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Java EE CDI Qualifiers: Quick Peek

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.

More on CDI

Thanks for reading!

Reference: Java EE CDI Qualifiers: Quick Peek from our JCG partner Abhishek Gupta at the Object Oriented.. blog.

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