Saturday, February 03, 2007

EJB 3.0 Outside the Container, Inside the JVM - Part 2: Configuring JPA Declaratively

Welcome to part two of this blog entry introducing using EJB3.0 inside the JVM. Click here to view the first part.

As we've seen, JPA can use our annotated POJO model to create a database for us, but first we need to give it some information on how to do it. We provide this information in a file called persistence.xml. Let's create a blank file with this name in {PROJECT_HOME_DIR}/expenses-tracker-model/
We also need to let maven know where this is so we add the following to our project.xml file:

This entry ensures that this file will be included on the build classpath.

Now lets add the content to persistence.xml. We'll start at the top:

The persistence-unit tag tells JPA about our (future) database. A single persistence.xml file can have mulitple persistence units but we will keep things simple and just stick to one. Attributes to this tag provide the persistence unit name (which we'll use to refer to it later in code) and the transaction-type. (We have the default). We'll only need the former as we'll see later on.

Enclosed within this top level is the tag which tells JPA which implementation will provide the actual persistence functionality - the "provider" tag. Our example uses the Toplink ORM tool but we could swap this to something like Hibernate simply by changing the contents of this tag.

Finally there is the tag where we declare our new entity class - "class". This tells JPA that we want it to be considered as part of this persistence unit. (Note: if you find JPA doing things you don't expect, check that you have listed all the classes you wish it to be aware of here. It can't work with what it has no idea about!)

Ignore the commented out tags. If you really want to know what they do then just google or yahoo! them.

This is all well and good but clearly my file is a little more complicated. Toplink will need some additional information in order to do its job and we can put this in this file as well. These go within the "properties" tags. You can see below that we use these to tell Toplink the driver class to use, the url for the database connection and well get to the third piece of magic later.

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