Find Java classes implementing an interface

Spring can do this for you...

BeanDefinitionRegistry bdr = new SimpleBeanDefinitionRegistry();
ClassPathBeanDefinitionScanner s = new ClassPathBeanDefinitionScanner(bdr);

TypeFilter tf = new AssignableTypeFilter(CLASS_YOU_WANT.class);
s.addIncludeFilter(tf);
s.scan("package.you.want1", "package.you.want2");       
String[] beans = bdr.getBeanDefinitionNames();

N.B. The TypeFilter is important if you want the correct results! You can also use exclusion filters here instead.

The Scanner can be found in the spring-context jar, the registry in spring-beans, the type filter is in spring-core.

Awhile ago, I put together a package for doing what you want, and more. (I needed it for a utility I was writing). It uses the ASM library. You can use reflection, but ASM turned out to perform better.

I put my package in an open source library I have on my web site. The library is here: http://software.clapper.org/javautil/. You want to start with the with ClassFinder class.

The utility I wrote it for is an RSS reader that I still use every day, so the code does tend to get exercised. I use ClassFinder to support a plug-in API in the RSS reader; on startup, it looks in a couple directory trees for jars and class files containing classes that implement a certain interface. It's a lot faster than you might expect.

The library is BSD-licensed, so you can safely bundle it with your code. Source is available.

If that's useful to you, help yourself.

Update: If you're using Scala, you might find this library to be more Scala-friendly.

I really like the reflections library for doing this.

It provides a lot of different types of scanners (getTypesAnnotatedWith, getSubTypesOf, etc), and it is dead simple to write or extend your own.

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.