In a project I’m currently working on, I need to make some parameters configurable, and they need to be outside the .war file that I’m deploying. For example, let’s say I’m creating a service which reads data from some other RESTful service. And let’s say that the other RESTful service has two URLs, one for test and one for production. I’d like to be able to deploy my .war, and then edit a file outside of that .war file to configure which URL my service should be using.

My first inclination was to try to do this with a JNDI Environment Entry, so I began researching that approach. However, while the EJB spec states in 20.2.4 that the container must “provide a deployment tool that allows the Deployer to set and modify the values of the enterprise bean’s environment entries” (thanks for finding that one, ipage), JBoss does not seem to have such a facility.

Soon I started to wonder if JNDI wasn’t a bit overkill for what I needed to do, anyway.  I didn’t want to specify my parameters on the command-line; I wanted to simplify deployment and wanted to be able to change these values at runtime without restarting the server. But perhaps a System Property was all I needed.

As it turns out, JBoss has the System Properties Management Service for such things.  Here’s what you need to do:

1. Make sure properties-plugin.jar is in your ${JBOSS_HOME}/server/<server>/lib directory.
2. Make sure the properties-service.xml is in your deploy directory (you can find a copy in the “default” server directory)
3. You now have two options, either edit the URLList to have a comma-separated list of locations of properties files, or you can specify your properties directly in properties-service.xml in the <attribute name=”Properties”> element.

Now, to access your property, all you need to do is call the venerable System.getProperty() method.

Photo Credit: Shelley Gibb

Okay,I admit that this one should have totally been obvious to me long ago. But I’m still a bit of a JEE newcomer (been doing it for almost five years), so perhaps I can be forgiven.

If you do a lot of ORM or EJB remoting, you probably deal with a lot of Serializable classes. And you’re probably used to the annoying warning message that you see all the time in your IDE when you’re working with Serializable classes:

The serializable class BlaBlaBla does not declare a static final serialVersionUID field of type long BlaBlaBla.java myProject/src/main/java/us/mikedesjardins/foo/domain/entity line 44

If you’re like me, you roll your eyes and politely add a @SuppressWarnings(“serial”) to the top of the class definition (or, worse, you just shut the warning message off in your IDE altogether. Even I don’t do that!). You reason with yourself that current versions of Java conveniently and automatically compute the serialVersionUID at run-time, so there’s no need to bother with the formality of a version number on your class – it’s just a nuisance holdover from days of Java yore.

IT’S A TRAP!
Now that I’ve found myself well into a new project with this lazy philosophy, I’m starting to run into problems. I have a client of my EJB that uses one of these Serializable objects, and I’m finding that when I make the most trivial changes to my shared classes, I need to compile both the server and the client components. The two components that were supposed to be loosely coupled are now hopelessly intertwined. So I did some further research on how the JVM computes the ad-hoc serialVersionUID at runtime when it isn’t provided.

This article over at JavaWorld does a far better and more thorough job of explaining it than I will. In a nutshell, backward-compatability with respect to serialization and de-serialization is a lot less fragile than the cases that the serialVersionUID generation is protecting you against. That version generation algorithm computes an SHA hash based on the class name, sorted member variables, modifiers, and interfaces.

In reality, serialization and de-serialization generally only breaks when one of the following things happens to your class (from the aforementioned article at JavaWorld):

• Delete fields
• Change class hierarchy
• Change non-static to static
• Change non-transient to transient
• Change type of a primitive field

Ensure Minimal Coupling Between Components
To ensure that your components which use Serialization have minimal runtime dependencies on each other, you have two options:

• Declare a specific serialVersionUID, and update it whenever you make a change that breaks backward compatability.
• Don’t rely on any classes for use as transfer objects which will potentially change. This one is pretty obvious, but sometimes you will be surprised down the road at which classes are modified more often than others.
• Don’t use your own objects at all when transferring data. Instead, rely on classes like Integers, Strings, or HashMaps to shuttle data around among components. (Obviously, protocols like SOAP and REST rely on XML documents for this to ensure maximum de-coupling, but you’re presumably using something like EJB remoting to avoid the complexity or overhead of these protocols).

Photo Credit: Mike Johnson