Question about C++ class instantiation and JNI
From what I read here
http://mindprod.com/jgloss/jni.html
and here
http://www.javaworld.com/javaworld/javatips/jw-javatip17.html
it seems that you cannot instantiate (call a constructor) a C++ class from a Java class. You need to have a C++ class that does it for you, and then the Java class maintains the pointer.
This article referenced above was written about JDK 1.0 and I cannot find any newer articles on the topic. Is this info still accurate? Is there anywhere I can find a more recent article that talks about this topic for JDK 1.6? Most of the JNI articles I've found talk straight C.
Secondly, I'm not sure if I am going to attempt to write my own JNI or use a toolkit. what is the best toolkit? I looked at SWIG but having to maintain all these extra input files seems like a waste of time to me.
Thanks!
[869 byte] By [
squiddoga] at [2007-10-3 6:07:28]
> From what I read here
> http://mindprod.com/jgloss/jni.html
> and here
> http://www.javaworld.com/javaworld/javatips/jw-javatip
> 17.html
>
> it seems that you cannot instantiate (call a
> constructor) a C++ class from a Java class. You need
> to have a C++ class that does it for you, and then
> the Java class maintains the pointer.
Yep.
>
> This article referenced above was written about JDK
> 1.0 and I cannot find any newer articles on the
> topic. Is this info still accurate? Is there
> anywhere I can find a more recent article that talks
> about this topic for JDK 1.6? Most of the JNI
> articles I've found talk straight C.
JNI hasn't changed much since 1.2. You still have to store the pointer on the Java side. I do it like this:
abstract class CppPeer {
protected long pointer;
protected CppPeer(long ptr) {
pointer = ptr;
}
public long getPtr() {
return pointer;
}
}
This is simply a placeholder for the pointer which can be returned from
the C++ code. I'm in the process of porting a C++/COM interface with
100 objects and about 2000 functions to Java and I'm using this
approach. Each C++ object has a corresponding Java object, and each C++
method has a corresponding Java method, and a native method to act as a
go-between:
public class WindowHandle extends CppPeer {
public WindowHandle(long ptr) {
super(ptr);
}
}
Anytime you need to pass the information back to the C++ side, you
simply pass the 'pointer' value:
public class HandleUser {
public void useHandle(WindowHandle wh) {
useHandle (wh.getPtr());
}
private native void useHandle(long ptr);
}
On the native side you cast to the appropriate type:
JNIEXPORT void JNICALL Java_HandleUser_useHandle(JNIEnv *env, jobject
obj,jlong ptr) {
WindowHandle *handle = reinterpret_cast<WindowHandle*>(ptr);
//do stuff with handle pointer
}
>
> Secondly, I'm not sure if I am going to attempt to
> write my own JNI or use a toolkit. what is the best
> toolkit? I looked at SWIG but having to maintain all
> these extra input files seems like a waste of time to
> me.
I haven't used any of the various toolkits. Usually the C/C++ APIs I've 'ported' are so un-object oriented that if I use a toolkit I would have to make a second layer of Java just to make the API work in a way that makes sense to Java folks.
Jim S.
