J2EE design decisions - POJOs in Action: Developing Enterprise Applications with Lightweight Frameworks - page 43

placeholders and to construct beans from a ResultSet . It also includes support for
database-generated primary keys, automatic loading of related objects, caching,
and lazy loading. In this way, i BATIS eliminates much of the drudgery of executing
SQL statements. As you will see in several chapters, including chapter 9, i BATIS can
considerably simplify code that executes SQL statements. Instead of writing a lot of
low-level JDBC code, you write an XML descriptor file and make a few calls to i BA-
TIS API s.

2.4.3 Using a persistence framework

Of course, i BATIS cannot address the overhead of developing and maintaining
SQL or its lack of portability. To avoid those problems you need to use a persis-
tence framework. A persistence framework maps domain objects to the database.
It provides an API for creating, retrieving, and deleting objects. It automatically
loads objects from the database as the application navigates relationships between
objects and updates the database at the end of a transaction. A persistence frame-
work automatically generates SQL using the object/relational mapping, which is
typically specified by an XML document that defines how classes are mapped to
tables, how fields are mapped to columns, and how relationships are mapped to
foreign keys and join tables.
EJB 2 had its own limited form of persistence framework: entity beans. How-
ever, EJB 2 entity beans have so many deficiencies, and developing and testing
them is extremely tedious. As a result, EJB 2 entity beans should rarely be used.
What's more, as I describe in chapter 10 it is unclear how some of their deficien-
cies will be addressed by EJB 3 .
The two most popular lightweight persistence frameworks are
JDO [ JSR12 ][ JSR243 ], which is a Sun standard, and Hibernate, which is an open
source project. They both provide transparent persistence for POJO classes. You
can develop and test your business logic using POJO classes without worrying
about persistence, then map the classes to the database schema. In addition, they
both work inside and outside the application server, which simplifies development
further. Developing with Hibernate and JDO is so much more pleasurable than
with old-style EJB 2 entity beans.
Several chapters in this topic describe how to use JDO and Hibernate effectively.
In chapter 5 you will learn how to use JDO to persist a domain model. Chapter 6
looks at how to use Hibernate to persist a domain model. In chapter 11 you will
learn how to use JDO and Hibernate to efficiently query large databases and pro-
cess large result sets.