使用Autowired和Qualifier解决多个相同类型的bean如何共存的问题

╰半橙微兮° 2022-05-25 09:42 223阅读 0赞

@Autowired是根据类型进行自动装配的。如果当spring上下文中存在不止一个A类型的bean时，就会抛出BeanCreationException异常;如果Spring上下文中不存在A类型的bean，而且我们又使用A类型，也会抛出BeanCreationException异常。

针对存在多个A类型的Bean，我们可以联合使用@Qualifier和@Autowired来解决这些问题。  
英文文档在[这里][Link 1]

例如

Caused by: org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type 'com.yq.demo.service.UserService' available: expected single matching bean but found 2: myUserServiceImpl,userServiceImpl
            at org.springframework.beans.factory.config.DependencyDescriptor.resolveNotUnique(DependencyDescriptor.java:173)
            at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1116)
            at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1066)

我们的service接口是

public interface UserService {
        public long userCount();
    
        public String showSpecificClassName();
    
        public List<User> getAllUser();
    
        public User addUser(User user);
    
        public void delUser(Integer id);
    
        public User updateUser(User user);
    
        public User getUserByName(String username);
    
        public User getUserByID(Integer id);
    }

我们有两个实现类，分别是MyUserServiceImpl和UserServiceImpl

@Service
    public class UserServiceImpl implements UserService { 
        @Autowired
        private UserJpaRepository userJpaRepo;
    
        private static final Logger log = LoggerFactory.getLogger(UserServiceImpl.class);
        @Override
        public long userCount(){
            return userJpaRepo.count();
        }
    
        @Override
        public String showSpecificClassName() {
            String clsName = this.getClass().getCanonicalName();
            log.info("impl class is " + clsName);
            return clsName;
        }
        ...

@Service
    public class MyUserServiceImpl implements UserService { 
        @Autowired
        private UserJpaRepository userJpaRepo;
    
        private static final Logger log = LoggerFactory.getLogger(MyUserServiceImpl.class);
        @Override
        public long userCount(){
            return userJpaRepo.count();
        }
    
        @Override
        public String showSpecificClassName() {
            String clsName = this.getClass().getCanonicalName();
            log.info("impl class is " + clsName);
            return clsName;
        }

然后我们在其他地方，如controller中这样写

@Controller   
    @RequestMapping(path="/user") 
    public class UserController { 
        @Autowired
        private UserService userService;
    
        @GetMapping(value = "/testAutowired", produces = "application/json;charset=UTF-8")
        public @ResponseBody String testAutowired (@RequestParam String name) {
            String name1 = userService.showSpecificClassName();
            return name1;
        }

当我们启动程序时就会报错，因为有两个UserService的实现类。程序不知道该选择那个。 这时可以使用Qualifier 来解决唯一性问题

@Autowired
        @Qualifier("userServiceImpl")
        private UserService userService;

具体代码在[这里][Link 2]，欢迎加星和fork。

[Link 1]: https://docs.spring.io/spring-framework/docs/5.0.1.RELEASE/spring-framework-reference/core.html#beans-autowired-annotation-qualifiers
[Link 2]: https://github.com/yqbjtu/springbootJpa.git