Qualifiers are the mainstay of type safety and loose coupling in Contexts and Dependency Injection (CDI). Why? Without CDI, we would be injecting Java EE components in a manner similar to below

Note:This will actually not compile and is just a hypothetical code snippet

Example 1

👁 cdi1

Example 2

👁 cdi2

What’s wrong with the above implementations?

• Not type safe – Uses a String to specify the fully qualified name of an implementation class (see Example 1)
• Tightly couples the BasicCustomerPortal class to the BasicService class (see Example 2)

This is exactly why CDI does not do Injection this way !

Qualifiers help promote

• Loose Coupling – An explicit class is not introduced within another. Detaches implementations from each other
• Strong Typing (type safety) – No String literals to define injection properties/metadata

 Qualifiers also serve as

• Binding components between beans and Decorators
• Event selectors for Observers (event consumers)

How to use Qualifiers? 

CDI Qualifiers Simplified

👁 cdi-qualifiers-simplified

 Simplified steps

• Create a Qualifier
• Apply Qualifiers to different implementation classes
• Use the Qualifiers along with @Inject to inject the instance of the appropriate implementation within a class

This was not a detailed or in-depth post about CDI Qualifiers. It’s more of a quick reference.

• Click for source code

More on CDI

• The Specification page (CDI 1.2)
• Official CDI page

Thanks for reading!