Dependency Injection

Dependency Injection (often called Inversion of Control) is a programming pattern that aims to keep the various components of a system independent, and couples them loosely at runtime. That is, instead of having code like:

public class Foo
{
    public void doer() {
        BarInterface b = new BarImplementation();
        b.doit();
        . . .

Which creates a direct dependency between the class Foo and the class BarImplementation, we allow the class to be given to us (injected), as in:

public class Foo
{
    BarInterface b;

    public void setBar (BarInterface b) {
        this.b = b;
    }

    public void doer() {
        b.doit();
        . . .

This pattern is also called the Hollywood Principle since it describes the "Don't call me, I'll call you" attitude. Yeah, geeks have to inject their own humor whereever they can.

In this presentation, I give a high-level overview of this programming pattern and how some IoC frameworks implement it. It is not a complete description of the concept, nor does it accurately describe these popular frameworks, like the Spring Framework or Google's Guice.

BTW: A really good article on this subject is from Martin Fowler.

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