Persisting a domain model with JDO 2.0 - POJOs in Action: Developing Enterprise Applications with Lightweight Frameworks - page 150

Now that you have gotten an overview of ORM framework concepts in JDO and
Hibernate, let's look at how you can persist a domain model with JDO 2.0 . As we
saw in chapter 4, JDO provides mostly transparent persistence for POJO s. Most
classes are unaware that they are persistent, and JDO supports the natural Java
programming style, including inheritance and recursive calls between objects. In
addition, because JDO can be used both inside and outside an application server,
you can develop and test business logic without having to first deploy it, which
accelerates development.
Of course, no technology is perfect; JDO has various shortcomings that must be
accommodated when you're developing an application. In this chapter we describe
the decisions you must make and the workarounds you must use when developing
with JDO . You will learn how to persist domain objects with JDO and implement
domain model repositories using Spring and JDO . This chapter also explains how
to optimize performance in a JDO application by using eager loading and caching.
We will use the Food to Go domain model developed in chapter 3 as an example.
For more detailed information about JDO , see Java Data Objects [Russell 2003].
5.1 JDO issues and limitations
JDO is one of those technologies that has never received a huge amount of atten-
tion and hype. Although it was the first standard for Java transparent persistence,
it has always has been overshadowed by Hibernate. And, to make matters worse,
politics led the EJB 3 expert group ( JSR-220 ) to define a totally new standard for Java
transparent persistence instead of using JDO . But JDO is an excellent ORM technol-
ogy that has multiple commercial and open source implementations. The trend is
for those implementations to also support EJB 3 , and as developers inevitably run
into EJB 3' s limitations (see chapter 10), it's likely they will find their way to JDO .
JDO provides a flexible ORM mechanism that makes it straightforward to per-
sist a typical domain model. It supports all of the ORM features I described in
chapter 4, including relationships, embedded value objects, and inheritance. You
rarely need to make significant changes to your domain model in order to persist
it unless, of course, you are working with a legacy schema that has some quirky
features (such as denormalized columns). Making a domain model persistent is
usually only a matter of writing the XML metadata that defines how it maps to the
database schema and perhaps adding ID fields to classes.
You must, however, decide which kind of JDO object identity to use for each of
the classes in the domain model. In addition, you need to work around a limita-
tion of how interfaces are mapped to the database.