Call back on JNI - java-native-interface

Question

How can I register call back on JNI from android app? My requirement is, I want to make a JNI call from Android application and want to register a call back, so that I can get call back on java application from JNI.
Thanks

Answer 1

First, check out Swig. It wraps C++ in Java and also has a "director" fcility that makes it easier to call C++ methods from Java.
With raw JINI, you cannot do this directly. Although the RegisterNatives() call can be made to bind a native method, it cannot be changed. If you want to call a C function by pointer, you will need to do this in two steps. I am glossing over a lot, because JNI is incredibly verbose and tedious. The basic trick is, wrap the C function pointer in a java long.
First declare a java class:
public class Callback {
public Callback(long cMethodPointer) { this.cMethod = cMethod; }
public void doCallback() { callCMethod(cMethod); }
public static native void callCMethod(long cMethod);
public long cMethod;
}
Run javah on that class file, and you'll get a stub header generated that looks like:
JNIEXPORT void JNICALL Java_Callback_callCMethod
(JNIEnv *, jclass, jlong);
Implement that method in a DLL/so:
typedef void (*FP)();
JNIEXPORT void JNICALL Java_Callback_callCMethod
(JNIEnv *, jclass, jlong pointer) {
((FP)(void*)pointer)();
}
and call System.loadLibrary() on that DLL/so in your Java main() method.
Finally, from your C/C++ code, you will need to create an instance of the Callback object via JNI, and pass the pointer to an actual method to it:
void MyFunction() { ... }
void Register() {
jclass cls = env->FindClass("Callback");
jmethodID mid = env->GetMethodID(cls, "<init>", "(J)V");
jvalue arg;
arg.j = (jlong)(void*)MyFunction;
jobject callback = env->NewObjectA(confCls, mid, &arg);
}
So this gives you a brand new Callback object that points to your C function! But then, how do you do anything with that? Well, you have to pass the Callback object to Java via JNI (step omitted), so that your Java code has the Callback object from which to call your C method:
public class Foo {
public Callback callback;
public void doSomeStuff() {
...
callback.doCallback();
}
}