Spring-boot读书笔记一@Component.vs.@bean

Component vs Bean in Spring Boot

Both are Spring-managed objects, but they differ in how and where they're defined.

@Component
Class-level annotation - marks the class itself as a Spring component

@Component
public class UserService {
    public void saveUser(String name) {
        // business logic
    }
}

Characteristics:

• Applied directly to the class
• Spring auto-detects during component scanning
• One instance per class (unless specified otherwise)
• Class controls its own creation

@Bean
Method-level annotation - creates beans through factory methods in @Configuration classes

@Configuration
public class AppConfig {
    
    @Bean
    public RestTemplate restTemplate() {
        return new RestTemplate();
    }
    
    @Bean
    public DatabaseService databaseService() {
        return new DatabaseService("localhost", 5432);
    }
}

Characteristics:

• Applied to methods in @Configuration classes
• You control object creation and configuration
• Can create multiple beans from same class
• External configuration of third-party classes

Key Differences
@Component @Bean
Class-level Method-level
Auto-detected Explicitly defined
Own classes Any classes (including third-party)
Simple creation Custom creation logic
One bean per class Multiple beans possible

When to Use Each

• @Component for:

  1. Your own business classes
  2. Services, repositories, controllers
  3. Simple object creation

• @Bean for:

  1. Third-party library objects
  2. Complex object configuration
  3. Conditional bean creation
  4. Multiple instances of same class

Both @Component and @Bean result in Spring-managed objects, but @Bean gives more control over the creation process.