@Autowired 애너테이션은 빈을 자동으로 연결하는 기능입니다.
@Autowired는 생성자, 메서드, 필드 수준에서 사용할 수 있습니다.

필드 수준

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

@Service
public class SpringTestServiceImpl implements SpringTestService {

    @Autowired
    private SpringTestDao springTestDao;

}

메서드 수준

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

@Service
public class SpringTestServiceImpl implements SpringTestService {

    private SpringTestDao springTestDao;

    @Autowired
    public void obtainSpringTestDao (SpringTestDao springTestDao) {
        this.springTestDao = springTestDao;
    }

}

생성자 수준

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

@Service
public class SpringTestServiceImpl implements SpringTestService {

    private SpringTestDao springTestDao;

    @Autowired
    public SpringTestServiceImpl (SpringTestDao springTestDao) {
        this.springTestDao = springTestDao;
    }

}

필드, 메서드, 생성자에 설정하면, AutowiredAnnotationBeanPostProcessor를 통해 자동 연결을 수행합니다.