如何理解Ribbon中的ServerList

public interface ServerList<T extends Server> {

    public List<T> getInitialListOfServers();
    
    /**
     * Return updated list of servers. This is called say every 30 secs
     * (configurable) by the Loadbalancer's Ping cycle
     * 
     */
    public List<T> getUpdatedListOfServers();   

}

public class StaticServerList<T extends Server> implements ServerList<T> {

   private final List<T> servers;

   public StaticServerList(T... servers) {
      this.servers = Arrays.asList(servers);
   }

   @Override
   public List<T> getInitialListOfServers() {
      return servers;
   }

   @Override
   public List<T> getUpdatedListOfServers() {
      return servers;
   }
}

public abstract class AbstractServerList<T extends Server> implements ServerList<T>, IClientConfigAware {   
       
    /**
     * Get a ServerListFilter instance. It uses {@link ClientFactory#instantiateInstanceWithClientConfig(String, IClientConfig)}
     * which in turn uses reflection to initialize the filter instance. 
     * The filter class name is determined by the value of {@link CommonClientConfigKey#NIWSServerListFilterClassName}
     * in the {@link IClientConfig}. The default implementation is {@link ZoneAffinityServerListFilter}.
     */
    public AbstractServerListFilter<T> getFilterImpl(IClientConfig niwsClientConfig) throws ClientException{
        try {
            String niwsServerListFilterClassName = niwsClientConfig
                    .getProperty(
                            CommonClientConfigKey.NIWSServerListFilterClassName,
                            ZoneAffinityServerListFilter.class.getName())
                    .toString();

            AbstractServerListFilter<T> abstractNIWSServerListFilter = 
                    (AbstractServerListFilter<T>) ClientFactory.instantiateInstanceWithClientConfig(niwsServerListFilterClassName, niwsClientConfig);
            return abstractNIWSServerListFilter;
        } catch (Throwable e) {
            throw new ClientException(
                    ClientException.ErrorType.CONFIGURATION,
                    "Unable to get an instance of CommonClientConfigKey.NIWSServerListFilterClassName. Configured class:"
                            + niwsClientConfig
                                    .getProperty(CommonClientConfigKey.NIWSServerListFilterClassName), e);
        }
    }
}

public class ConfigurationBasedServerList extends AbstractServerList<Server>  {

   private IClientConfig clientConfig;
      
   @Override
   public List<Server> getInitialListOfServers() {
       return getUpdatedListOfServers();
   }

   @Override
   public List<Server> getUpdatedListOfServers() {
        String listOfServers = clientConfig.get(CommonClientConfigKey.ListOfServers);
        return derive(listOfServers);
   }

   @Override
   public void initWithNiwsConfig(IClientConfig clientConfig) {
       this.clientConfig = clientConfig;
   }
   
   protected List<Server> derive(String value) {
       List<Server> list = Lists.newArrayList();
      if (!Strings.isNullOrEmpty(value)) {
         for (String s: value.split(",")) {
            list.add(new Server(s.trim()));
         }
      }
        return list;
   }
}

public class DiscoveryEnabledNIWSServerList extends AbstractServerList<DiscoveryEnabledServer>{

    @Override
    public List<DiscoveryEnabledServer> getInitialListOfServers(){
        return obtainServersViaDiscovery();
    }
    
    @Override
    public List<DiscoveryEnabledServer> getUpdatedListOfServers(){
        return obtainServersViaDiscovery();
    }
    
    private List<DiscoveryEnabledServer> obtainServersViaDiscovery() {
        List<DiscoveryEnabledServer> serverList = new ArrayList<DiscoveryEnabledServer>();
    
        if (eurekaClientProvider == null || eurekaClientProvider.get() == null) {
            logger.warn("EurekaClient has not been initialized yet, returning an empty list");
            return new ArrayList<DiscoveryEnabledServer>();
        }
    
        EurekaClient eurekaClient = eurekaClientProvider.get();
        if (vipAddresses!=null){
            for (String vipAddress : vipAddresses.split(",")) {
                // if targetRegion is null, it will be interpreted as the same region of client
                List<InstanceInfo> listOfInstanceInfo = eurekaClient.getInstancesByVipAddress(vipAddress, isSecure, targetRegion);
                for (InstanceInfo ii : listOfInstanceInfo) {
                    if (ii.getStatus().equals(InstanceStatus.UP)) {
    
                        if(shouldUseOverridePort){
                            if(logger.isDebugEnabled()){
                                logger.debug("Overriding port on client name: " + clientName + " to " + overridePort);
                            }
    
                            // copy is necessary since the InstanceInfo builder just uses the original reference,
                            // and we don't want to corrupt the global eureka copy of the object which may be
                            // used by other clients in our system
                            InstanceInfo copy = new InstanceInfo(ii);
    
                            if(isSecure){
                                ii = new InstanceInfo.Builder(copy).setSecurePort(overridePort).build();
                            }else{
                                ii = new InstanceInfo.Builder(copy).setPort(overridePort).build();
                            }
                        }
    
                        DiscoveryEnabledServer des = new DiscoveryEnabledServer(ii, isSecure, shouldUseIpAddr);
                        des.setZone(DiscoveryClient.getZone(ii));
                        serverList.add(des);
                    }
                }
                if (serverList.size()>0 && prioritizeVipAddressBasedServers){
                    break; // if the current vipAddress has servers, we dont use subsequent vipAddress based servers
                }
            }
        }
        return serverList;
    }


}