How is bean injection done in Spring?

2 years ago
Olivia Parker
2 minutes

In Spring, there are three common ways to inject beans: constructor injection, setter injection, and field injection.

  1. Constructor injection: Dependency injection is achieved by using the @Autowired or @Inject annotation on the class constructor. When the Spring container creates a bean, it automatically checks the constructor parameters and injects the matching dependencies into the constructor. For example:
@Service
public class UserService {
    private UserRepository userRepository;

    @Autowired
    public UserService(UserRepository userRepository) {
        this.userRepository = userRepository;
    }
}
  1. Setter injection: Dependency injection can be achieved by using the @Autowired or @Inject annotation on the setter methods of a class. When the Spring container creates a bean, it automatically calls the corresponding setter method and injects the matching dependency into the property. For example:
@Service
public class UserService {
    private UserRepository userRepository;

    @Autowired
    public void setUserRepository(UserRepository userRepository) {
        this.userRepository = userRepository;
    }
}
  1. Field injection: Dependency injection is achieved by using the @Autowired or @Inject annotation on the fields of a class. When the Spring container creates a bean, it automatically injects the matching dependencies into the fields. For example,
@Service
public class UserService {
    @Autowired
    private UserRepository userRepository;
}

In addition to the common injection methods mentioned above, you can also use the @Inject annotation from the JSR-330 specification or the @Qualifier annotation from Spring to specify specific dependency injections. Additionally, you can use the @Bean annotation in Java configuration classes to define beans and inject them into other beans.

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