Service locator pattern

The service locator pattern is a design pattern or anti-pattern used in software development to encapsulate the processes involved in obtaining a service with a strong abstraction layer. This pattern uses a central registry known as the "service locator", which on request returns the information necessary to perform a certain task. The main criticism of service location is that it obscures dependencies. Meanwhile, its proponents say the approach should not be discarded as it simplifies component-based applications where all dependencies are cleanly listed at the beginning of the whole application design, consequently making traditional dependency injection a more complex way of connecting objects. The usual criticism of service locator pattern as being the one which is "much harder to test" actually becomes a benefit in such applications.

Advantages

• The "service locator" can act as a simple run-time linker. This allows code to be added at run-time without re-compiling the application, and in some cases without having to even restart it.
• Applications can optimize themselves at run-time by selectively adding and removing items from the service locator. For example, an application can detect that it has a better library for reading JPG images available than the default one, and alter the registry accordingly.
• Large sections of a library or application can be completely separated. The only link between them becomes the registry.
• An application may use multiple structured service locators purposed for particular functionality/testing. Service locator does not mandate one single static class per process
• The solution may be simpler with service locator (vs. dependency injection) in applications with well-structured component/service design. In these cases the disadvantages may actually be considered as an advantage (e.g. no need to supply various dependencies to every class and maintain dependency configurations)

Disadvantages

• The registry hides the class' dependencies, causing run-time errors instead of compile-time errors when dependencies are missing (similar to using Dependency injection).
• The registry makes the code more difficult to maintain (similar to using Dependency injection), because it becomes unclear when you would be introducing a breaking change.
• The registry makes code harder to test, since all tests need to interact with the same global service locator class to set the fake dependencies of a class under test. However, this is easily overcome by injecting application classes with a single service locator interface.

See also

• Dependency injection
• Dependency inversion principle
• Java Naming and Directory Interface

External links

• Sample code
• In Defense of Service Locator
• Game Programming Patterns: Service Locator
• Dependencies In Disguise