Qualifier kincolle’s blogs

@Autowired is done according to type of bean. If more than one bean with the same tyep, then BeanCreationException will happen. To solve this problem, we use @Autowired and @Qualifier together.

Interface

public interface EmployeeService {
    public EmployeeDto getEmployeeById(Long id);
}

We have 2 implementations:

@Service("service")
public class EmployeeServiceImpl implements EmployeeService {
    public EmployeeDto getEmployeeById(Long id) {
        return new EmployeeDto();
    }
}

@Service("service1")
public class EmployeeServiceImpl1 implements EmployeeService {
    public EmployeeDto getEmployeeById(Long id) {
        return new EmployeeDto();
    }
}

Then invoke it:

@Controller
@RequestMapping("/emplayee.do")
public class EmployeeInfoControl {

    @Autowired
    EmployeeService employeeService;

    @RequestMapping(params = "method=showEmplayeeInfo")
    public void showEmplayeeInfo(HttpServletRequest request, HttpServletResponse response, EmployeeDto dto) {
        ...
    }
}

When we star the tomcat, the BeanCreationException will happen:

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'employeeInfoControl': Injection of autowired dependencies failed; nested exception is org.springframework.beans.factory.BeanCreationException: Could not autowire field: com.kincolle.service.EmployeeService com.kincolle.controller.EmployeeInfoControl.employeeService; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No unique bean of type [com.kincolle.service.EmployeeService] is defined: expected single matching bean but found 2: [service1, service2]

From the exception we know we have 2 implementions, @Autowired won’t know which one should be chosen. To solve this problem we can do like the following:

@Controller
@RequestMapping("/emplayee.do")
public class EmployeeInfoControl {

    @Autowired
    @Qualifier("service")
    EmployeeService employeeService;

    @RequestMapping(params = "method=showEmplayeeInfo")
    public void showEmplayeeInfo(HttpServletRequest request, HttpServletResponse response, EmployeeDto dto) {
        ...
    }
}