Dependency Injection An overview Jason Ragsdale – DPUG 4/13/2010 

Basics ● DI is a technique for supplying an external dependency to a software component ● DI involves at least three elements ● A dependent ● Dependencies ● An injector (provider / container) 

Our Interface 

Highly coupled dependency 

Manually-injected dependency 

Framework-managed dependency injection 

Benefits and Drawbacks ● Reduction of boiler-plate code ● Flexibility in changing out external dependencies ● Testing in general, ability to inject mock or dev dependencies 

Types of DI ● Interface Injection ● in which the exported module provides an interface that its users must implement in order to get the dependencies at runtime. ● Setter Injection ● in which the dependent module exposes a setter method that the framework uses to inject the dependency. ● Constructor Injection ● in which the dependencies are provided through the class constructor. 

References ● http://en.wikipedia.org/wiki/Dependency_injection ● http://martinfowler.com/articles/injection.html