dto对象转json报错，是什么原因？

HTTP Status 500 - Could not write content: No serializer found for class org.seckill.dto.Exposer and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS) ) (through reference chain: org.seckill.dto.SeckillResult["data"]); nested exception is com.fasterxml.jackson.databind.JsonMappingException: No serializer found for class org.seckill.dto.Exposer and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS) ) (through reference chain: org.seckill.dto.SeckillResult["data"])