Service Locator vs. Dependency Injection Container

I am NOT as crazy as I thought!

Over the last few years of learning to incorporate Java-like patterns into my PHP development practices one area seemed to drive me crazy, almost to the point of second guessing myself about dependency injection altogether. Years ago I learned from Misko Hevery from Google that a service locator is really just global state or a bad singleton in sheep’s clothing. He states that a service locator violates the Law of Demeter and calls singletons are pathological liars.

Needless to say I have developed a bad taste for singletons and service locators. So when I saw that many PHP projects had dependency injection containers (DIC) that were used as a service locator I became sick. I discussed the issues with many PHP developers who didn’t seem to understand the negative issues that dependency injection containers create for testing and loosely-coupling. That is when the second guessing started. So I did some research on the internet and found only two others that felt the way I did. Here are their links:

• Service Locator vs Dependency Injection Container (or Tell, Don’t Ask Part 2)
• Inversion of Control, Dependency Injection, Dependency Injection Container and Service Locator

Basically, in my opinion, a service locator and dependency injection container are for all intents and purposes identical. The difference is in how they are used. Typically, a service locator is passed into a class which then asks the locator for a dependency. A dependency injection container is used to get the dependencies and then pass them into the class that will use them. Essentially, this is a similar view point that Adam Brett holds. Read his article for code examples of both a service locator and dependency injection container.