How can i convert jstring to char array?

Question

I cannot convert jstring to char array

JNIEXPORT jlong JNICALL Java_com_jnitest_JNI_getUid(JNIEnv * env,jobject thiz,jstring user){
const char *userName = (*env)->GetStringUTFChars(env,user,0);
(*env)->ReleaseStringUTFChars(env,user,userName);
FILE * f = fopen("/data/data/com.jnitest/p.txt","w");
if(f != NULL){
    fprintf(f,"%c%c%c",userName[0],userName[1],userName[2]);
fclose(f);
}
return getUser(userName);
}

And in file p.txt i have this:

Answer 1

I have a mistake:

I'm releasing string before printing it to file.

This code:

(*env)->ReleaseStringUTFChars(env,user,userName);

should be at end of function.