Can C++ call Java code?

Question

I know that Java code can call C++ code through JNI. Is it possible, however, to call Java code from C++ again through JNI or with any other method?

Answer 1

Yep you certainly can. Here's an example:

Here's the java file:

public class InvocationHelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
        System.out.println("Arguments sent to this program:");
        if (args.length == 0) {
            System.out.println("(None)");
        } else {
            for (int i=0; i<args.length; i++) {
                System.out.print(args[i] + " ");
            }
            System.out.println();
        }
    }
}

And heres some C++ that uses it:

void invoke_class(JNIEnv* env) {
  jclass helloWorldClass;
  jmethodID mainMethod;
  jobjectArray applicationArgs;
  jstring applicationArg0;

  helloWorldClass = (env)->FindClass("InvocationHelloWorld");
  if(! helloWorldClass )
  {
    std::cerr<<"Couldn't get \"InvocationHelloWorld\""<<std::endl;
    return;
  }

  mainMethod = (env)->GetStaticMethodID(helloWorldClass, "main", "([Ljava/lang/String;)V");
  if(! mainMethod )
  {
    std::cerr<<"Coulnd't get \"InvocationHelloWorld::main\""<<std::endl;
    return;
  }

  applicationArgs = (env)->NewObjectArray(1, (env)->FindClass("java/lang/String"), NULL);
  applicationArg0 = (env)->NewStringUTF("From-C-program");
  (env)->SetObjectArrayElement(applicationArgs, 0, applicationArg0);

  (env)->CallStaticVoidMethod(helloWorldClass, mainMethod, applicationArgs);
}

Answer 2

You can also use SWIG to automatically generate the JNI files. The setup can be a bit tricky but it's very useful when you have a lot of C++ code to expose

http://www.swig.org/index.php