JPA 2.0 提供的解决办法通过静态地公开相同的持久化属性实例化名为 Person_的元模型类（对应于 Person）。
The solution that JPA 2.0 provides is the ability to instantiate a metamodel class named Person_ that corresponds to Person by exposing the same persistent attributes statically.
某些实例对象就是类本身，并且所有的类都是相应元类的实例，甚至类也只能通过实例化机制生成。
Some instance objects are themselves classes; and all classes are instances of a corresponding metaclass. Even classes only come into existence through the instantiation mechanism.
为了提高可读性，通过在实体类文件中直接指定的对象或关系元数据表达场景中的关系。
For better readability, the relationships in our scenario are expressed through object or relational metadata specified directly in the entity class files.
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