Jakarta EE @Qualifier Anotasyonu

Giriş

Açıklaması şöyle. Yani seçilebilecek iki bean arasından hangisini istediğimizi belirtiriz.

The @Qualifier annotation can be applied to another annotation for creating a custom qualifier:

...

In contrast to Spring’s @Qualifier, the Jakarta EE @Qualifier does not possess a String value attribute and hence, cannot be applied directly to bean definitions. Custom qualifiers can be further refined with additional attributes. An example for such additional attributes is the built-in @Named qualifier, which can be used both in Spring and Jakarta EE.

Açıklaması şöyle

If we don't have @Qualifier set, CDI won't know the default implementation and will throw the AmbiguousResolutionException. You can solve this problem using the Named annotation or a Qualifier. In our case, we will use the Qualifiers.

Örnek - Custom Qualifier

Şöyle yaparız

// custom qualifier definition:

@Qualifier
@Retention(RUNTIME)
@Target({ TYPE, METHOD, FIELD, PARAMETER })
public @interface Feline { }

// bean definition:

@Feline
class Cat implements Pet { }

// injection point:

@Inject
@Feline
Pet felinePet;

Örnek - Custom Qualifier

Elimizde şöyle bir kod olsun

public interface Instrument {
  String sound();
}

public enum InstrumentType {
  STRING, PERCUSSION, KEYBOARD;
}

@Qualifier
@Retention(RUNTIME)
@Target({TYPE, METHOD, FIELD, PARAMETER})
public @interface MusicalInstrument {
    InstrumentType value();
}

Farklı sınıfları tanımlayan şöyle bir kod olsun

@MusicalInstrument(InstrumentType.KEYBOARD)
@Default
class Piano implements Instrument {
  ...
}

@MusicalInstrument(InstrumentType.STRING)
class Violin implements Instrument {
  ...
}

@MusicalInstrument(InstrumentType.PERCUSSION)
class Xylophone implements Instrument {
  ...
}

Şöyle yaparız

@ApplicationScoped
public class Orchestra {

  @Inject
  @MusicalInstrument(InstrumentType.PERCUSSION)
  private Instrument percussion;
  @Inject
  @MusicalInstrument(InstrumentType.KEYBOARD)
  private Instrument keyboard;

  @Inject
  @MusicalInstrument(InstrumentType.STRING)
  private Instrument string;

  @Inject
  private Instrument solo;

  @Inject
  @Any
  private Instance<Instrument> instruments;

}

Default Qualifier

Açıklaması şöyle

In the absence of a qualifier, the built-in @Default qualifier is assumed both for bean 
definition and injection point. So the following, simple example 
without explicit qualifiers

// bean definition without explicit qualifier:

class Cat implements Pet { }

// injection point without explicit qualifier

@Inject
Pet felinePet;

is equivalent to the following example with an explicit @Default qualifier:
// bean definition with @Default made explicit:

@Default
class Cat implements Pet { }

// injection point with @Default made explicit:

@Inject
@Default
Pet felinePet;