@Override
    public void setListener(ResolverListener<DiscoveryResult> listener) {
        if (this.listener != null) {
            LOG.warn("Ignoring call to setListener, because a listener was already set");
            return;
        }

        this.listener = Objects.requireNonNull(listener);
        this.loadBalancer.addServerListChangeListener(this::onUpdate);
    }
@author Argha C
@since 2/25/21
<p>
Implements a resolver, wrapping a ribbon load-balancer.
zuul-discovery/src/main/java/com/netflix/zuul/discovery/DynamicServerResolver.java