protected Class getMethodArgumentType(Method method) {
        Class[] paramTypes = method.getParameterTypes();
        if (paramTypes.length != 1) {
            //the default implementation expects a single typed argument and nothing more:
            String msg = "Event handler methods must accept a single argument.";
            throw new IllegalArgumentException(msg);
        }
        return paramTypes[0];
    }
A event listener that invokes a target object's method that accepts a single event argument.

@since 1.3
event/src/main/java/org/apache/shiro/event/support/SingleArgumentMethodEventListener.java