Java Platform, Enterprise Edition (Java EE) 8 The Java EE Tutorial

## Using Qualifiers

You can use qualifiers to provide various implementations of a particular bean type. A qualifier is an annotation that you apply to a bean. A qualifier type is a Java annotation defined as `@Target({METHOD, FIELD, PARAMETER, TYPE})` and `@Retention(RUNTIME)`.

For example, you could declare an `@Informal` qualifier type and apply it to another class that extends the `Greeting` class. To declare this qualifier type, use the following code:

    package greetings;

    import static java.lang.annotation.ElementType.FIELD;
    import static java.lang.annotation.ElementType.METHOD;
    import static java.lang.annotation.ElementType.PARAMETER;
    import static java.lang.annotation.ElementType.TYPE;
    import static java.lang.annotation.RetentionPolicy.RUNTIME;

    import java.lang.annotation.Retention;
    import static java.lang.annotation.RetentionPolicy.RUNTIME;
    import java.lang.annotation.Target;

    import javax.inject.Qualifier;

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

You can then define a bean class that extends the `Greeting` class and uses this qualifier:

    package greetings;

    @Informal
    public class InformalGreeting extends Greeting {
        public String greet(String name) {
            return "Hi, " + name + "!";
        }
    }

Both implementations of the bean can now be used in the application.

If you define a bean with no qualifier, then the bean automatically has the qualifier `@Default`. The unannotated `Greeting` class could be declared as follows:

    package greetings;

    import javax.enterprise.inject.Default;

    @Default
    public class Greeting {
        public String greet(String name) {
            return "Hello, " + name + ".";
        }
    }

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