• Java Code:  Iching3.java

import java.lang.*;
import java.io.*;

class Iching3 {

  String msgs[] = new String[] {
    "Don King: Plain dealing will keep you out of trouble.",
    "IMF: The situation is about to change for the better.",
    "Clinton: Pay attention to small warnings.  Hard times lie ahead.",
    "Starr: Don't keep hitting your head against a stone wall.\nChannel your energy to attainable goals.",
    "Tyson: Overconfidence will lead to misadventure and regret.",
    "Bill Gates: For the moment, you are free of worry.",
    "Javasoft: Do what you have to do, but if it ain't broken, don't fix it.",
    "Smith: Honors, wealth, health and long life are in store as problems disappear.",
    "Sadam: Accept defeat and repent.  You will become at peace with yourself.",
    "Kiduk: Never give up.  Good times lie ahead." };

  private native void getfortune(String s, int cnt);

  static {
    System.loadLibrary("iching3");
  }

  private void findfortune(String s, int cnt) {
    if (cnt < 10) {
        if (msgs[cnt].startsWith(s))
            System.out.println("\n...fortune found!...\n\n" + msgs[cnt] + "\n\n");
        else {
            System.out.println(cnt + " : Java going to C");
            getfortune(s,cnt);
            System.out.println(cnt + " : Java returning from C");
        }
    }
    else
         System.out.println(cnt + " : Search limit exceeded. Java Gone!\n\n");
  }

  public static void main(String args[]) {

    Iching3 ich = new Iching3();
    byte buffer[]= new byte[255];

    // prompt for input
    System.out.println("\nPlease enter your name.");
    String prompt= new String ("Please enter your name.");
    try { System.in.read(buffer,0,255); }
    catch (Exception e) { System.out.println(e.toString()); }

    // pass input string to recursive method call
    String name= new String (buffer);
    ich.findfortune(name.trim(),0);

  }

}
 

• Header File:  Iching3.h

#ifndef _Included_Iching3
#define _Included_Iching3
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     Iching3
 * Method:    getfortune
 * Signature: (Ljava/lang/String;I)V
 */
JNIEXPORT void JNICALL Java_Iching3_getfortune
  (JNIEnv *, jobject, jstring, jint);

#ifdef __cplusplus
}
#endif
#endif
 
 

• Native Method using C++:  Ich3.cc

extern "C" /* specify the C calling convention */

JNIEXPORT void JNICALL Java_Iching3_getfortune
  (JNIEnv *env, jobject obj, jstring s, jint cnt) {

  jclass cls = env->GetObjectClass(obj);
  jmethodID mid = env->GetMethodID(cls, "findfortune", "(Ljava/lang/String;I)V");
  if (mid == 0)  return;
  printf("In C, cnt = %d, about to enter Java\n", cnt);
  env->CallVoidMethod(obj, mid, s, cnt+1);
  printf("In C, cnt = %d, back from Java\n", cnt);

}
 
 
 

• 5 steps to calling the Java method from the Native Method

1. get the class type of the invoking object
2. get the Method ID
3. initialize variables to be passed as parameters to the Java method
4. use appropriate CallMethod() accessor funtion to invoke the method
5. convert the returned object field for use within the native method if needed.
 
• JNI uses method signatures to denote the type of Java methods. The signature (I)V, for example, denotes a Java method that takes one argument of type int and has return type void. The general form of a method signature argument is: