How Relationships are Represented
One of the most exciting aspects of the EJB 2.0 persistence specification is the comprehensive support
for relationships. Relationships are between two or more entity beans that use EJB 2.0 container
managed persistence. They can be unidirectional:
or bi-directional:
(can be navigated in one or both directions), and can have a multiplicity of one-to-one:
one-to-many:
or many-to-many:
The target of a relationship must be represented by a local interface, although the source of a
unidirectional relationship does not absolutely need a local interface.
Relationships are used by the EJB component developer in the following three ways:
-
Accessing objects and collections of objects that are associated, based on the business logic of
the application
For instance, there might be a Collection class that represents the line items associated with
a particular order. The related objects and collections of objects are retrieved by using abstract
accessor methods, as described earlier. For instance, the Order object would have a
getLineItems() method and a setLineItems() method that had a java.util.Set or
java.util.Collection as a return value or parameter.
-
Navigating between associated objects using the new EJB query language syntax
A query can declaratively traverse the relationships of objects defined in an EJB JAR file. For
instance, a query might retrieve only those orders that have a line item for the product
"Spam", without knowing anything about the structure of the underlying database.
-
Managing the lifespan of entity objects
An entity object in a relationship can be cascade-deleted when its containing object is deleted.
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