Eclipse project layout classpath issues

Question

I have a large scale project I am working on at the moment using Eclipse. Normally, as a one man team, these problems would not be an issue, but as our team is not one person we need to be able to break up pieces of the project to be worked on by certain team members.

In simplicity, let's say I have two layers to be separated apart: 1. Each DAO is a separate Java project, to be worked upon individually 2. The web-tier service layer contains all of our service endpoints and must be able to reference all of the DAOs. This layer runs on Tomcat as a dynamic web project, and utilizes Adobe LiveCycle Data Services as the piece that handles creation and management of endpoints.

Now, the issue we are running into is that when we create a DAO and unit test it individually it runs great. But when we reference it into our service project and try to run it we begin to get all kinds of issues related to the fact that we have two different versions of certain jars referenced in and as such we begin to have errors when running the server.

As a result, we know we can solve the issue by pulling the problem jars and ensuring that this is not an issue again in the future, but as I said before this is a large scale project with multiple people working on it and we don't want to be spending our time weeding out dependency issues when under the gun.

We are looking for recommendations on where to proceed for alternative solutions? Our team is new to JavaEE and as such we don't have much of a bearing on what we can use to tie everything together in it, or if it is a viable solution. Should we be looking at turning our DAOs into EJBs and deploying them in an EAR library? If so, where would our service layer lie, and would the service layer be able to reference the DAO classes since the EJB maintains it's own classpath (from what we have read?) Are we looking down the wrong path, or are we completely wrong in our current understanding of JavaEE?

Any assistance is greatly appreciated. We are still in the framework stage of this project and we want to be sure that we will be able to maintain it in the long run.

Answer 1

I second the Maven recommendation. That can add all sorts of sanity to your project structure.

Maven can even generate Eclipse workspaces via mvn eclipse:eclipse

An important clarification on the EJBs note. As of ava EE 6 is you no longer need to separate EJBs from Servlets and can use them together in the very same jar in the war file.

So understand from that that using EJBs or not no longer has any impact on packaging or classloaders as it once did. These are now separate decisions. EARs and classloader separation should now be viewed as a feature you might want to use should you want classloader separation and the complexity it brings. Most applications simply do not need that and are more than fine with just a war file containing servlets, ejbs, jpa entities, cdi beans, jaxrs services and whatever else you need. You are free to decide how you want to separate them or if you want to bother separating them at all.

EJBs do make great DAOs due to transaction management, something you don't get from plain Tomcat but can be made available in Tomcat via TomEE and works fine in Eclipse. You should consider EJBs for that reason, not for dependency reasons.

Side note, as you're new to Java EE, you might find this helpful:

• http://openejb.apache.org/examples-trunk/index.html

Answer 2

In order to have things organized when working with Java EE in teams of 1+ people I could suggest:

• Use Maven to manage your build process and library dependencies.

  Maven has a small learning curve, but once you grasp it you will be grateful. By using Maven you no longer depends on Eclipse to manage your classpath.

  A thing about it that I think is really helpful when working in teams is the install feature. Suppose you are woking on the version 1.0 of an EJB module, say core-ejb-module-1.0, and you've got it to a stable state and want everyone working in the project to refer to it from now on.

  You then run a maven command like this on it: mvn clean package install

  Maven will clean this module, compile it, run tests, create the jar and then install it to a repository that you define. Could be any computer in your company.

  Now you may tell the guys working on other projects to update this dependency version on their .pom file and in the next build they run, before compiling, maven will download this library and then use it. Really neat. No more classpath hell. (There are other ways to always automatically refer to the latest library as stated in this post, but there are some caveats. Anyway it's just an example.)

• Use JPA/EJB instead of DAO Pattern.

  Some people say DAO meaning any sort of data access, others really mean that they use the DAO Pattern to access objects. If that is your case, you no longer need to use it when using JPA. (At least for most common scenarios).

  In my case, I have a generic EntityService which is capable of doing CRUD operations on any Entity and has a centralized query management. Then every EJB's that should perform database related operations may inject this guy and do its job.

As a suggestion, with Maven, you project could be organized as such:

• core project structure
  • core (The pom root)
  • core-ejb-module (Includes all generic EJB's, like the EntityService for instance.)
  • core-jpa-module (Includes all JPA generic definitions, like Interfaces, MappedSuperclasses and such.)
  • core-jsf-module (Includes all JSF generic definitions, like abstract controllers, generic converters and wrappers for FacesContext, etc..)

Now that you have a core generic module setup, you could create:

• an application structure
  • app (The pom root)
  • app-ear-module (Includes all other modules in this application. Shared jars goes in the ear /lib folder, so all other modules could reference to them.)
  • app-ejb-module-a (Includes EJB's for the business layer of your application. It uses the core-ejb-module)
  • app-ejb-module-b (You may have lots of ejb modules. You may even have a project that contains only ejb modules. Other apps will declare their dependency on them via Maven.)
  • app-jpa-module (Contains definitions for JPA Entities that represents you database tables. Depends on the core-jpa-module)
  • app-web-module (Holds the pages, Controllers and Converters for this application.)

I think you got the idea. Things tend to be loosely coupled and you may organize your projects as you like.

This is just a simple example to illustrate. I didn't explained a lot about Maven but if you're interested I think it will help you indeed.

I hope it gives you some ideas and may help you in any way.

[]'s

Answer 3

If you can run all the sub-components using the same set of dependencies, you may find it helpful to migrate to a Maven build.

With Maven, you can define a top-level project that manages all the 3rd party dependency versions in one place, so all modules are built, tested and deployed against the same library versions. You are also likely to find Maven a good fit for the multi-module approach you have adopted, as it ensures that a project is rebuilt correctly if one of its dependencies changes.

You would still be able to use dynamic web projects as before; Eclipse will automatically deploy the DAOs as part of the service project (IIRC you need to characterise the DAOs as utility modules).

If you do go down the EJB root, you are correct that each EAR will get its own class-loader, and can therefore use a varying set of dependencies. However, in your position I would tend to look at improving your dependency management first - it'll probably be cheaper and easier.