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Using container-managed persistence with jBoss
By Kevin Boone (k.boone@web-tomorrow.com)
What this article is about
his article presensts a step-by-step example of
creating, deploying and testing an Entity JavaBean that uses `container-managed
persistence' with the jBoss EJB server. This example is very simple, and is a
variation of the old favourite `music CD' case study. I have chosen this
example because the application is likely to be familiar to most people without
much explanation. In short, the `CD' EJB models a music CD. Applications will
want to add and delete CDs to a collection, and search the collection for
specific CDs. There is also a `CDCollection' EJB that lists and searches
collections of CDs.
The full source code to accompany this article can be found
here (in gzipped tar format). This archive contains
the Java source code, JavaDoc documentation, and a text file of test data
(CDs.txt) to initialize the database.
Although I will occasionally make reference to Linux, this is only because
that's what I use for EJB development; most of the material in this article
will work with any Unix version, and even with Windows NT if the directory
structure is adjusted accordingly.
Pre-requisites
You will need a fully-functioning jBoss installation to follow this tutorial,
which must have a working database connection. I will assume that you are
basically familiar with EJBs
and know how to structure and compile JavaBeans as Java packages. Also I will
assume you know the basic structure of a deployment descriptor, and the
run-time descriptor that jBoss uses (jboss.xml). I will assume that you know
how to package and deploy EJBs using jBoss. If you don't know about these
things, you might want to look at my introductory article
on jBoss first.
jBoss hints
jBoss is written entirely in Java, as is the `Hypersonic' database with which
it is supplied. Searching a database requires lots of repetitive operations,
and an interpreting Java system will be extremely slow if the database is
large. If you are using the Sun JDK, you will need to ensure that it is
configured to use the `Hotspot' virtual machine, to achieve anything close to
acceptable performance. For the purposes of study, you could get along by
ensuring that the database size is kept small, but with even a hundred objects
you will find it too slow to use. Some Linux systems are reluctant to use the
`Hotspot' JVM, but they can normally be coerced to.
Persistence: review
There are, in essence, two kinds of Enterprise JavaBean: session and entity.
Entity EJBs contain information that is persistent across different client-Bean
interactions, while session EJBs don't. For example, a class called `Customer'
that represents the customers of a particular service will contain persistent
information (about the customer). A class called `CustomerFinder', say, that
finds Customer instances that match certain criteria is likely to be a session
EJB, because it does not require information that is maintained between
different client-server interactions.
Session EJBs can be further divided into `stateless' and `stateful'. A stateful
session EJB has a state that is persistent between invocations of its methods.
A stateless EJB does not even have to retain its state between method
invocations. The stateless session EJB is therefore the type of EJB that
exhibits the least persistence.
The entity EJBs contain information that persists even when no clients are
using any of the Bean's services; the information should persist even if the
server is restarted. There is a high degree of correspondence between instances
of an entity EJB and rows of a database table. In practice, all EJB servers
implement entity instances as table rows. This correspondence is so strong that
the notion of a `primary key' is relevant to an entity
EJB. Of course, a primary key is a database concept, not an object-orientation
concept at all.
The persistence of an entity EJB may be managed by the Bean itself, or by the
server (technically by the `container'). The latter technique is called
`Container-managed persistence', and is the subject of the rest of this
article.
Begin tutorial...
After the tutorial, you might like to read the
discussion
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