I notice that the method org.apache.dubbo.registry.support.AbstractRegistry#getRegistered just returns the internal reference of registered, and everyone can modify this Set outside the Registry, is this a good practice?

An example is that in org.apache.dubbo.registry.support.AbstractRegistryTest#testRegister, there is something like:

//test multiple urls
abstractRegistry.getRegistered().clear();

we clear the registered urls outside the Registry, maybe it's not a big deal because we just do some logs and remove the url in unregister method:

    @Override
    public void unregister(URL url) {
        if (url == null) {
            throw new IllegalArgumentException("unregister url == null");
        }
        if (logger.isInfoEnabled()) {
            logger.info("Unregister: " + url);
        }
        registered.remove(url);
    }

but still, it'll lose some logs and if we add more logics in the unregister method in the future, it could be dangerous.

I suggest returning a unmodifiable set in the getRegistered method and unregister all urls one by one or providing a method such as unregisterAll.

There is the same problem with method getSubscribed, getNotified, etc.

What do you think?