Choosing the Right Spring Bean Injection: @Autowired vs @Resource vs @Qualifier

Three ways to specify which bean to inject in a Spring controller

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

Method 2: Use @Resource(type = ClassName.class) to specify the bean by its class.

Method 3: Give each @Service implementation a name (e.g., @Service("teacherService")) and inject it with @Resource(name="teacherService") or @Autowired @Qualifier("teacherService").

What @Service actually does

1. Declares the class as a Spring bean, allowing other components to autowire it.

2. Registers the bean with an id equal to the class name with the first letter lower‑cased (e.g., teacherServiceImpl).

Note: Bean names must be unique; duplicate names cause errors.

Meaning of @Autowired

When Spring encounters @Autowired, it searches the application context for a matching bean (by type by default) and injects it.

Meaning of @Resource

@Resource

performs the same injection as @Autowired but follows J2EE semantics.

Differences between @Autowired and @Resource

@Autowired

is a Spring annotation; @Resource is a J2EE annotation. @Autowired matches beans by type by default; @Resource matches by name by default. @Autowired requires the dependency to exist; you can make it optional with @Autowired(required=false).

Example code

Interface:

public interface HumanService {
    String name();
}

Implementations:

@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";
    }
}

Controller using @Qualifier:

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

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

Controller using @Resource:

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

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