Googe Guice's documentation has a nice explanation of how to scope objects (from http://code.google.com/p/google-guice/wiki/Scopes). How they word it is specific to Guice but the underlying points are valid for any Java application

"Choosing a scope

If the object is stateful, the scoping should be obvious. Per-application is @Singleton, per-request is @RequestScoped, etc. If the object is stateless and inexpensive to create, scoping is unnecessary. Leave the binding unscoped and Guice will create new instances as they're required.

Singletons are popular in Java applications but they don't provide much value, especially when dependency injection is involved. Although singletons save object creation (and later garbage collection), getting a handle to the single instance requires synchronization. Singletons are most useful for:

* stateful objects, such as configuration or counters

* objects that are expensive to construct or lookup

* objects that tie up resources, such as a database connection pool."

I have to admit that when I first started coding Java/Guice I had 'singleton addiction'. Luckily with Guice, as someone else mentioned, it's just matter of deleting the annotation on a class to change your mind about that.

Anyway, if you use Guice and dependency inject your singleton classes, then the issue of being hard to test and disguising dependencies goes away, doesn't it?