Native CDI Qualifiers: @Any and @Default

Abhishek GuptaSeptember 29th, 2015Last Updated: September 29th, 2015

0 134 1 minute read

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!

Reference:

Native CDI Qualifiers: @Any and @Default from our JCG partner Abhishek Gupta at the Object Oriented.. blog.