Java Reference
In-Depth Information
HIBERNATE
Despite their eventual unpopularity, entity beans did much to popularize
Object-
Relational Mapping (
ORM
)
. The
ORM
ideas were picked up by a range of lightweight
alternatives to
EJB
s. The dominant competitor was an open source product called
Hibernate
. Hibernate uses an
XML
file (or, in later versions, annotations) to define
the mapping between data objects and database content. The Hibernate framework
does all the
JDBC
work so that the application doesn't have to.
JPA
The
Java Persistence
API
(
JPA
)
is a Java
EE
5 specification that's a natural evolution of
Hibernate and another
ORM
technology known as
JDO
. Many Hibernate ideas were
incorporated into
JPA
, and more recent versions of Hibernate implement the
JPA
spec-
ification (as do many other persistence providers such as EclipseLink and Open
JPA
).
The standardization of
JPA
allows different persistence providers to be used without
rewriting applications.
Although
JPA
(and its precursors, like Hibernate) is now the persistence program-
ming model of choice,
JDBC
hasn't gone away.
JPA
implementations are all built inter-
nally on
JDBC
(figure 3.1).
Although it's a Java
EE
specification,
JPA
also works well in a conventional Java envi-
ronment. Unfortunately, until recently, neither
JDBC
nor
JPA
worked well in an
OSG
i
environment. This was a shame, because these technologies had some limitations that
could be fixed by
OSG
i. Although
JPA
gives good vendor-independence, because
there's no support for dynamic replacement,
JPA
providers can't be swapped without
restarts. Similarly,
JDBC
drivers can't be removed from a running system.
3.1.2
The problems with traditional persistence in OSGi
Why don't
JDBC
and
JPA
work well as designed in an
OSG
i environment? After all,
OSG
i is Java with a fancy classpath.
JPA
JDBC
Database
Figure 3.1
JDBC provides a useful Java-language access layer on top of a raw database. JPA and other
ORM frameworks add support for transparently moving between database entries and Java objects.
