Qualifier and Primary

Definition

When two beans implement the same interface, Spring does not know which to inject and fails. @Primary marks the default one. @Qualifier("name") picks a specific one at the injection point.

Example code

Two notifiers: one default, one chosen by name

import org.springframework.beans.factory.annotation.Qualifier;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Primary;
import org.springframework.stereotype.Component;

interface Notifier { String send(String text); }

@Component
@Primary                                                         // the default choice
class EmailNotifier implements Notifier {
    public String send(String text) { return "email: " + text; }
}

@Component("sms")
class SmsNotifier implements Notifier {
    public String send(String text) { return "sms: " + text; }
}

@Component
class Alerts {
    private final Notifier normal;
    private final Notifier urgent;

    Alerts(Notifier normal, @Qualifier("sms") Notifier urgent) {   // default, and a named one
        this.normal = normal;
        this.urgent = urgent;
    }

    void run() {
        System.out.println(normal.send("report ready"));
        System.out.println(urgent.send("server down"));
    }
}

@ComponentScan
class Demo {
    public static void main(String[] args) {
        try (var context = new AnnotationConfigApplicationContext(Demo.class)) {
            context.getBean(Alerts.class).run();
        }
    }
}

// Output:
// email: report ready
// sms: server down