Public Review version of the Java Persistence API (JPA) spec suggested a new extension par be used to bundle EJB 3 persistence entity beans. It also suggested a new module type called persistence to be used in application.xml, if user uses an application.xml in their ear file containing persistence entity beans. (Note application.xml is optional starting with Java EE 5.)

Lot of developers still believe that .par is needed. Looking at the comments in theserverside.com to an earlier posted tutorial showing how to use JPA in web application it is very clear that user community do not want a proliferation of extensions used in Java EE platform. Here is a good news:
.par is not needed any more -- take a look at latest XML schema of application.xml posted in project glassfish web site. There is no mention of persistence module type.

Here is a simple convention (rule you may say) followed in Java EE platform from early days:
Any jar-format file that packages classes that can be directly loaded by a class loader ends with a .jar extension. e.g. jar files containing EJB classes, application client classes, utility classes all end with .jar extension (not even .zip). This is precisely why, bundled optional libraries must only be .jar files as mentioned in chapter #8 of the Java EE 5 platform spec available here .

As you can see .ear, .war and .rar do not fall into this category, as they do not package classes directly inside them which are used by a standard class loader. Note that although .war sometimes directly contains classes that are used by an applet, but the applet container is not given the .war file, instead it downloads the classes from the web server.

I must say having this convention prevented introduction of .par file and it is a relief. This kind of convention is also getting into Java SE, because I have heard in Java SE 6 (mustang) there is going to be a facility to specify a directory in the CLASSPATH environment variable and VM will look for all *.jar files in that directory and add it to CLASSPATH automatically.

So if you want to write portable applications, stick to these principles. Sun's next version of application server which is being develped in open sorce project named glassfish does not require any such extensions like .ejb3 or .par.

As a result of this not requiring .par files, the most notable simplification is:
You don't always need an .ear file to use EJB 3 persistece entity beans in Java EE platform. You can bundle them in .war files and deploy .war directly if you are developing such an app. e.g. see tutorial showing how to use JPA in web application. You could even take such a war file and use it in a Java web server, if that supports JPA. You can bundle the entity classes along with ejb classes in an ejb-jar and deploy the ejb-jar directly.

Similarly many developers think EJB 3 session beans have to be packaged in a jar file with extension .ejb3. Let me tell you, that was JBoss specific. I am not up-to-date with recent developments in JBoss, so not sure if they still require this and what are their plans.More blogs about glassfish persistence.