Persistence Fields
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 implement the
java.io.Serializable
interface. Most deployment tools easily
handle java.lang.String
,
java.util.Date
, and the primitive wrappers
(Byte
, Boolean
,
Short
, Integer
,
Long
, Double
, and
Float
), because these types of objects are part of
the Java core and map naturally to database fields.
The CustomerEJB
declares three serializable
fields, id
, lastName
, and
firstName
, which map naturally to the
INT
and 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 necessaryâthey 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 value of a serializable object must be updated
using the set<field-name>
method.
The primitive types (byte
,
short
, int
,
long
, double
,
float
, and boolean ...
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