How can I find all implementations of interface in classpath?

At best, this will be expensive. At worst (depending on the classloaders) it may be impossible.

I strongly suggest that you look for an alternative approach to the underlying problem that you are trying to address.

The Reflections library will let you do that (to an extent):

Set<Class<? extends SomeClassOrInterface>> subTypes = 
     reflections.getSubTypesOf(SomeClassOrInterface.class);

However I wouldn't recommend that. Imagine a typical classpath with 50 external jars, each of which being a big framework like spring, hibernate, aspectj, jsf, etc. It would take much time.

If you want to have some kind of plugin mechanism, so that others can implement your interfaces and supply jars with the implementation, then look at java.util.ServiceLoader