Google Data

Five years ago, Spring 1.0 brought Java dependency injection into the mainstream. Three years later, Google Guice 1.0 introduced annotation-based dependency injection and made Java programming a little easier. Since then, developers have had to choose between a) writing external configuration or b) importing vendor-specific annotations.

Today, we hope to give developers the best of both worlds. Google Guice and SpringSource have partnered to standardize a proven, non-controversial set of annotations that make injectable classes portable across frameworks. At the moment, the set of specified annotations consists of:

• @Inject - Identifies injectable constructors, methods, and fields
• @Qualifier - Identifies qualifier annotations
• @Scope - Identifies scope annotations
• @Named - String-based qualifier
• @Singleton - Identifies a type that the injector only instantiates once

One additional interface is specified for use in conjunction with these annotations:

• Provider - Provides instances of a type T. For any type T that can be injected, you can also inject Provider.

You can check out an early draft of the specification. We deliberately left external dependency configuration out, so as not to quash ongoing innovation. We haven't formally submitted this standard to the JCP yet, but we plan to do so shortly. Standards wonks can read a draft of our JSR proposal.

The expert group will be inclusive and will work in the open. For example, our mailing list is publicly readable, and we host the specification at Google Code. Several industry players have already expressed interest in supporting this effort. Contact us if you'd like to help out.

Interested in learning more about dependency injection? Don't miss Jesse and Dhanji's Big Modular Java with Guice session at Google I/O!

By Bob Lee, Google Guice founder