@Override
  public List<InetSocketAddress> getOnlineLocators() {
    if (PoolImpl.TEST_DURABLE_IS_NET_DOWN) {
      return Collections.emptyList();
    }
    return Collections.unmodifiableList(new ArrayList<>(onlineLocators.get().getLocators()));
  }
key is the InetSocketAddress of the locator. value will be an exception if we have already
found the locator to be dead. value will be null if we last saw it alive.
geode-core/src/main/java/org/apache/geode/cache/client/internal/AutoConnectionSourceImpl.java