code: public static void destroy() {
          if (Mongo.DEFAULT_DATABASE != null) {
              synchronized (LOCK) {
                  if (Mongo.DEFAULT_DATABASE != null) {
                      //销毁客户端，并将引用置空
                      Mongo.DEFAULT_CLIENT.close();
                      Mongo.DEFAULT_DATABASE = null;
                      Mongo.DEFAULT_CLIENT = null;
                  }
              }
          } else {
              LOG.warn("Mongodb client is not initialized yet, nothing is need to destroy.");
          }
      }
docs: Destroy MongoDB client if the client is initialized or else nothing will be done.
  This method is thread-safe to be used concurrently with {@link #getOrInitDefaultDatabase()} and {@link #getOrInitDefaultDatabase(String, String)}