docs: public String getKey() {
          String dbkey = ThreadContext.getContext(dbName);
          if (dbkey == null) {
              Integer userid = ThreadContext.getShardKey();
              if (userid == null) {
                  userid = 0;
                  // throw new
                  // RuntimeException("No userid contexted,so you can't know how to routing!!");
              }
              dbkey = router.getTargetDataSourceKey(userid, true);
  
              StackTraceElement[] elements = Thread.currentThread().getStackTrace();
  
              logTrace(dbkey, elements);
  
          }
          return dbkey;
      }