Why was dependency injection pattern not included in the Gang of four?

Question

Why was the dependency injection pattern not incluided in the gang of four? Did GOF pre-date widespread automated testing? Is dependency injection now considered a core pattern?

Answer 1

I was Editor of Software Development magazine when the Gang of Four book came out and I can say with total confidence that unit-testing was not a widespread practice in 1994, when Design Patterns was originally published.

In 1994, C++ was the most commonly used object-oriented language, and most people programming it were coming from a C background. One of the "thinking in objects" things that people simply didn't have is the idea of hundreds or thousands of entry points into your program. You thought about the main(). If you worked on a large project, you might have a (usually quite elaborate) makefile to create a module-based program. But "unit-testing"? Starting a process, building the necessary memory context, executing it, and tearing it down, on a per method basis? That was very radical.

Java made multiple-entry-point programming more obvious. By the time of the original Dot-Com boom, unit-testing was a well-known technique, but it was really JUnit (circa 2001?) that caused it to catch fire and become a universal practice.

Although Strategy and the general concept of programming to an interface were part of GoF and the mid-90s zeitgeist, the idea of injection came quite late to the party (circa '03-'05?). Honestly, my gray hairs are still quite dubious about that aspect of DI ("Get off my lawn, you darn configuration files!").

Answer 2

They called it Strategy.

Their Strategy seems to have all the features of dependency injection without the complex-sounding name.

Answer 3

I think Dependency Injection is more relevant when separating implementation in tiers. Another area where we think about dependency injection is unit testing. And your pre-date suggestion seems to be correct. If the gang were to collect and segregate patterns in 2012, definitely dependency injection will be there.

Strategy could come up in discussions but Strategy does not talk about dependency injection. But when using strategy pattern in a single project or dll(all classes and interfaces remain in one project) it appears that we are doing dependency injection. In fact we are not.

Now, if the classes and interfaces mentioned in strategy pattern are separated in different projects or tiers then WE will have to use dependency injection techniques. We could use unity configuration files(no runtime change possible though). But Strategy pattern does not say how to inject a dependency.

If there is a pattern that resembles closely to Dependency injection then it is Abstract Factory Method pattern. This pattern could be used inside a strategy pattern to inject dependency.

Answer 4

the answer Strategy is 100% correct. I up voted it but can comment.

"Strategy lets the algorithm vary independently from clients that use it.[1] Strategy is one of the patterns included in the influential book Design Patterns by Gamma et al. that popularized the concept of using patterns to describe software design."

A design pattern is not dependent on its use. Dependency Injection is implemented by using the Strategy pattern. If we named each pattern based on the use case we would have to rename alot of patterns.

The repository pattern is not a new pattern, it is the Template Pattern.

"In the template method of this design pattern, one or more algorithm steps can be overridden by subclasses to allow differing behaviors while ensuring that the overarching algorithm is still followed."

Often the patterns are multiple patterns combined and named such as the MVC pattern.

The GOF didn't have interfaces the used Pure Abstract classes, and also took advantage of C++'s ability to inherit from more than one class.