Bean

• Bean 本质就是“被 Spring 容器(IOC container)管理的对象”。 Bean = 普通 Java 对象 + 交给 Spring 管理生命周期和依赖关系
• Spring creates it, manages it, injects it where needed

//Spring 自动把 UserService 注入进来,不需要手动传参或 new
@Service
public class OrderService {
    @Autowired
    private UserService userService;
	}

IOC(Spring Container)—called Bean

• The container is like a factory + warehouse of objects.
• Spring scans your code, Finds classes with annotations, Creates objects, Stores them in a map (Bean container)

  Example internal structure:
  BeanContainer:
  "couponService" -> CouponService instance
  "orderService"  -> OrderService instance

How a Bean is created

1. Annotation-based (most common)

Spring scans and registers them as Beans.

@Service
@Component
@Controller
@Repository

2. Configuration class

Manually define the Bean.

@Configuration
public class AppConfig {
	@Bean
	public CouponService couponService() {
	    return new CouponService();
	}
}

3.Third-party libraries

Spring Boot auto-config creates Beans for you:

• DataSource
• RedisTemplate
• KafkaTemplate

What Bean really gives you (core value)

1. Dependency Injection (DI)

Instead of:

CouponService service = new CouponService();

You write:

@Autowired
private CouponService service;

Spring injects dependencies automatically.

2. Loose coupling

@Autowired
private PaymentService paymentService;

You don’t care which implementation is used.

3. Lifecycle management

full lifecycle:

1. Bean definition loaded

2. Bean instantiated实例化 (new)

3. Dependency injection

4. Initialization (@PostConstruct)

5. Ready for use

6. Destroy (@PreDestroy)

7. AOP support (very important)

Because Spring controls Beans, it can add:

• transactions (@Transactional)
• logging
• security

Without Beans → these don’t work

Bean Scope

1. Default is singleton

   @Service

   • only ONE instance in the container
   • shared everywhere

2. Prototype

   	@Scope("prototype")

   • new object every time you request

3. Other scopes (web)

   • request
   • session

Bean Injection methods

1. Field字段 injection

   @Autowired
   private CouponService service;

2. Constructor injection (recommended in real projects)—easier testing, immutable

   private final CouponService service;
   public OrderService(CouponService service) {
   this.service = service;
   }

3. Setter injection (rare)