JNI loses reference to native methods
- by lhw
As an example for later use in Android I wrote a simple callback interface. While doing so i ran into the following error or bug or whatever. In C the two commented lines are supposed to be executed resulting in calling the C callback onChange. But instead i get an UnsatisfiedLinkError. Calling the native Method directly in Java works just fine. Calling it directly from C as presented here in the example also produces the UnsatisfiedLinkError. I'm open for any advice concerning this issue or work arounds and so on.
The Java Part:
import java.util.LinkedList;
import java.util.Random;
interface Listener {
public void onChange(float f);
}
class Provider {
LinkedList<Listener> all;
public Provider() {
all = new LinkedList<Listener>();
}
public void registerChange(Listener lst) {
all.add(lst);
}
public void sendMsg() {
Random rnd = new Random();
for(Listener l : all) {
try {
l.onChange(rnd.nextFloat());
}
catch(Exception e) {
System.out.println(e);
}
}
}
}
class Inheritance implements Listener {
static public void main(String[] args) {
System.load(System.getProperty("user.dir") + "/libinheritance.so");
}
public native void onChange(float f);
}
The C Part:
#include "inheritance.h"
jint JNI_OnLoad(JavaVM *jvm, void *reserved) {
JNIEnv *env;
(*jvm)->GetEnv(jvm, (void**)&env, JNI_VERSION_1_4);
inheritance = (*env)->FindClass(env, "Inheritance");
o_inheritance = (*env)->NewObject(env, inheritance, (*env)->GetMethodID(env, inheritance, "<init>", "()V"));
provider = (*env)->FindClass(env, "Provider");
o_provider = (*env)->NewObject(env, provider, (*env)->GetMethodID(env, provider, "<init>", "()V"));
(*env)->CallVoidMethod(env, o_inheritance, (*env)->GetMethodID(env, inheritance, "onChange", "(F)V"), 1.0);
//(*env)->CallVoidMethod(env, o_provider, (*env)->GetMethodID(env, provider, "registerChange", "(LListener;)V"), o_inheritance);
//(*env)->CallVoidMethod(env, o_provider, (*env)->GetMethodID(env, provider, "sendMsg", "()V"));
(*env)->DeleteLocalRef(env, o_inheritance);
(*env)->DeleteLocalRef(env, o_provider);
return JNI_VERSION_1_4;
}
JNIEXPORT void JNICALL Java_Inheritance_onChange(JNIEnv *env, jobject self, jfloat f) {
printf("[C] %f\n", f);
}
The header file:
#include <jni.h>
/* Header for class Inheritance */
#ifndef _Included_Inheritance
#define _Included_Inheritance
#ifdef __cplusplus
extern "C" {
#endif
jclass inheritance, provider;
jobject o_inheritance, o_provider;
/*
* Class: Inheritance
* Method: onChange
* Signature: (F)V
*/
JNIEXPORT void JNICALL Java_Inheritance_onChange(JNIEnv *, jobject, jfloat);
jint JNI_OnLoad(JavaVM *, void *);
#ifdef __cplusplus
}
#endif
#endif
Compilation:
gcc -c -fPIC -I /usr/lib/jvm/java-6-openjdk/include -I /usr/lib/jvm/java-6-openjdk/include/linux/inheritance.c inheritance.h
gcc -g -o -shared libinheritance.so -shared -Wl,-soname,libinheritance.so -lc inheritance.o
