Why implementing a Singleton pattern in Java code is (sometimes) considered an anti-pattern in Java world?

There are many requirements that you might have on the singleton:

1. lazy initialization;
2. proper disposal;
3. scoping (one per thread, for example).

Typically, also, you'll have a whole lot of singletons in your app, and the Singleton pattern doesn't allow reusable code. So, if you want to implement all these concerns for all your singletons, you'll immediately see its anti-pattern quality.