How do I run a Java program from a C program?

Question

I have researched this, but the results are either 10+ years old and do not explain what JNI is or if it is the only approach, or the results are for C++ or C#. So here is my question:

How do I run a Java program from a C program, using the following code as an example? What specific changes to I make to the following code to get the C program to successfully call the Java program with parameters?

In the CentOS terminal, I am able to successfully run a Java program when I type the following in the command line:

java -cp . my.package.SomeClass 1 2 3

Similarly, from the same folder in the terminal, I am able to successfully run a C program when I type the following in the command line:

./hello  

The code for hello.c is:

#include <stdio.h>

main() {
    printf("Hello World from C!\n");
}  

How do I modify the code for hello.c so that it also runs my.package.SomeClass with the parameters 1 2 3 ?

For example, how do I accomplish the following, but without throwing errors:

#include <stdio.h>

main() {
    printf("Hello World from C!\n");
    java -cp . my.package.SomeClass 1 2 3  //What is the right syntax here?
}  

Edit

I am most interested in answers that show how to call a method, such as SomeClass.someMethod(1,2,3).

Answer 1

If all you want to do is run java -cp . my.package.SomeClass 1 2 3 from your program, you could just use system which runs an external command:

system("java -cp . my.package.SomeClass 1 2 3");

This is the simplest solution, but not very flexible - you can't interact with the Java program apart from giving it arguments, for example.

Answer 2

You do need to use JNI (as far as I know). I use the documentation here all the time:

http://docs.oracle.com/javase/7/docs/technotes/guides/jni/

It's a bit terse but it's workable. It handles C and C++. I don't know C#, so I cannot comment on that (although if forced to guess I would say no to C#).

Running your java program from C will require you to start a JVM programmatically using JNI functions, then find your class, then find your method, then call it. There are examples here to get you started:

http://docs.oracle.com/javase/7/docs/technotes/guides/jni/spec/invocation.html#wp9502

If you need more help, you might need to find someone who's done it before to walk you through the steps.

Answer 3

I believe you are looking for the exec family of functions. Something like,

#include <stdio.h>
#include <unistd.h>

int main(int argc, char *argv[])
{
        const char *file = "java";
        const char *arg = "-cp . my.package.SomeClass 1 2 3";
        printf("Hello World from C!\n");
        execlp(file, arg, (char *) NULL);
}