@Autowired,@Resource,@Qualifier,@Primary的作用和区别

@Autowired注解的用法:可以用于构造器,方法,参数,字段进行属性注入,有一个required属性,默认是true,当改成false时,如果注入的属性在容器中不存在也不会报错
@Resource该注解来源于javax包,有个属性name可以指定注入哪一个bean,当容器中某个类存在2个bean时,该注解就非常有用了,但没有required属性
@Qualifier和@Autowired搭配使用,可以实现@Resource指定注入某个bean的功能,当某个类存在2个bean时,这样也是一种解决方案
@Primary 代表优先使用某个类,例如UserDAO向spring注入2个bean,dao1,和dao2;如果注入dao1时加了@Primary 注解,则,其他类注入UserDAO会优先注入dao1

public class UserDAO {
    private String id="1";

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }
}
@Configuration
@Import({UserService.class})
public class AppConfigurate {
    @Bean("userDAO1")
    public UserDAO userDAO(){
        UserDAO userDAO = new UserDAO();
        userDAO.setId("userDAO1");
        return userDAO;
    }
    @Bean("userDAO2")
    public UserDAO userDAO2(){
        UserDAO userDAO = new UserDAO();
        userDAO.setId("userDAO2");
        return userDAO;
    }
}
public class UserService {
    @Autowired
    private UserDAO userDAO;

    public void sayHi(){
        System.out.println(userDAO.getId());
    }

}
public class APP {
    public static void main(String[] args) {
        ApplicationContext ctx = new AnnotationConfigApplicationContext(AppConfigurate.class);
        UserService userService =(UserService) ctx.getBean("com.yang.xiao.config.UserService");
        userService.sayHi();
        String[] beanDefinitionNames = ctx.getBeanDefinitionNames();
        for (String definitionName : beanDefinitionNames) {
            System.out.println(definitionName);
        }

    }
}

上面代码直接使用@Autowired注入UserDAO,执行后会报错,因为spring区分不了要注入哪个bean;解决方案如下:

//1.Autowired注入的属性优先按照名字去匹配,名字匹配不到才会按照类型去匹配
public class UserService {
    @Autowired
    private UserDAO userDAO1;

    public void sayHi(){
        System.out.println(userDAO1.getId());
    }

}

//2.@Qualifier("userDAO2") 使用该注解去指定
public class UserService {
    @Autowired
    @Qualifier("userDAO2")
    private UserDAO userDAO;

    public void sayHi(){
        System.out.println(userDAO.getId());
    }

}

//3.使用@Primary去指定优先级
@Configuration
@Import({UserService.class})
public class AppConfigurate {
    @Bean("userDAO1")
    @Primary
    public UserDAO userDAO(){
        UserDAO userDAO = new UserDAO();
        userDAO.setId("userDAO1");
        return userDAO;
    }
    @Bean("userDAO2")
    public UserDAO userDAO2(){
        UserDAO userDAO = new UserDAO();
        userDAO.setId("userDAO2");
        return userDAO;
    }
}

//4.使用@Resource注解
public class UserService {
    @Resource(name = "userDAO2")
    private UserDAO userDAO;

    public void sayHi(){
        System.out.println(userDAO.getId());
    }

}