What would the world be like if there was only a single object with no dependencies? Well, it certainly wouldn't be called object-oriented, to begin with. In programming, you normally have your objects depend on other objects. The responsibility of obtaining these other dependencies is owned by the owing object itself. In Inversion of Control (IoC), the container is responsible for handing these dependencies to the object during its creation. Context and Dependency Injection (CDI) allows us to set the dependencies on an object without having to manually instantiate them; a term often used to describe this is called injection. It does this with the added advantage of type-safe injection, so there’s no string matching done to get the dependency, but instead its done based on the existing Java object model. Most of the CDI features have been driven by the community and input from expert group members. Many of the features in CDI have been influenced by a number of existing Java frameworks such as Seam, Guice, and Spring.
While Java EE developers have enjoyed this flexible yet powerful API, SE developers were deprived of it, as CDI was part of Java EE alone. That's changed since this version, as this powerful programming model is now going to be available for Java SE as well. As of the 2.0 release, CDI can be used in both Java SE and Java EE. To make use of CDI in SE, you can pick a reference implementation such as Weld to get started. CDI can be broken down into three parts:
- Core CDI
- CDI for Java SE
- CDI for Java EE
Given how important CDI has become to the Java EE platform, it's a key programming model to familiarize oneself with. It can be considered the glue between the other specifications and is heavily used in JAXRS and Bean Validation specs. It's important to note that CDI is not just a framework but a rich programming model with a focus on loose coupling and type safety. A reference implementation for CDI is Weld, which is an open source project developed by JBoss/Red Hat. The primary theme for this release was to add support for Java SE, as earlier versions were targeted specifically at Java EE alone. CDI provides contexts, dependency injection, events, interceptors, decorators, and extensions. CDI services provide for an improved life cycle for stateful objects, bound to well-defined contexts. Messaging between objects can be facilitated by using event notifications. If all this sounds a little overwhelming then don't worry, as we will be covering all of it and much more in the next chapter.
If there's any feature that might draw developers towards CDI, then that in all probability must be the event notification model of CDI. CDI events are pretty much what you would refer to as an implementation of the observer pattern. This feature largely influences the decoupling of code to allow for greater flexibility in object communication. When we talk about events in CDI, this mainly involves two functions; one is to raise an event and the other would be to catch an event. Now isn't that simple? Events can be synchronous or asynchronous in nature. Event firing is supported by the Event interface; the earlier version was only supported by firing synchronous events, but with the 2.0 release you can now fire async events as well. Now, in case you are wondering what this event is and why we would use it, an event is just a Java object that can be passed around. Consider a plain old Java object called LoginFailed. Based on a certain scenario or method invocation, we want to notify an observer of this event, what just happened. So, here's how you can put this together in code:
public class LoginFailed {}
public class LoginController {
@Inject Event<LoginFailed> loginFailedEvent;
public void loginAttempt() {
loginFailedEvent.fire(new LoginFailed());
}
}
We will discuss the specifics of events and more in the next chapter, which is dedicated to CDI and JPA. For now, we have just scratched the surface of what CDI has to offer, but nevertheless this should serve as a good starting point for our journey into the exciting world of CDI-based projects.