JNI-Can't access variables in native methods

Question

Hello,
I wrote a simple JNI code in which the native method accesses an instance
method, prints it and modifies it. The java method is names Sample.java
and the native method is called Sample.c.
The problem is I don't seem to get the variables correctly accessed in the
native code. I am reproducing Sample.java, Sample.c and Sample.h. here. I
am also reproducing the output.
Please let me know if there are any mistakes..
//Sample.java
public class Sample
int i;
public native void modify();
Sample()
i = 10;
System.out.println("In Java: i = "+ i);
static
System.loadLibrary("samp");
public static void main(String [] args)
Sample s = new Sample();
s.modify();
System.out.println("Again in Java: " +s.i);
//Sample.c
#include <jni.h>
#include "Sample.h"
#include <stdio.h>
JNIEXPORT void JNICALL Java_Sample_modify(JNIEnv * env, jobject obj)
jclass cls = (*env)->GetObjectClass(env, obj);
jfieldID fid;
jint ci;
fid = (*env)->GetFieldID(env, cls, "i", "I");
ci = (*env)->GetIntField(env, cls, fid);
printf("Accessed in c\n: i = %d\n", ci);
(*env)->SetIntField(env, cls, fid, 200);
printf("modified in c\n i = %d\n", ci);
//Sample.h
/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class Sample */
#ifndef IncludedSample
#define IncludedSample
#ifdef __cplusplus
extern "C" {
#endif
* Class: Sample
* Method: modify
* Signature: ()V
JNIEXPORT void JNICALL Java_Sample_modify
(JNIEnv *, jobject);
#ifdef __cplusplus
#endif
#endif
and the output is
In Java : i=10
Accessed in c
: i=196653
modified in c
i=196653
Again in Java : 10
Thanks,
Aditya

Answer 1

GetFieldID(..) expects an obj variable, not a cls. The
obj passed in is a reference to the java object you
are trying to modify, so you need to pass this to
GetFieldID(..) etc.
See below:
JNIEXPORT void JNICALL Java_Sample_modify(JNIEnv *
env, jobject obj)
jclass cls = (*env)->GetObjectClass(env, obj); /// NOT
REQUIRED
jfieldID fid;
jint ci;
fid = (*env)->GetFieldID(env, cls, "i", "I"); // USE
THE obj PASSED in NOT cls
...hope this helps :)Hi
From what I got I modified the code as
fid = (*env)->GetFieldID(env, obj, "i", "I");//obj instead of cls
Well, the program kinda crashed :-(
Moreover I used the same code I found in the jni tutorials on the sun website..which I reproduce. I noted that they used cls instead of obj.
#include <stdio.h>
#include <jni.h>
#include "FieldAccess.h"
JNIEXPORT void JNICALL
Java_FieldAccess_accessFields(JNIEnv *env, jobject obj)
jclass cls = (*env)->GetObjectClass(env, obj);
jfieldID fid;
jstring jstr;
const char *str;
jint si;
printf("In C:\n");
fid = (*env)->GetStaticFieldID(env, cls, "si", "I");
if (fid == 0) {
return;
si = (*env)->GetStaticIntField(env, cls, fid);
printf(" FieldAccess.si = %d\n", si);
(*env)->SetStaticIntField(env, cls, fid, 200);
fid = (*env)->GetFieldID(env, cls, "s", "Ljava/lang/String;");
if (fid == 0) {
return;
jstr = (*env)->GetObjectField(env, obj, fid);
str = (*env)->GetStringUTFChars(env, jstr, 0);
printf(" c.s = \"%s\"\n", str);
(*env)->ReleaseStringUTFChars(env, jstr, str);
jstr = (*env)->NewStringUTF(env, "123");
(*env)->SetObjectField(env, obj, fid, jstr);
I may not have got your exact message :-(. But something seems to be wrong with yhe code and being a begining jni programmer, I am absolutely clueless:-(
Any help would be appreciated :-)
Thanks,
Aditya