java.lang.IllegalArgumentException: DefaultSerializer requires a Serializable payload but received

要緩存的 Java 對象必須實現 Serializable 接口，因為 Spring 會將對象先序列化再存入 Redis，比如的com.defonds.bdp.city.bean.City 類，
如果不實現 Serializable 的話將會遇到類似這種錯誤：
nested exception is java.lang.IllegalArgumentException: DefaultSerializer requires a Serializable payload but received an object of type [com.defonds.bdp.city.bean.City]]。