Most of the examples I've seen for JNI seem to allow access to 'top
level' instance variables like this:

public class HelloNative{

   private String str;
   private native void setObjLong();

   public static void main(String[] args) {
       HelloNative test = new HelloNative();
       test.setObjLong();
       System.out.println("The java s is: "+ test.str);
   }
}

The string is created in the c code:
JNIEXPORT void JNICALL
Java_HelloNative_setObjLong (JNIEnv* env, jobject obj) {

   /* get the class object */
   jclass cls = (*env)->GetObjectClass(env, obj);
   /* get the field id */
   jfieldID fid = (*env)->GetFieldID(env, cls, "str",
"Ljava/lang/String;");
   /* make new string */
   jstring myStr = (*env)->NewStringUTF(env,"1234");
   /* set the value */
   (*env)->SetObjectField(env, obj, fid, myStr);
   return 0;
}

This all works fine, the question is how do you get to user-defined
object's instanace variables?
So if I change the java bit thus:
public class HelloNative {

   private MyClass myObj = new MyClass();
   private native void setObjLong();

   class MyClass{
       private long address;
       // getters and setters omitted
   }

   public static void main(String[] args) {
       HelloNative test = new HelloNative();
       test.setObjLong();
       System.out.println("The java long is: "+
test.myObj.getAddress());
   }
}

How can I set the address field of myObj?

If I try:
  jfieldID fid = (*env)->GetFieldID(env, cls, "myObj",
"Ljava/lang/Object;");

I get:
java.lang.NoSuchFieldError: HelloNative.myObj

...and anyway this would only get me to the object and not to the
address instance which I wish to set.

Thanks,
Andy