Spring Singleton Bean Scope

Direct Answer

Singleton is Spring's default scope: one object is created and cached for one bean definition in one BeanFactory/ApplicationContext, then the same object is returned to callers. It is not one object per JVM. Stateless services are a simple safe use, but a singleton may hold shared state when that state and its collaborators are designed for concurrency and lifecycle requirements.

Code Example

import org.springframework.stereotype.Service;
import org.springframework.stereotype.Repository;

@Service // singleton by default
public class OrderService {
    private final OrderRepository repository; // injected once

    public OrderService(OrderRepository repository) {
        this.repository = repository; // constructor runs once
        System.out.println("OrderService created: " + this);
    }

    public void placeOrder(Long userId, String item) {
        // called by multiple threads concurrently
        var order = new Order(userId, item); // local variable, thread-safe
        repository.save(order);
    }
}

@Repository // also singleton
public class OrderRepository {
    // no mutable instance state, thread-safe
    public void save(Order order) {
        // save to database
    }
}

record Order(Long userId, String item) {}

Common Mistakes & Pitfalls

• Thinking singleton means one per JVM: Freshers often say singleton creates one instance per JVM, but it's one per ApplicationContext. With a parent-child context hierarchy or multiple Spring apps in one JVM (like integration tests), you'll have multiple singleton instances.
• Storing mutable state in instance variables: Adding a field like private String currentUser to a singleton service causes thread-safety bugs. Thread A sets it to Alice, then Thread B sets it to Bob before Thread A's request finishes — now Thread A sees Bob's data.
• Expecting singleton state to survive restarts: Singleton instances are created when the ApplicationContext starts and destroyed on shutdown. They're not persisted, so state is lost between application restarts.