Let’s take a look at the out-of-the-box qualifiers in CDI

There are three qualifiers declared by the CDI specification – @Any, @Default, @New

• @Any: Think of it as an omnipresent qualifier. It’s there even if its not ;-)
• @Default: As the name suggests, this qualifier treated as a default when none other qualifiers have been specific. The only exception to this rule is when the @Named (javax.inject) qualifier is used as well
• @New: Used to obtain a new instance of a bean on-demand. The new instance is scope independent. This has been deprecated since CDI 1.1

Here are some simple examples:

Qualifiers at Bean (class) level

//Explicit qualifier not specified, hence @Default is assumed

public class CSVParser implements Parser{
//implementation ...
}

//Explicit qualifier specified

@XMLParser
public class XMLParser implements Parser{
//implementation ...
}

Qualifiers at Injection point

public class ParsingService{
 @Inject
 Parser parser //an implementation of the CSVParser class is injected (default behavior)
 //business logic
}

public class ParsingService{
 @Inject
 @XMLParser
 Parser parser //an implementation of the XMLParser class is injected
 //business logic
}

What’s so special about @Any ?

As stated earlier, the @Any qualifier is omnipresent i.e. it is always there, no matter what. The interesting part is that if you explicitly mention this annotation, it opens up the following options

• You have access to all possible implementations of a bean

  public class ParsingService{
   @Inject
   @Any
   Instance<Parser> parsers; //javax.enterprise.inject.Instance handles the magic behind the scenes
   
   public void availableParsingOptions(){
   for(Parser parser : parsers){
   System.out.println("Parser implementation class: "+ parser.getConcreteImplName());
   System.out.println("Supported format: "+ parser.getSupportedFormat().getName());
   }
   }
  }

• It does not suppress the default bean (if any) or any of the explicit (qualified) implementations. You can still look them up dynamically (at run time)

  public class ParsingService{
   @Inject
   @Any
   Instance<Parser> parsers;
   
   public Parser chooseParser(){
   //@XMLParser (qualifier annotation) being enforced programmatically
   Parser selected = parsers.select(new XMLParserQualifier()).get();
   return (selected.isUnsatisfied() || selected.isAmbiguous()) ? null : selected;
   }
   
   //javax.enterprise.util.AnnotationLiteral project qualifiers as objects
   private static class XMLParserQualifier extends AnnotationLiteral<XMLParser> implements XMLParser{}
  }

That’s all for a quickie on default CDI qualifiers. You might want to check out one of my earlier posts on basics of custom qualifiers in CDI

Cheers!