Comments on: Singletons Are Evil Part 2

By: codematrix

codematrix — Sat, 11 Aug 2012 16:42:00 +0000

It’s not that singletons themselves are bad but the GOF design pattern is. The only really argument that is valid is that the GOF design pattern doesn’t lend itself in regards to testing, especially if tests are run in parallel.

Using a single instance of an class is a valid construct as long as you apply the following means in code:

1. Make sure the class that will be used as a singleton implements an interface. This allows stubs or mocks to be implemented using the same interface

2. Make sure that the Singleton is thread-safe if necessary. That’s a given.

3. The singleton should be simple in nature and not overly complicated.

4. During the RUNTIME of you application, where singletons need to be passed to a given object, use a class factory that builds the depending object and have the class factory pass the global singleton instance to the depending object that requires it.

5. During TESTING and to ensure deterministic behavior, create the singleton class as separate instance as either the actual class itself or a stub/mock that implements its behavior and pass it as is to the object that requires it. Don’t USE the class factor that creates that object under test that needs the signleton during test as it will pass the global singleton instance, which defeats the purpose.

We’ve used Singletons in our solutions with a great deal of success that are not tightly bound, extensible, replaceable and are testable, ensuring deterministic behavior in parallel test run streams.

codematrix

By: shopping cart software

shopping cart software — Tue, 24 Jan 2012 14:10:00 +0000

Well great addition to my knowledge, its good to keep learning all the time.

By: Domain register

Domain register — Fri, 23 Dec 2011 06:54:00 +0000

Great content I keep learning new things every day from post like these. Good stuff!

By: kerrrtivolsafdf

kerrrtivolsafdf — Sat, 26 Mar 2011 16:43:22 +0000

Thanks pal. This is nice knowing

By: Asics Gel

Asics Gel — Sat, 26 Mar 2011 04:04:15 +0000

great blog post, i surely adore this site, keep on it.

By: Claudio Lassala

Claudio Lassala — Fri, 28 Aug 2009 22:19:07 +0000

Yeah, Deliverance by Opeth is awesome!! *That* is evil.

By: CNemo

CNemo — Fri, 21 Aug 2009 14:37:37 +0000

How does it prove that singleton is evil? :-0

By: Harry M

Harry M — Fri, 14 Aug 2009 08:44:21 +0000

You can use a static constructor on the class to initialise the instance: http://stackoverflow.com/questions/7095/is-the-c-static-constructor-thread-safe

By: Eric Smith

Eric Smith — Fri, 14 Aug 2009 08:12:05 +0000

Shouldn’t line 19 of the third listing be:

instance = new Foo(new Bar(new Baz()))