Howto: Callback from C++ to Java, initiated from the Java side (not C++ to JVM function call)?

[vbag...@gmail.com]

Hello,

I am new to JavaCPP and trying to do a do a callback from C++ to Java. 

I have seen the example in the JavaCPP readme on creating callback functions, but if I understand correctly, it shows how to call a Java function from C++, whereby the JVM is initiated from C++,  using JavaCPP_init(). 

I would like to implement the callback function in Java, and I to set up everything from the Java side, while the callback is executed from the C++ side.

Here's how my Java calling code could look like:

Caller caller = new Caller();

Callback callback = new JavaCallback();

 

caller.setCallback(callback);

caller.call(); // calls C++ which does the callback in Java

where JavaCallback is a Java class with my implementation of the callback:

class JavaCallback extends Callback {

    @Override

    public void run() {

        System.out.println("this is my java-side callback");

    }

}

And the C++ side that executes the callback would look like this:

// C++

class Callback {
public:
	virtual void run() { ... }
};


class Caller {
private:
	Callback *_callback;
public:
	Caller(): _callback(0) {}
	void setCallback(Callback *cb) { _callback = cb; }
	void call() { _callback->run(); }
};

The above code is taken from this SWIG callback example.

Your help is very much appreciated!

Best regards,
Vito

[Samuel Audet]

The example in the README.md file isn't just for when calling explicitly from C++, it works the same when passing pointers to a native library.

In your case, with new Info("Callback").virtualize() the Parser outputs something like the following:

public static class Callback extends Pointer {
    public Callback() { allocate(); }
    private native void allocate();
    @Virtual public native void run();
}

public static class Caller extends Pointer {
    public Caller() { allocate(); }
    private native void allocate();
    public native void setCallback(Callback cb);
    public native void call();
}

And we can use those peer classes pretty much in the way you describe.

Samuel

[vbag...@gmail.com]

Dear Samuel,

many thanks for your reply - I'll try that out over the weekend!

Best regards,

Vito