private T get() {
      if (!valid) {
        value = supplier.get();
        valid = true;
      }
      return value;
    }
Return the cached value, recomputing it if the cache is invalid.

@return the current value, recomputed if necessary
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/subscription/agent/SubscriptionBrokerAgent.java