Is it possible to create two Singletons in Spring…

Yes, it is possible:

Different contexts: Each Spring context (for example, two AnnotationConfigApplicationContext) will manage its own set of beans, including singletons. Each context will have its own singleton instance.

// Singleton bean
public class MySingleton {
    private static int counter = 0;
    private int instanceId;

    public MySingleton() {
        instanceId = ++counter;
    }

    public int getInstanceId() {
        return instanceId;
    }
}

// Bean configuration
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
public class AppConfig {
    @Bean
    public MySingleton mySingleton() {
        return new MySingleton();
    }
}

// Using different contexts
import org.springframework.context.ApplicationContext;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;

public class Main {
    public static void main(String[] args) {
        ApplicationContext context1 = new AnnotationConfigApplicationContext(AppConfig.class);
        MySingleton singleton1 = context1.getBean(MySingleton.class);
        System.out.println("Singleton 1 from context 1, ID: " + singleton1.getInstanceId()); // Output: 1

        ApplicationContext context2 = new AnnotationConfigApplicationContext(AppConfig.class);
        MySingleton singleton2 = context2.getBean(MySingleton.class);
        System.out.println("Singleton 2 from context 2, ID: " + singleton2.getInstanceId()); // Output: 2
    }
}

Different bean IDs (if the bean is defined multiple times): If the same class is registered in one context under different bean IDs, Spring by default will create a separate singleton for each ID. This is not exactly "two singletons of this class," but rather two different beans, each with singleton scope and using the same implementation class.

// Configuration with different IDs for the same class
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
public class DifferentIdConfig {
    @Bean(name = "singletonA")
    public MySingleton mySingletonA() {
        return new MySingleton();
    }

    @Bean(name = "singletonB")
    public MySingleton mySingletonB() {
        return new MySingleton();
    }
}

// Using beans with different IDs
import org.springframework.context.ApplicationContext;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;

public class MainDifferentId {
    public static void main(String[] args) {
        ApplicationContext context = new AnnotationConfigApplicationContext(DifferentIdConfig.class);
        MySingleton singletonA = context.getBean("singletonA", MySingleton.class);
        MySingleton singletonB = context.getBean("singletonB", MySingleton.class);

        System.out.println("Singleton A ID: " + singletonA.getInstanceId()); // Output: 1
        System.out.println("Singleton B ID: " + singletonB.getInstanceId()); // Output: 2
    }
}

Despite the possibilities, the standard and most common use of Singleton in Spring implies a single instance of a bean of a specific class within one ApplicationContext.

Is it possible to create two Singletons in Spring?