Martin Fowler's article on dependency injection summarizes the choices between a service locator and dependency injection:

The important difference between the two patterns is about how that implementation is provided to the application class. With service locator the application class asks for it explicitly by a message to the locator. With injection there is no explicit request, the service appears in the application class - hence the inversion of control.

Inversion of control is hard to understand, especially in a code base that you are wading through and not intimately familiar with. It does not help that IDEs such as eclipse are still not quite up to snuff on recognizing IoC and DI. Fowler writes:

Inversion of control is a common feature of frameworks, but it's something that comes at a price. It tends to be hard to understand and leads to problems when you are trying to debug. So on the whole I prefer to avoid it unless I need it. This isn't to say it's a bad thing, just that I think it needs to justify itself over the more straightforward alternative.

In my experience, dependency injection in spring just feels like more and more configuration, something that Tomcat used to have in spades and has slowly been decreasing. Setting up Tomcat 3 in comparison to Tomcat 5 is way harder because there were xml files galore in 3. Spring feels like that. When it craps out at runtime or startup, it isn't into the code you go, it is into the xml configuration which tends to be outside of the domain expertise of software developers, though most accept it quite cheerily, unless they don't fully understand spring. Fowler summarizes with:

The choice between Service Locator and Dependency Injection is less important than the principle of separating service configuration from the use of services within an application.

Then again if the code is an impenetrable blob that the end user is going to interact with in a manner that doesn't change the source code itself, the only place you are left to wire things in is in the configuration.

More Reading on Software engineering

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• User Experience and Front-End Engineering
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• The Physical Effects of Working on Projects
• Do you need a University Degree for a Software Engineering Position
• Banning the Word Just and Should
• Value Objects in PureMVC and Object Naming Conventions
• Debugger vs System.out.println()
• Open Closed Principle in Object Oriented Programming
• Classic Mistakes in Software Engineering
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