Container-managed persistence (CMP) fields are virtual fields whose values map directly to the database. Persistence fields can be Java serializable types and Java primitive types.
Java serializable types can be any class that implements the
interface. Most deployment tools handle
java.util.Date, and the primitive wrappers
Float) easily, because these types of objects are
part of the Java core and map naturally to fields in relational and
CustomerEJB declares three serializable
firstName, which map naturally to the
CHAR fields of the
CUSTOMER table in the database.
You can also define your own serializable types, called dependent value classes , and declare them as CMP fields. However, I recommend that you do not use custom serializable objects as persistence field types unless it is absolutely necessary—they are usually recommended for unstructured types, such as multimedia data (images, blobs, etc.). Arbitrary dependent value classes usually will not map naturally to database types, so they must be stored in their serializable forms in some type of binary database field.
Serializable objects are always returned as copies and not
references, so modifying a serializable object will not impact its
database value. The entire value must be updated using the abstract
The primitive types (