Qualifying By Annotation Members

Since Micronaut 3.0, annotation qualifiers can also use annotation members to resolve the correct bean to inject. For example, consider the following annotation:

import io.micronaut.context.annotation.NonBinding;
import jakarta.inject.Qualifier;
import java.lang.annotation.Retention;
import static java.lang.annotation.RetentionPolicy.RUNTIME;
@Qualifier (1)
@Retention(RUNTIME)
public @interface Cylinders {
    int value();
    @NonBinding (2)
    String description() default "";
}

import io.micronaut.context.annotation.NonBinding
import jakarta.inject.Qualifier
import java.lang.annotation.Retention
import static java.lang.annotation.RetentionPolicy.RUNTIME
@Qualifier (1)
@Retention(RUNTIME)
@interface Cylinders {
    int value();
    @NonBinding (2)
    String description() default "";
}

import io.micronaut.context.annotation.NonBinding
import jakarta.inject.Qualifier
import kotlin.annotation.Retention
@Qualifier (1)
@Retention(AnnotationRetention.RUNTIME)
annotation class Cylinders(
    val value: Int,
    @get:NonBinding (2)
    val description: String = ""
)

You can then use the @Cylinders annotation on any bean and the members that are not annotated with @NonBinding are considered during dependency resolution:

@Singleton
@Cylinders(value = 6, description = "6-cylinder V6 engine")  (1)
public class V6Engine implements Engine { (2)
    @Override
    public int getCylinders() {
        return 6;
    }
    @Override
    public String start() {
        return "Starting V6";
    }
}

@Singleton
@Cylinders(value = 6, description = "6-cylinder V6 engine")  (1)
class V6Engine implements Engine { (2)
    @Override
    int getCylinders() {
        return 6
    }
    @Override
    String start() {
        return "Starting V6"
    }
}

@Singleton
@Cylinders(value = 6, description = "6-cylinder V6 engine") (1)
class V6Engine : Engine { (2)
    (2)
    override val cylinders: Int
        get() = 6
    override fun start(): String {
        return "Starting V6"
    }
}

@Singleton
@Cylinders(value = 8, description = "8-cylinder V8 engine") (1)
public class V8Engine implements Engine { (2)
    @Override
    public int getCylinders() {
        return 8;
    }
    @Override
    public String start() {
        return "Starting V8";
    }
}

@Singleton
@Cylinders(value = 8, description = "8-cylinder V8 engine") (1)
class V8Engine implements Engine { (2)
    @Override
    int getCylinders() {
        return 8
    }
    @Override
    String start() {
        return "Starting V8"
    }
}

@Singleton
@Cylinders(value = 8, description = "8-cylinder V8 engine") (1)
class V8Engine : Engine { (2)
    override val cylinders: Int
        get() = 8
    override fun start(): String {
        return "Starting V8"
    }
}

You can then use the @Cylinders qualifier on any injection point to select the correct bean to inject. For example:

@Inject Vehicle(@Cylinders(8) Engine engine) {
    this.engine = engine;
}

@Inject Vehicle(@Cylinders(8) Engine engine) {
    this.engine = engine
}

@Singleton
class Vehicle(@param:Cylinders(8) val engine: Engine) {
    fun start(): String {
        return engine.start()
    }
}