I wanted to take some time to put together a summary of some popular dependency injection (DI) frameworks for Java. This is a high-level overview of what’s available.
First off, what is dependency injection?
“Dependency injection is a software design pattern that allows the removal of hard-coded dependencies and makes it possible to change them, whether at run-time or compile-time.” – Wikipedia
If you’ve ever had a constructor with 12 parameters, you’ve already encountered a compelling reason to use dependency injection.
Most objects require references to other objects to be useful. Dependency injection allows the system to provide those references without you having to manage all of those dependencies at the application level.
This sounds complicated, but it’s really not. Since this is not an in-depth discussion of dependency injection, check out the Wikipedia page for several examples in various languages.
Beginning with one of the simplest, most lightweight frameworks available, Square Dagger is a tiny library (<100KB) that is useful for programs that must minimize their footprint, as in the case of mobile applications. It lacks many of the features of the larger frameworks, but it makes it up in speed and a neat compile-time validation tool.
- Square Dagger
- Dependency Injection with Dagger on Android
- Dagger – A new Java dependency injection framework
Guice is Google’s attempt to create a feature-rich, independent DI facility. It is useful in most applications, especially web development. It provides a lot more features than Dagger, but it is slower and can be complex.
- Guice – Getting Started
- Creating a simple google guice application
- Advanced dependency injection with Guice
Spring is huge. It’ll do everything for you, even write perl so that the perl can then do your laundry. Of course it has dependency injection.
The upside is that if you are already using Spring, enabling and using DI is very easy and makes for a very well integrated approach.
The downside is that if Spring isn’t your thing or you are trying to add DI to an existing enterprise application, it may be challenging to isolate the small subset of features that you want to use.
Spring DI can be configured either through XML or through annotations. I strongly recommend the annotated approach. It is much easier to traverse in code and can provide significant insights to developers that need to understand what’s going on.
Java EE6 CDI
CDI is the Context and Dependency Injection framework that is included in Java Enterprise Edition. On the surface, it looks very similar to Spring’s annotation-based DI mechanism and, in reality, it is. There are some things that it does differently under the covers, but for the most part it is attempting to solve exactly the same problems for the Java EE crowd.
- CDI Dependency Injection Introductory Tutorial. Note: this is a very detailed tutorial.
- Overview of CDI
I mention this one last because I just came across it while poking around the internet. PicoContainer, like Dagger, is a very small framework that intends to do a few things very well. It doesn’t have the features or complexity of Spring or CDI, and in exchange you get simplicity.
I don’t know much about this one but I am now aware of its existence. A new adventure every day!