code: @Override
      public Set<ProviderInfo> currentProviderList() {
          providerLock.lock();
          try {
              ConcurrentHashSet<ProviderInfo> providerInfos = new ConcurrentHashSet<ProviderInfo>();
              providerInfos.addAll(aliveConnections.keySet());
              providerInfos.addAll(subHealthConnections.keySet());
              providerInfos.addAll(retryConnections.keySet());
              return providerInfos;
          } finally {
              providerLock.unlock();
          }
      }
docs: 获取当前的Provider列表（包括连上和没连上的）
  
  @return 当前的Provider列表 set