Singleton to read properties file in a Java webapp; correct approach?

Question

Hi, all.

My spaghetti monster consumes XML from several different SOAP services, and the URL for each service is hardcoded into the application. I'm in the process of undoing this hardcoding, and storing the URLs in a properties file.

In terms of reading the properties file, I'd like to encompass that logic in a Singleton that can be referenced as needed.

Change this:
accountLookupURL ="http://prodServer:8080/accountLookupService";

To this:
accountLookupURL =urlLister.getURL("accountLookup");

The Singleton would be contained within the urlLister.

I've tended to shy away from the Singleton pattern, only because I've not had to use it, previously. Am I on the right track, here?

Thanks!
IVR Avenger

Answer 1

To restate the obvious, Singleton is to be used when all client code should talk to a single instance of the class. So, use a Singleton IFF you are certain that you would not want to load multiple properties files at once. Personally, I would want to be able to have that functionality (loading multiple properties files).

Answer 2

Singletons are mutable statics and therefore evil. (Assuming a reasonably useful definition of "singleton".

Any code that uses the static (a transitive relationship), is has assumptions about pretty much everything else (in this case, a web server and the internet). Mutable statics are bad design, and bad design makes many aspects go rotten (dependency, understandability, testing, security, etc).

As an example, the only thing stopping late versions of JUnit 3 being used in a sandbox was loading a configuration file in one static initialiser. If it had used Parameterisation from Above, there would have been no issue.

Answer 3

You haven't said why you need only one of whatever it is which will be getting the URL. If that just involves reading a properties file, I don't think you do need only one. Seems to me that having two threads read the same properties file at the same time isn't a problem at all.

Unless you were thinking of having some object which only reads the properties file once and then caches the contents for future use. But this is a web application, right? So the way to deal with that is to read in the properties when the application starts up, and store them in the application context. There's only one application context, so there's your "only one" object.

Answer 4

Singletons are appropriate for this scenario, BUT you have to make sure you're doing the singleton right.

So, for example, what Bozhno suggests is not a singleton, it's an ugly concoction of nasty statics that's not mockable, not easily testable, not injectable, and generally comes back to bite you in the ass.

An acceptable singleton is just your average class with one notable exception that it is guaranteed either by itself or by some external factory/framework (e.g Spring IoC) to exist in only one instance. If you go with the first approach, you do something like

private MyUberSingletonClass() {
    //..do your constructor stuff, note it's private
}

private static MyUberSingletonClass instance = null;

public static synchronized MyUberSingletonClass instance() {
    if (instance == null) {
        instance = new MyUberSingletonClass();
    }
    return instance;
}

public String getUberUsefulStuff(){
    return "42";
}

That's acceptable if you don't really feel the need for a factory otherwise, and aren't using any IoC container in your app (good idea to think about using one though). Note the difference from Bozhno's example: this is a good vanilla class where the only static is an instance var and a method to return it. Also note the synchronized keyword required for lazy-initialization.

update: Pascal recommends this very cool post about a better way to lazy-init singletons in the comments below: http://crazybob.org/2007/01/lazy-loading-singletons.html

Answer 5

As an alternative, did you consider using something like Apache Commons Configuration (or maybe another configuration framework)?