Spring注解之@Qualifier

@Qualifier：限定描述符，用于细粒度选择候选者；

@Autowired默认是根据类型进行注入的，因此如果有多个类型一样的Bean候选者，则需要限定其中一个候选者，否则将抛出异常

场景如下：

现有有接口：

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

同时有下述两个实现类 EmployeeServiceImpl和EmployeeServiceImplOne：

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

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

调用代码如下：

@Controller
@RequestMapping("/emplayee.do")
public class EmployeeInfoControl {
    
    @Autowired
    EmployeeService employeeService;
     
    @RequestMapping(params = "method=showEmplayeeInfo")
    public void showEmplayeeInfo(HttpServletRequest request, HttpServletResponse response, EmployeeDto dto) {
        ... ...
    }
}

在启动Tomcat时报错如下：

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.test.service.EmployeeService 
com.test.controller.EmployeeInfoControl.employeeService; nested exception is 
org.springframework.beans.factory.NoSuchBeanDefinitionException: No unique bean of type
 [com.test.service.EmployeeService] is defined: expected single matching bean but found 
2: [service1, service2]

其实报错信息已经说得很清楚了，在@autowired时，由于有两个类实现了EmployeeService接口，所以Spring不知道应该具体绑定那个实现类，所以抛出了如上错误。

这个时候就该用到@Qualifier注解了，被@Qualifier标注的意为合格者，通过标识才知道那个是我们所需要的，我们修改调用代码，添加@Qualifier注解：

@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) {
       ...  ...
    }
}

Spring注解之@Qualifier

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