How to structure a Java EE system? How is the term application and thus the content of an EAR defined?

Question

I am in the process of designing a build system and the directory structure for a large software system developed with JEE 5. The build system will be implemented using ant.

We have multiple different services, which are grouped thematically. Each service offers either a web service or EJBs. Each service runs on a dedicated application server cluster. Thus, we have multiple clusters and some of these clusters can be grouped logically by topics.

I did read generic definitions and examples, but I am still confused about the Java EE terminology:

• What is a Java EE application? And thus, what is the content of an EAR file?
• What is a Java EE Project? (the term is used by Netbeans as well as in the Java Blueprints Guidelines Project Conventions for Enterprise Applications)

Do I have to put all EJB and WAR-module-package-files into one single EAR, so that this single EAR contains our complete system?

Or do I put each group of services into one EAR, despite the fact that these services are only grouped logically but not technically?

Or do I assemble a separate EAR for each service, i.e. most often only containing a single EJB jar file and sometimes and EJB and a WAR file?

Or do I dismiss the concept of applications and merely build EJB and WAR files, so that I have exactly one deployment file for each application server cluster?

I guess, my main question is: What are the advantages of packaging EAR files?

As I see it at the moment, there is only the need for EAR-EJB and WAR files and additionally the concept of nested subproject in the ant-build-system and the directory structure of our source?

Edit: Thanks a lot for the answers! It seems to me that an application packaged into an ear is a rather atomic subsystem. So I guess, that I will have a nested subproject-structure (only logical, visible only to the build system and in the directory structure of the source) and a rather large amount of EARs, each of those containing mostly only one ejb-jar and/or war module and implementing a single service (which is deployed on a single application server cluster).

Answer 1

Lots of questions here.

Java EE projects are either EAR or WAR deployments that use Java EE technology. If you have a WAR with JSPs and JDBC access of a relational database, that's a Java EE project. The original intent was that EAR files were "enterprise", and that meant EJBs. An EAR file an contain EJBs, WARs, JARs, the whole enchilada.

Thinking in terms of services are a little different. I think deployment deserves careful consideration, because components that are packaged together must be brought down and up together if any maintenance has to be done.

So think carefully about how you package your services. It's not an all or none blanket answer, IMO. You should look at what your services are doing and how they might be used together to decide how they should be packaged and deployed.

Answer 2

I think what you decide to put in each EAR is governed by organizational and technical issues.

I think most important technical role of an EAR is a classloader root in a runtime environment. This normally means you can deploy different versions of libraries and your own classes in different EAR's. This means you should keep your container root classpath fairly empty (or as supplied by the container vendor), because it may allow one phsyical container to service multiple applications using possibly conflicting libraries. This is great news if you're developing a number of different applications using a common code-base. You can have a number of projects deploying to the same farm of servers without messing up for eachother.

You will normally never deploy a smaller unit of software than the EAR. Each EAR can be more or less fully self-contained. I would normally let the content of these refelect what the owning organization thinks of as applications or subsystems. You can usually package the same components in multiple EAR's.