# @Qualifier的作用
###### tags: `SpringMVC-會用到的註解`

Spring 使用@Autowired注入的介面有多個@Bean類時，可以搭配@Qualifier控制要注入哪個實例。

#### 介面

public interface BookService {
    //增加一本書
    int addBook(Books books);
    //刪除一本書
    int deleteBookByID(int id);
    //更新一本書
    int updateBook(Books books);
    //查詢一本書
    Books queryBookById(int id);
    //查詢全部的書
    List<Books> queryAllBook();
}

#### 引用BookService介面的BookServiceImpl

public class BookServiceImpl implements BookService{

    //service調dao層：組合dao層

    private BookMapper bookMapper;

    public void setBookMapper(BookMapper bookMapper) {
        this.bookMapper = bookMapper;
    }

    @Override
    public int addBook(Books books) {
        return bookMapper.addBook(books);
    }

    @Override
    public int deleteBookByID(int id) {
        return bookMapper.deleteBookByID(id);
    }

    @Override
    public int updateBook(Books books) {
        return bookMapper.updateBook(books);
    }

    @Override
    public Books queryBookById(int id) {
        return bookMapper.queryBookById(id);
    }

    @Override
    public List<Books> queryAllBook() {
        return bookMapper.queryAllBook();
    }
}

#### controller

@Controller
@RequestMapping("/book")
public class BookController {
    //controller調 service層
    @Autowired
    @Qualifier("BookServiceImpl")
    private BookService bookService;
}

### 問題：當有複數的impl引用service層的BookService來實現，控制層BookController在使用@Autowired，該如何判斷使用哪個impl來進行注入？

### 答案：利用@Qualifier來決定注入的impl

### 相關閱讀
[@Autowired的用法](https://hackmd.io/mAVPK0IjSa6Ge19t86FsIA)