java_nmi3

NMI with arguments

Java Code: Iching1.java

// prompts for user's birth year
// and returns his/her fortune for the day

import java.lang.*;

class Iching1 {

private native String getfortune(int i, String s);

    static {
        System.loadLibrary("iching1");
    }

    public static void main(String args[]) {
        Iching1 ich = new Iching1();
        int seed= (int) (10*Math.random());
        String fortune = ich.getfortune(seed,"Please enter your birth year.");
        System.out.println("Your today's fortune is...\n\n" + fortune + "\n\n");
    }

}

Header file: Iching1.h

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class Iching1 */

#ifndef _Included_Iching1
#define _Included_Iching1
#ifdef __cplusplus
extern "C" {
#endif
/*
* Class: Iching1
* Method: getfortune
* Signature: (ILjava/lang/String;)Ljava/lang/String;
*/
JNIEXPORT jstring JNICALL Java_Iching1_getfortune
(JNIEnv *, jobject, jint, jstring);

#ifdef __cplusplus
}
#endif
#endif

jstring: return value of the method

jint, jstring

To learn more about how to reference Java types in your native method, see Mapping between Java and Native types, JNI Types and Data Structures , and Accessor Function Tables. This is useful when you want to access the arguments passed in from Java, create new Java objects, and return results to the caller.

Native Method using C: Ich1.c

#include "Iching1.h"
#include <stdio.h>

JNIEXPORT jstring JNICALL Java_Iching1_getfortune
(JNIEnv *env, jobject obj, jint i, jstring s) {

    char buf[128];
    int byr, predict;
    char fortune[10][200]=
        { "Plain dealing will keep you out of trouble.",
          "The situation is about to change for the better.",
          "Pay attention to small warnings. Hard times lie ahead.",
          "Don't keep hitting your head against a stone wall.\nChannel your energy to attainable goals.",
          "Overconfidence will lead to misadventure and regret.",
          "For the moment, you are free of worry.",
          "Do what you have to do, but if it ain't broken, don't fix it.",
          "Honors, wealth, health and long life are in store as problems disappear.",
          "Accept defeat and repent. You will become at peace with yourself.",
          "Never give up. Good times lie ahead." };

/* Obtain a C-copy of the Java string */
const char *str = (*env)->GetStringUTFChars(env, s, 0);

/* Display input prompt */
printf("\n\n%s\n", str);

/* Release string buffer */
(*env)->ReleaseStringUTFChars(env, s, str);

    /* Process user and method argument input (jint i) to select today's fortune */
    scanf("%s", buf);
    byr= atoi(buf);
    predict= (byr*i)%10;

/* Return today's fortune to the invoking object */
return (*env)->NewStringUTF(env, fortune[predict]);

}

Java objects are manipulated using the interface pointer env .

jstring type passed from Java to native code is not the regular C string type (char *). Therefore, the native method code must use JNI functions (GetStringUTFChars to print the string passed in from Java, NewStringUTF to construct a new string to pass to Java) to convert Java strings to native strings.

ReleaseStringUTFChars informs the VM that the native method is finished with the string, so that the memory taken by the string can then be freed. Failing to call ReleaseStringUTFChars results in a memory leak, which will ultimately lead to system memory exhaustion.

Other JNI Functions for Accessing Java Strings

GetStringChars -- takes the Java string and returns a pointer to an array of Unicode characters that comprise the string.
ReleaseStringChars -- releases the pointer to the array of Unicode characters.
NewString -- constructs a new java.lang.String object from an array of Unicode characters.
GetStringLength -- returns the length of a string that is comprised of an array of Unicode characters.
GetStringUTFLength -- returns the length of a string if it is represented in the UTF-8 format.

Native Method using C++: Ich1.cc

#include "Iching1.h"
#include <stdio.h>
#include <stdlib.h>

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

JNIEXPORT jstring JNICALL Java_Iching1_getfortune
(JNIEnv *env, jobject obj, jint i, jstring s) {

/* Obtain a C-copy of the Java string */
const char *str = env->GetStringUTFChars(s, 0);

/* Display input prompt */
printf("\n\n%s\n", str);

/* Release string buffer */
env->ReleaseStringUTFChars(s, str);

    /* Process user and method argument input (jint i) to select today's fortune */
    scanf("%s", buf);
    byr= atoi(buf);
    predict= (byr*i)%10;

/* Return today's fortune to the invoking object */
return env->NewStringUTF(fortune[predict]);

}

With C++, the extra level of indirection and the interface pointer argument disappear from the source code. However, the underlying mechanism is exactly the same as with C. In C++, JNI functions are defined as inline member functions that expand to their C counterparts.

NMI using Arrays

//-------------------------------------------
// filename: Iching2.java
//-------------------------------------------
// prompts for user's birth year
// and returns his/her fortune for the day

import java.lang.*;

class Iching2 {

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

private native String getfortune(int i, String s, String[] ss);

static {
System.loadLibrary("iching2");
}

public static void main(String args[]) {
    Iching2 ich = new Iching2();
    int seed= (int) (10*Math.random());
    String fortune = ich.getfortune(seed,"Please enter your birth year.",ich.msgs);
    System.out.println("\n...Your today's fortune is...\n\n" + fortune + "\n\n");
}

}

//---------------------------------
// filename: Iching2.h
//---------------------------------

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class Iching2 */

#ifndef _Included_Iching2
#define _Included_Iching2
#ifdef __cplusplus
extern "C" {
#endif
/*
* Class: Iching2
* Method: getfortune
* Signature: (ILjava/lang/String;[Ljava/lang/String;)Ljava/lang/String;
*/
JNIEXPORT jstring JNICALL Java_Iching2_getfortune
(JNIEnv *, jobject, jint, jstring, jobjectArray);

#ifdef __cplusplus
}
#endif
#endif

//---------------------------------
// filename: Ich2.cc
//---------------------------------

#include "Iching2.h"
#include <stdio.h>
#include <stdlib.h>

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

JNIEXPORT jstring JNICALL Java_Iching2_getfortune
(JNIEnv *env, jobject obj, jint i, jstring s, jobjectArray ss) {

char buf[128];
int byr, predict;

/* Obtain a C-copy of the Java string */
const char *str = env->GetStringUTFChars(s, 0);

/* Display input prompt */
printf("\n\n%s\n", str);

/* Release string buffer */
env->ReleaseStringUTFChars(s, str);

    /* Process user and method argument input (jint i) to select today's fortune */
    scanf("%s", buf);
    byr= atoi(buf);
    predict= (byr*i)%10;

/* Return today's fortune to the invoking object */
return (jstring) env->GetObjectArrayElement(ss,predict);

}