Basic Injection / Qualifiers, Scope

Alvin ReyesAugust 28th, 2013Last Updated: August 28th, 2013

1 202 3 minutes read

This is a continuation of the DI/CDI Basics tackled last week – in this post, I’ll be discussing Basic Injection, Qualifiers and Scope.

In our last topic, there we’re a great deal of information regarding the concepts of DI/CDI, we also discussed how these beans or class loaded using the annotations -that makes up the composition of the object and created samples on how can we declare them programmatically. In this post, we will tackle more on the injection part, e.i how to acquire a certain resource and inject them on another resource for usage.

So how to inject a bean in the first place?

Basic Injection

How do you inject a Bean?

Use @Inject <Java-Type> <variable> for field injection
<Java-Type> can be Java class or Java interface

public class MyGreeter {
   // Inject Greeting object for field injection
   @Inject Greeting greeting;
   public sayGreeting(String name){
      // You can then used the injected Greeting object
      System.out.println(greeting.greet(name)); 
   }
}

Where can beans be injected?

Bean can be injected at “Injection points”

Field
Method parameter

Method can be:

Constructor (useful for created immutable object) > Initializer
Setter method
Producer
Observer

public class MyGreeter {
   private Greeting greeting;
   // Use constructor method injection
   @Inject
   public MyGreeter(Greeting greeting) {
      this.greeting = greeting; 
   }
   public sayGreeting(String name){
      System.out.println(greeting.greet(name)); 
   }
}

Qualifiers

Qualifiers are used to assign a specific bean if there are multiple implementation type (interface and child classes). Say you have two candidates (implementation class) for a specific interface:

Interface is Person
Class 1 is Regular Person
Class 2 is NonRegular Person

We can use Qualifiers to cleanly designate the implementation of classes by introducing / creating an Qualifier annotation of your own:

RegularPerson Qualifier Annotation:

NonRegularPerson Qualifier Annotation:

Now we need to create an implementation class and tag it to the Qualifier we created.

RegularPersonImpl:

NonRegularPersonImpl:

Note: You can have as many qualifiers as possible – remember that Qualifiers are meant to separate logical objects implementation from its common interface.

Now that the implementation is set, we can now Inject the class and use its qualifier.

In this example, you can clearly see the usage, by introducing a Qualifier, developer can clearly specify the implementation type by annotation.

Download the example here.

Other things you can do with Qualifiers?

I won’t be discussing further what else can you do with qualifiers, instead, I’ll give let you check this examples for you to try and explore:

Scope

Why in the first place we need Scope on our Objects? – For Web applications, we need our beans to hold state over the duration of the user’s interaction with the application, for example, across multiple requests to the server.

There are 5 types of scopes that developers can assign to an object:

@Dependent (default)
- The default scope if none is specified; it means that an object exists to serve exactly one client (bean) and has the same lifecycle as that client (bean).
@RequestScoped
- State of an object is maintained during a user’s interaction with web application in a single HTTP request.
@SessionScoped
- State of an object is maintained during user’s interaction with a web application across multiple HTTP requests.
@ApplicationScoped
- Shared state across all users’ interactions with a web application.
@ConversationScoped
- Conversation context is demarcated explicitly by the application
  - Spans multiple requests
  - But “Smaller” than session
- Used when you want to have explicit boundaries of multiple conversations within a single session

I won’t be diving into the details here but what I’ll provide you are actual examples that you can run. Download the projects here.

Next Stop: DI / CDI – Advance

With this new features, you can clearly observe and imagine how it will improve developer productivity by simplifying a lot of processes to setup a Web Application – Qualifiers can be created to clearly separate logical / business beans with a common goal (by interface), scoping let developers control the flow and manage the state of objects – and the simplification of calling beans directly from the UI thru EL makes it more straightforward.

Please do check the samples and try them on your own, it would be better to check an actual example than by just reading thru the details and concepts. Enjoy!

Reference: Basic Injection / Qualifiers, Scope from our JCG partner Alvin Reyes at the Alvin “Jay” Reyes Blog blog.