Dynamic, reflective SignalHandler in Java

Question

How do I install signal handling logic iff sun.misc.Signal is available?

Background First generation of my code, which assumed signal handling availability, looked something like this:

class MyApp {
    public static void main(String[] args) {
        ...
        Signal.handle(term_sig, new SignalHandler() {
            public void handle(Signal sig) { ... }
        });
        ...
    }
}

I believe I understand how to reflectively test for and use signal handlers -- Class.forName("sun.misc.Signal"), reflectively call Signal.handle, and so forth.

My impulse was simply to instantiate another anonymous inner class with the dynamically obtained SignalHandler class, but I think that's just wishful syntax.

Answer 1

You need to use a Dynamic Proxy to implement the SignalHandler interface. The rest is just basic reflection.

Update

Here's how you do it. Note, I've omitted the try-catch that needs to wrap all of this

        Class<?> handlerCl = Class.forName("sun.misc.SignalHandler");
        Class<?> signalCl = Class.forName("sun.misc.Signal");

        Constructor signalCtor = signalCl.getConstructor(String.class);
        Method signalHandle = signalCl.getMethod("handle", signalCl, handlerCl);

        // Create a proxy class that implements SignalHandler
        Class<?> proxyClass = Proxy.getProxyClass(signalCl.getClassLoader(),
            handlerCl);

        // This is used by the instance of proxyClass to dispatch method calls
        InvocationHandler invHandler = new InvocationHandler()
        {
            public Object invoke(Object proxy,
                Method method, Object[] args) throws Throwable
            {
                // proxy is the SignalHandler's "this" rederence
                // method will be the handle(Signal) method
                // args[0] will be an instance of Signal
                // If you're using this object for multiple signals, you'll
                // you'll need to use the "getName" method to determine which
                // signal you have caught.
                return null;
            }
        };

        // Get the constructor and create an instance of proxyClass
        Constructor<?> proxyCtor = proxyClass.getConstructor(InvocationHandler.class);
        Object handler = proxyCtor.newInstance(invHandler);

        // Create the signal and call Signal.handle to bind handler to signal
        Object signal = signalCtor.newInstance("TERM");
        signalHandle.invoke(null, signal, handler);