code: public ApplicationInstance getInstance(String nodeId) throws ApplicationInstanceOfflineException {
          if (ApplicationDiscovery.singleton == null)
              throw new RuntimeException("Service discovery is disabled! No service discovery plugin is provided.");
          ApplicationInstance instance = ApplicationDiscovery.singleton.instance(nodeId);
          if (instance == null)
              throw new ApplicationInstanceOfflineException(nodeId);
          return instance;
      }
docs: 获取指定id的节点；本方法扫描本地内存缓存数据，支持适当高频访问.