Callback from C to Java method

#include <jni.h>
#include <stdlib.h>
#include <string.h>

#include "api.h"

static JNIEnv* genv = NULL;
static jclass gclass = NULL;
static jmethodID gcallbackid = 0;

static void msg_callback(const char* msg) {
  printf("c/c++ callback !!!\n");
  jstring message = (*genv)->NewStringUTF(genv, msg);
  (*genv)->CallStaticVoidMethod(genv, gclass, gcallbackid, message);
}


SWIGEXPORT void JNICALL Java_JNI_1set_1api_1msg_1callback(JNIEnv *jenv, jclass jcls) {
  genv = jenv;
  gclass = jcls;
  gcallbackid = (*jenv)->GetStaticMethodID(jenv, jcls, "callMsgCallback", "(Ljava/lang/String;)V");
  api_msg_callback(msg_callback);
}

public class MyLibraryJNI {
  private static ApiMsgCallback msgCallback = null;
  public final static native int set_api_msg_callback();
  public final static void setMsgCallback(ApiMsgCallback callback) {
    msgCallback = callback;
  }
  public final static void callMsgCallback(String message) {
    System.out.println("java callMsgCallback");
    msgCallback.call(message);
  }
}

public class MyLibrary {
  public static void set_api_msg_callback(ApiMsgCallback callback) {
    MyLibraryJNI.setMsgCallback(callback);
    MyLibraryJNI.slon_application_set_api_msg_callback();
  }
}