How does Lazy annotation work in Spring?

@Lazy is an annotation used to delay the initialization of a Spring bean until it is actually needed (i.e., first used).

By default, Spring creates all singleton beans at startup (eager initialization). With @Lazy, you say: “Don’t create this bean now — create it only when someone asks for it.”

Why Use @Lazy?

• Improve startup time (especially for heavy or rarely used beans)
• Avoid circular dependencies in some cases
• Save resources by loading beans on-demand

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How to Use @Lazy

1. On a @Bean method (in @Configuration)

@Configuration
public class AppConfig {

    @Bean
    @Lazy
    public HeavyService heavyService() {
        System.out.println("Creating HeavyService... (this will print later)");
        return new HeavyService();
    }

    @Bean
    public LightService lightService(HeavyService heavyService) {
        System.out.println("Creating LightService...");
        return new LightService(heavyService);
    }
}

Supporting Classes

public class HeavyService {
    public HeavyService() {
        System.out.println("HeavyService constructor called!");
    }
}

public class LightService {
    private final HeavyService heavyService;

    public LightService(HeavyService heavyService) {
        this.heavyService = heavyService;
    }
}

2. On a @Component class

@Component
@Lazy
public class DatabaseConnection {
    public DatabaseConnection() {
        System.out.println("DatabaseConnection created! (only when used)");
    }
}

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Example: See the Difference

@SpringBootApplication
public class LazyDemoApplication {

    public static void main(String[] args) {
        var context = SpringApplication.run(LazyDemoApplication.class, args);

        System.out.println("--- App started ---");

        // This triggers lazy bean creation
        HeavyService service = context.getBean(HeavyService.class);
        System.out.println("Got HeavyService: " + service);
    }
}

Output:

--- App started ---
Creating HeavyService... (this will print later)
HeavyService constructor called!
Got HeavyService: com.example.HeavyService@...

Notice: HeavyService is not created at startup — only when getBean() is called.

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@Lazy on @Configuration Class?

Yes! You can apply it to the whole config:

@Configuration
@Lazy
public class LazyConfig { ... }

This makes all beans in that config class lazy.

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Important Notes

Point	Explanation
Only works for singleton beans	Default scope is singleton
Proxy-based	Spring creates a proxy that initializes the real bean on first use
@Autowired + @Lazy	If you inject a lazy bean, it won’t be created until the dependent bean is used

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Summary

Annotation	When is bean created?
No @Lazy	At application startup
@Lazy	On first use (lazy)

Use @Lazy when:

• Bean is expensive to create
• Bean is rarely used
• You want faster startup