@Configuration vs @Bean

In Spring Boot and the broader Spring Framework, @Configuration and @Bean are used to define and manage beans within the Spring container. They are closely related but serve different purposes and are used in different contexts.

@Configuration

Purpose:

• @Configuration is a class-level annotation that indicates that a class contains one or more @Bean definitions and can be used by the Spring container as a source of bean definitions.

Usage:

• @Configuration classes are used to define and configure beans that should be managed by the Spring container.
• Typically, @Configuration classes are used to provide a centralized place for bean definitions and application configuration.

Key Features:

• Class-Level Annotation: It is applied at the class level.
• Component Scanning: It is a specialized form of @Component, meaning it will be picked up by component scanning if scanning is enabled.
• Configuration Classes: You can define multiple @Bean methods within a single @Configuration class to configure different beans.
• @Bean Methods: Each @Bean method in a @Configuration class creates and configures an instance of the specified bean and is managed by the Spring container.

Example:

@Configuration
public class AppConfig {

    @Bean
    public MyService myService() {
        return new MyServiceImpl();
    }

    @Bean
    public MyRepository myRepository() {
        return new MyRepositoryImpl();
    }
}

In this example, AppConfig is a configuration class that defines two beans: myService and myRepository.

@Bean

Purpose:

• @Bean is a method-level annotation used to indicate that a method produces a bean to be managed by the Spring container.

Usage:

• @Bean is used inside a @Configuration class to define individual beans and their configurations.
• The method annotated with @Bean returns an instance of the bean that Spring should manage.

Key Features:

• Method-Level Annotation: It is applied to methods within a @Configuration class.
• Bean Creation: Each @Bean method is responsible for creating and configuring an instance of the bean.
• Dependency Injection: Beans defined with @Bean methods can depend on other beans, and Spring will manage the dependencies.

Example:

@Configuration
public class AppConfig {

    @Bean
    public MyService myService() {
        return new MyServiceImpl();
    }
}

In this example, the myService method is annotated with @Bean, meaning it will create and return an instance of MyServiceImpl, which Spring will manage.

Comparison and Relationship

• @Configuration: Used at the class level to indicate that the class provides Spring configuration. It can contain multiple @Bean methods and is part of the application context configuration.
• @Bean: Used at the method level within a @Configuration class to define individual beans. Each @Bean method is a source of bean definitions and creates an instance of the bean.

Usage Scenarios

• @Configuration:
• Use when you want to define a configuration class to group multiple bean definitions together.
• Useful for organizing and managing bean definitions in a modular way.
• @Bean:
• Use within @Configuration classes to define specific beans.
• Useful for creating and configuring beans that are managed by the Spring container.

Example Integration

@Configuration
public class AppConfig {

    @Bean
    public DataSource dataSource() {
        // Configure and return the DataSource bean
        return new DataSource();
    }

    @Bean
    public MyService myService(DataSource dataSource) {
        // Inject DataSource into MyService and return the bean
        return new MyServiceImpl(dataSource);
    }
}

In this example, dataSource and myService are beans defined within the AppConfig configuration class. The myService bean method depends on the dataSource bean, and Spring will handle the dependency injection automatically.

Summary

• @Configuration is used to define configuration classes that contain bean definitions.
• @Bean is used within @Configuration classes to define and configure individual beans.

By using @Configuration and @Bean, you can efficiently manage and configure your Spring application’s beans, ensuring a clean and modular approach to configuration.