an entity is usually a rather fine-grained business object corresponding to a DB table, ofet having relationships to other entities and associated with some metainformation, in addition to otherwise being a POJO.
Characteristics:
- persistability = unique identity
- (transactionality)
- granularity
2 annotations are needed to turn a POJO with a no-arg ctor to a JPA entity @Entity and @Id
The default table name is the entity name The default column is the name of the property
Persistence context - set of managed entity instances
There is a one-to-one corresponence between a persistence unit and it's EntityManangerFactory. A persistence Unit can have multiple contexts, a context is tied to a single unit. an EMF can produce many EMs, each EM can have a single PC, a PC can be attached to many EMs - see p.23
- creating EMs - p23
- persisting, finding - p.24
em.find(Empoyee.class, 158) - if the object was not found, em returns null
removing entities is not very common
java emp = em.find(Emp.class, 123); if (emp!= null) em.remove(emp)
using user transactions outside of the container - p27
don't forget to close the em AND the emf when working without a container