Choosing Between @Autowired, @Resource, and @Qualifier for Service Injection in Spring

The article introduces three methods to specify which service implementation should be injected into a Spring controller.

Method 1: Use @Autowired together with @Qualifier("beanId") to indicate the exact bean to inject.

Method 2: Use @Resource(type = ClassName.class) (or @Resource(name="beanName")) to specify the target bean by type or name.

Method 3: Assign a name to each @Service implementation (e.g., @Service("teacherService")) and inject by that name with @Resource(name="teacherService") or @Qualifier("teacherService").

The @Service annotation registers the class as a Spring bean and sets its default bean ID to the class name with a lowercase first letter; naming it explicitly avoids conflicts.

The @Autowired annotation performs type‑based injection by default, while @Resource follows J2EE semantics and defaults to name‑based injection. @Autowired can be made optional by setting required=false.

Below are the complete code examples used in the article.

public interface HumanService {
    public String name();
}

@Service
public class TeacherServiceImpl implements HumanService {
    @Override
    public String name() {
        System.out.println("teacher");
        return "teacher";
    }
}

@Service
public class DoctorServiceImpl implements HumanService {
    @Override
    public String name() {
        System.out.println("doctor");
        return "doctor";
    }
}

@RestController
public class HumanController {
    // Method 1 example
    @Autowired
    @Qualifier("teacherServiceImpl")
    private HumanService humanService;

    // Method 2 example (commented)
    // @Resource(type = DoctorServiceImpl.class)
    // private HumanService humanService;

    @RequestMapping("/name")
    public String name(){
        return humanService.name();
    }
}

@Service("teacherService")
public class TeacherServiceImpl implements HumanService {
    @Override
    public String name() {
        System.out.println("teacher");
        return "teacher";
    }
}

@Service("doctorService")
public class DoctorServiceImpl implements HumanService {
    @Override
    public String name() {
        System.out.println("doctor");
        return "doctor";
    }
}

@RestController
public class HumanController {
    @Resource(name="doctorService")
    private HumanService humanService;

    @RequestMapping("/name")
    public String name(){
        return humanService.name();
    }
}

Finally, the article reminds readers that bean names must be unique to avoid runtime errors.