读hibernate in action中发现了一段关于持久层的阐述，有些疑惑。Persistence layer―The persistence layer is a group of classes and components
responsible for data storage to, and retrieval from, one or more data stores.
This layer necessarily includes a model of the business domain entities
(even if it’s only a metadata model).
疑惑在于域模型和域实体这两个概念上，实体似乎只是一个数据表达（metadata），不知道理解对否
