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Inversion of Control Containers and the Dependency Injection pattern

In the Java community there's been a rush of lightweight containers that help to assemble components from different projects into a cohesive application. Underlying these containers is a common pattern to how they perform the wiring, a concept they refer under the very generic name of "Inversion of Control". In this article I dig into how this pattern works, under the more specific name of "Dependency Injection", and contrast it with the Service Locator alternative. The choice between them is less important than the principle of separating configuration from use.

Last significant update: 23 Jan 04

Contents

Components and Services

A Naive Example

Inversion of Control

Forms of Dependency Injection

Constructor Injection with PicoContainer

Setter Injection with Spring

Interface Injection

Using a Service Locator

Using a Segregated Interface for the Locator

A Dynamic Service Locator

Using both a locator and injection with Avalon

Deciding which option to use

Service Locator vs Dependency Injection

Constructor versus Setter Injection

Code or configuration files

Separating Configuration from Use

Some further issues

Concluding Thoughts

One of the entertaining things about the enterprise Java world is the huge amount of activity in building alternatives to the mainstream J2EE technologies, much of it happening in open source. A lot of this is a reaction to the heavyweight complexity in the mainstream J2EE world, but much of it is also exploring alternatives and coming up with creative ideas. A common issue to deal with is how to wire together different elements: how do you fit together this web controller architecture with that database interface backing when they were built by different teams with little knowledge of each other.A number of frameworks have taken a stab at this problem, and several are branching out to provide a general capability to assemble components from different layers. These are often referred to as lightweight containers, examples include PicoContainer, and Spring.