Embed Java into a C++ application?

Question

I got an application written in C++ and I am able to extend the applications functionality by writing plugins in C++ for it.

What I basically want to do is to embed Java into this application. This has already been done with Python (not done by me).

I read something about JNI but there is always the speech from a full programm that uses Java classes.

What I'd like to do is, to use classes from C++ in Java to interact with the application.
It's a 3D app in this case, called Cinema 4D.

Is there a way to compile and evaluate Java code while the application is running (in some sort of scripting language) using JNI or anything like it ?

Example imaginary code after the embedding was done:

import c4d.documents.*;

class Main {
  public static void main() {
    BaseDocument doc = GetActiveDocument();
    BaseObject op = doc.GetActiveObject();
    if (op != null) {
      op.Remove();
    }
  }
}

This code should interact with Cinema 4D to delete the selected object.

Answer 1

You can embed a JVM within your application. Oracle's official reference book has some more details. The synopsis of it is:

#include <jni.h>       /* where everything is defined */

int main() {
  JavaVM *jvm;       /* denotes a Java VM */
  JNIEnv *env;       /* pointer to native method interface */
  JDK1_1InitArgs vm_args; /* JDK 1.1 VM initialization arguments */
  vm_args.version = 0x00010001; /* New in 1.1.2: VM version */
  /* Get the default initialization arguments and set the class 
   * path */
  JNI_GetDefaultJavaVMInitArgs(&vm_args);
  vm_args.classpath = ...;
  /* load and initialize a Java VM, return a JNI interface 
   * pointer in env */
  JNI_CreateJavaVM(&jvm, &env, &vm_args);
  /* invoke the Main.test method using the JNI */
  jclass cls = env->FindClass("Main");
  jmethodID mid = env->GetStaticMethodID(cls, "test", "(I)V");
  env->CallStaticVoidMethod(cls, mid, 100);
  /* We could have created an Object and called methods on it instead */
  /* We are done. */
  jvm->DestroyJavaVM();
}

You can do far more sophisticated things if you want (e.g. custom class loaders) but that's about it in terms of the bare minimum needed to get a JVM working within your application.