (*env)->GetStaticMethodID(env, hello_world_class, "main", "([Ljava/lang/String;)V");
    square_method = (*env)->GetStaticMethodID(env, hello_world_class, "square", "(I)I");
    power_method = (*env)->GetStaticMethodID(env, hello_world_class, "power", "(II)I");
    (*env)->CallStaticVoidMethod(env, hello_world_class, main_method, NULL);
    printf("%d squared is %d\n", number,
        (*env)->CallStaticIntMethod(env, hello_world_class, square_method, number));
    printf("%d raised to the %d power is %d\n", number, exponent,
        (*env)->CallStaticIntMethod(env, hello_world_class, power_method, number, exponent));
}

int main(int argc, char **argv)
{
    JavaVM *jvm;
    JNIEnv *env;
    env = create_vm(&jvm);
    if(env == NULL)
        return 1;
    invoke_class(env);
    return 0;
}

Makefile

all: run

helloWorld.class: helloWorld.java
	/usr/java/latest/bin/javac helloWorld.java

hello_world: hello_world.c
	gcc -o hello_world \
	-L /usr/java/latest/jre/lib/amd64/server/ \
	-ljvm \
	-I /usr/java/latest/include/ \
	-I /usr/java/latest/include/linux/ \
	hello_world.c

run: helloWorld.class hello_world
	export LD_LIBRARY_PATH="/usr/java/latest/jre/lib/amd64/server/"
	./hello_world

clean:
	rm -f helloWorld.class hello_world

Building and Test Output

[BUILDROOT-RHEL5 hello_world]# make
/usr/java/latest/bin/javac helloWorld.java
gcc -o hello_world \
        -L /usr/java/latest/jre/lib/amd64/server/ \
        -ljvm \
        -I /usr/java/latest/include/ \
        -I /usr/java/latest/include/linux/ \
        hello_world.c
export LD_LIBRARY_PATH="/usr/java/latest/jre/lib/amd64/server/"
./hello_world
Launched JVM! :)
Hello, World
20 squared is 400
20 raised to the 3 power is 8000