Bean Lifecycle & Scopes

What Is a Spring Bean?

A Spring bean is an object that is created, managed, and destroyed by the Spring IoC container. Every class marked with a stereotype annotation (@Component, @Service, @Repository, etc.) is a Spring bean.

Bean Creation Lifecycle

Here are the stages when Spring creates a bean:

1. Container detects the class definition
2. Create an instance (constructor)
3. Set properties (dependency injection)
4. Call @PostConstruct
5. Bean is ready to use
6. (at shutdown) Call @PreDestroy
7. Bean is destroyed

Lifecycle Callback Example

@Component
public class DatabaseMonitor {

    @PostConstruct
    public void init() {
        // Called AFTER the bean is created and its dependencies are injected
        System.out.println("DatabaseMonitor initialized");
    }

    @PreDestroy
    public void cleanup() {
        // Called BEFORE the bean is destroyed by the container
        System.out.println("DatabaseMonitor destroyed");
    }
}

Full Order

Bean Scopes

A scope determines how many instances of a bean are created and when they are used.

Singleton (Default)

@Component  // Default: singleton
public class UserService {
    // Only 1 instance across the entire application
}

Prototype

@Component
@Scope("prototype")
public class ShoppingCart {
    // Every @Autowired creates a new instance
}

Request & Session (Web)

@Component
@RequestScope  // Shorthand for @Scope(value="request", proxyMode=ScopedProxyMode.TARGET_CLASS)
public class RequestContext {
    private String ipAddress;
}

@Component
@SessionScope
public class UserSession {
    private String username;
}

@Scope Annotation

@Component
@Scope(value = ConfigurableBeanFactory.SCOPE_PROTOTYPE)
public class TaskExecutor {
    // Prototype scope using a constant
}

Lazy Initialization

By default, Spring creates all beans at startup (eager initialization). With @Lazy, a bean is created the first time it's accessed:

@Component
@Lazy
public class ExpensiveReportGenerator {
    public ExpensiveReportGenerator() {
        // Constructor is not called at startup
        // It's called the first time the bean is injected/accessed
        System.out.println("ReportGenerator created on demand");
    }
}

Enable lazy loading globally in application.properties:

spring.main.lazy-initialization=true

Summary

With an understanding of the bean lifecycle and scopes, you can control how Spring manages the objects in your application. In the next chapter, we'll put it all into practice: building a first REST API with Spring Boot.