Question

I wondered if Java7’s new invokedynamic bytecode instruction could be used to implement multiple dispatch for the Java language. Would the new API under java.lang.invoke be helpful to perform such a thing?

The scenario I was thinking about looked as follows. (This looks like an application case for the visitor design pattern, but there may be reasons that this is not a viable option.)

class A {} 
class A1 extends A {}
class A2 extends A {}

class SomeHandler {
    private void doHandle(A1 a1) { ... }
    private void doHandle(A2 a2) { ... }
    private void doHandle(A a) { ... }

    public void handle(A a) {
        MultipleDispatch.call(this, "doHandle", a);
    }
}

The library class MultipleDispatch would then do something of the kind:

class MultipleDispatch {

    public static Object call(Object receiver, String method, Object...arg) {
        // something like that in byte code
        #invokeDynamic "doHandle" "someBootstrap"
    }

    static CallSite someBootstrap {
        // resolve that dynamic method call.
    }
}

(I am aware of MultiJava, but can this be achieved in a Java-pure fashion?)

Answer 1

Since I have no experience with invokedynamic, I do not know how good the performance and type-safety would be, but can only give some pointers:

• the Da Vinci Machine Project offers multiple dispatch via invokedynamic (see Multiple Dispatch/src/invokedynamicmultipledispatch/);
• Charles Oliver Natter has talked about applications of invokedynamic on JAX2012. The slides do not go into details at all, but I think I came across more details in a video or podcast talking about JAX2012, which I cannot find right now.
• Christopher Dutchyn’s JVM Multiple Dispatch is an alternative approach without invokedynamic. His COOTS ’01 Paper and these slides have a lot of performance information/benchmarking (and are a good read after you’ve finished your disseration 😉