Are immutable/stateless singletons bad?

Question

Lately there have been some kind of revolution against singletons, but is there something wrong with them if they are stateless?

I know the overuse talk and all... this applies to everything not just singletons.

Answer 1

  > Are immutable/stateless singletons bad?

• No if they do not depend on other external Systems.
  • Example: A Stringutility that escapes html inside a string.
  • Reason: In unittests there is no need to replace this with a mock/simulator.
• Yes if your immutable/stateless singleton depend on other external Systems/Services and if you want to do unittesting (testing in Isolation)
  • Example: a Service that depends on an External Tax-Calculator-Webservice.
  • Reason: In order to do unittests with this Service (in isolation) you need to simulate/mock the external Systems/Services.

For more details see the-onion-architecture

Answer 2

It always depends on the usage. I think the revolution comes from the fact, that every programmer learns this pattern as the object oriented pattern. Most forget to think about where it makes sense and where it doesn't.
This, of course, is true for every pattern. Just by using patterns you don't create good code or good software.

If you have a stateless singleton, why not use a class offering only static methods (or use a static class)?

Here some post regarding global variables and singletons in general.

I wouldn't be as strict as the author but he shows that for most cases where you think you need a singleton, you don't really need it.

Answer 3

There is nothing an immutable stateless singleton can do that a static class can't.

There is simply no reason to add the extra level of complexity that ->Instance() creates, while plain call to a static method will be clearer, more conservative in terms of resources and probably faster.

It's not that they are wrong. It's that there is a better way to do it. There are scenarios where normal ("stateful") singletons are the right way to go. The evil with singleton is that they are often abused, with same bad results as global variables, but there are specific cases where using a singleton is simply correct. There are none such cases for the stateless ones.