1. @Autowired
- Used for dependency injection (Spring automatically injects the required bean).
- By default, it matches by type.
@Component
class Engine {
public String start() { return "Engine started"; }
}
@Component
class Car {
@Autowired
private Engine engine;
public void drive() {
System.out.println(engine.start() + " - Car is driving");
}
}
2. @Qualifier
- Used when multiple beans of the same type exist, to avoid ambiguity.
- Works with
@Autowired.
@Component("dieselEngine")
class DieselEngine implements Engine {}
@Component("petrolEngine")
class PetrolEngine implements Engine {}
@Component
class Car {
@Autowired
@Qualifier("dieselEngine")
private Engine engine;
}
3. @Primary
- If multiple beans are available, Spring uses the @Primary bean unless
@Qualifieris used.
@Component
@Primary
class PetrolEngine implements Engine {}
@Component
class DieselEngine implements Engine {}
@Component
class Car {
@Autowired
private Engine engine; // PetrolEngine will be injected (because of @Primary)
}
4. @Resource
- From Jakarta (javax.annotation), not Spring-specific.
- By default, injects beans by name (unlike @Autowired which is by type).
@Component("myEngine")
class Engine {}
@Component
class Car {
@Resource(name = "myEngine")
private Engine engine;
}
5. @Value
- Injects literal values or values from
application.properties.
application.properties
car.name=Tesla
Java:
@Component
class Car {
@Value("${car.name}")
private String carName;
}
6. @ConfigurationProperties
- Maps external configuration properties (from
.ymlor.properties) to a POJO.
application.yml
car:
name: Tesla
model: Model-S
year: 2025
Java:
@Component
@ConfigurationProperties(prefix = "car")
class CarProperties {
private String name;
private String model;
private int year;
// getters & setters
}
Now you can inject it:
@Component
class CarService {
private final CarProperties carProperties;
public CarService(CarProperties carProperties) {
this.carProperties = carProperties;
}
}
7. @Lazy
- Delays bean creation until it’s needed (by default, Spring creates beans eagerly).
@Component
@Lazy
class HeavyEngine {
public HeavyEngine() {
System.out.println("HeavyEngine initialized");
}
}
Only when you call context.getBean(HeavyEngine.class), it will initialize.
8. @Order
- Defines the execution order of beans when multiple beans implement the same interface (used in Lists).
public interface Engine {}
@Component
@Order(1)
class DieselEngine implements Engine {}
@Component
@Order(2)
class PetrolEngine implements Engine {}
@Component
class Car {
@Autowired
private List<Engine> engines; // Will inject [DieselEngine, PetrolEngine] in order
}
✅ Summary Table
| Annotation | Purpose |
|---|---|
@Autowired | Auto-inject dependency (by type). |
@Qualifier | Resolve ambiguity when multiple beans exist. |
@Primary | Mark default bean when multiple exist. |
@Resource | Inject by name (JSR annotation). |
@Value | Inject values from properties. |
@ConfigurationProperties | Bind external properties to POJO. |
@Lazy | Create bean only when needed. |
@Order | Define order for bean execution. |