It never fails. There is some great piece of code that will save you tons of time, but it is written in Java and your project is written in C or C++. Okay, I've really only needed this a few times, but sometimes calling Java class methods from C code with JNI is the best approach. I leveraged existing examples and web pages heavily, but many of them did not include a Makefile or any sort of build notes. This is what worked for me: hello_world.tar.gz.
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;
}
