What is the core criticism of the Singleton pattern, and what would a better alternative look like in a Spring application?

A Singleton is really just global, shared mutable state given a more respectable name, and it inherits all of global state's usual problems — it is hard to unit test, since there is no way to swap in a fake instance for a test when access is hardcoded to one static method, and it hides a class's real dependencies, since any code can silently reach for getInstance() without that dependency ever showing up in a constructor signature. It also directly works against dependency inversion, since code ends up depending on one concrete, globally accessible class instead of an injected abstraction. In a Spring Boot application, a @Service or @Component class is a singleton-scoped bean by default, meaning the framework guarantees exactly one shared instance, but that instance is injected wherever it is needed rather than fetched through a global static method — giving you the exactly-one-instance benefit without the hidden-dependency, hard-to-test cost of a hand-rolled Singleton.