code: @SuppressWarnings("unchecked")
      public static <E> ServiceRegistry<E> getInstance() {
          if (instance == null) {
              synchronized (ServiceRegistry.class) {
                  if (instance == null) {
                      instance = new ServiceRegistry<E>();
                  }
              }
          }
          return instance;
      }
docs: 获取服务注册对象
  
  @return 服务注册信息保存者