What's the difference between the Service Locator and the Factory Design pattern?

Question

I'm using unity and I'm creating a class that wrapps it and I dont' know how to call it, service locator or factory, both encapsulate the creation of the objects, so.... what's the difference?

Answer 1

A factory creates objects for you, when requested.

Service locator returns objects that may already exist, that is services that may already exist somewhere for you.

Just think about the meaning of the names:

• Factory: is a place where objects are created.
• Service: is something that can do something for you as a service.
• Service locator: is something that can find something that can perform a service.

Answer 2

Actually there is a clear separation between this both pattern. It's common known that both pattern are used for avoid dependencies from concrete types.

However after read

• Agile Software Development, Principles, Patterns, and Practices [book] by Rober C. Martin
• Inversion of Control Containers and the Dependency Injection pattern [article] by Martin Fowler at http://martinfowler.com/articles/injection.html
• Pattern Recognition: Abstract Factory or Service Locator? [article] by Mark Seemann at http://blog.ploeh.dk/2010/11/01/PatternRecognitionAbstractFactoryorServiceLocator/
• Design Pattern [book] by Erich Gamma et al

Some severe contradictions arises:

Seemann said: "An Abstract Factory is a generic type, and the return type of the Create method is determined by the type of the factory itself. In other words, a constructed type can only return instances of a single type."

While Rober C. Martin didn't mention anything about generic types and furthermore, factory example in his book allow to create instance of more than one type of objects distinguish between them using a key string as parameter in the Factory.Make().

Gamma said that intent of Abstract Factory is to "Provide an interface for creating families of related or dependent objects without specifying their concrete classes". Is worth to mention that Gamma Abstract Factory example violate Interface Segregation Principle (ISP) stated by Martin. ISP and SOLID in general are more moderns principles or maybe for simplicity where omitted.

Gamma and Martin's works precede Seemann's, so I think he should follow definition already made.

While Fowler propose Service Locator as a way to implement Dependency Inversion, Seemann consider it as an anti-pattern. Neither Gamma or Martin mention Service Locator.

However, Seemann and Fowler agreed in that Service Locator needs a configuration step to register an instance of a concretes class, that instance is what will be later returned when an object of that kind be requested. This configuration step is not mentioned by Martin or Gamma in their definition of Abstract Factory. Abstract Factory pattern suppose a new object to be instantiated every time an object of that kind be requested.

Conclusion

The main difference between Service Locator and Abstract Factory is that Abstract Factory suppose a new object be instantiated an returned at each requested and Service Locator needs to be configured with an object instance and every time the same instance will be returned.