面试题与答案

• Entity manager javax.persistence.EntityManager provides the operations from and to the database, e.g. find objects, persists them, remove objects from the database, etc.
• Entities which are managed by an EntityManager will automatically propagate these changes to the database (if this happens within a commit statement). These objects are known as persistent object. 
• If the Entity Manager is closed (via close()) then the managed entities are in a detached state. These are known as the detached objects. If you want synchronize them again with the database, the a Entity Manager provides the merge() method. Once merged, the object(s) becomes perstent objects again.
• The EntityManager is the API of the persistence context, and an EntityManager can be injected directly in to a DAO without requiring a JPA Template. The Spring Container is capable of acting as a JPA container and of injecting the EntityManager by honoring the @PersistenceContext (both as field-level and a method-level annotation).

• Entities may use persistent fields, persistent properties, or a combination of both.
• If the mapping annotations are applied to the entity's instance variables, the entity uses persistent fields.
• If the mapping annotations are applied to the entity's getter methods for JavaBeans-style properties, the entity uses persistent properties.

Valid attribute types for Entity classes are:

1. primitive types and their Java wrappers,
2. strings,
3. any Java serializable types,
4. enums;
5. entity types;
6. embeddable classes
7. collection types