Overview of persisting a domain model - POJOs in Action: Developing Enterprise Applications with Lightweight Frameworks - page 121

Spring framework provides AOP interceptors that call the transaction manage-
ment API s on behalf of the application.
In addition, JDO and Hibernate provide JTA integration. A Hibernate Session
or JDO PersistenceManager that is opened within a JTA transaction will automati-
cally participate in the transaction.

4.3.5 Lazy and eager loading

By default, JDO and Hibernate load objects lazily, but you can configure them to
load objects eagerly. The details differ by framework but they provide roughly
equivalent functionality.

Configuring eager loading in JDO
You configure eager loading in JDO by using fetch groups. A JDO fetch group
specifies a graph of interconnected objects and is defined in the XML mapping
document. When an application loads an object or executes a query, it can iden-
tify the objects to eagerly load by specifying one or more active fetch groups.
You can use fetch groups to configure eager loading in one of two ways:

Statically configure a relationship to be always loaded.

■

Activate particular fetch groups at runtime to dynamic control which rela-
tionships to eagerly load.

■

For more information on JDO fetch groups, see chapter 5.

Configuring eager loading in Hibernate
Hibernate provides two ways to configure eager loading. You can specify in the
O/R mapping which relationships should be eagerly loaded. This approach is
useful when a relationship is always traversed by the application. Alternatively,
you can use a Hibernate query with what is termed a fetch join to specify which
objects should be eagerly loaded by a query. By using different queries at
runtime, you can dynamically control which objects are eagerly loaded when
handling a request. See chapter 6 for more information about Hibernate and
eager loading.

4.3.6 Object caching

Hibernate and JDO cache objects, albeit with some minor differences. By default,
Hibernate caches objects in the Session for the lifetime of the session, which can
span multiple transactions, and JDO caches objects in the PersistenceManager for
the duration of a transaction. However, this difference is not usually that important