code: public static Logger getLogger(String key) {
          FailsafeLogger logger = LOGGERS.get(key);
          if (logger == null) {
              LOGGERS.putIfAbsent(key, new FailsafeLogger(LOGGER_ADAPTER.getLogger(key)));
              logger = LOGGERS.get(key);
          }
          return logger;
      }
docs: 获取日志输出器
  
  @param key 分类键
  @return 日志输出器, 后验条件: 不返回null.