void invoke(Request req) {
        try {
            handler.invoke(req);
        } catch (Exception e) {
            req.setError(ErrorCode.METHOD_FAILED, e.toString());
        }
    }
Invoke this method. The given request holds the parameters and
will be given as parameter to the handler method.

@param req the request causing this invocation
jrt/src/com/yahoo/jrt/Method.java