The code you are talking about sounds like ServiceLoader, which was introduced in Java 6 to support a feature that has been defined since Java 1.3 or earlier. For performance reasons, this is the recommended approach to find interface implementations at runtime; if you need support for this in an older version of Java, I hope that you'll find my implementation helpful.
There are a couple of implementations of this in earlier versions of Java, but in the Sun packages, not in the core API (I think there are some classes internal to ImageIO that do this). As the code is simple, I'd recommend providing your own implementation rather than relying on non-standard Sun code which is subject to change.
