限定注入

使用注解@Qualifier限定

通过Bean名称限定

	@Autowired
    @Qualifier("user") // 指定 Bean 名称或 ID
    private User namedUser;

通过分组限定

使用@Qualifier进行逻辑分组，@Qualifier与非@Qualifier形成两个分组

	@Autowired
    private Collection<User> allUsers; // 2 Beans = user + superUser

    @Autowired
    @Qualifier
    private Collection<User> qualifiedUsers; // 2 Beans = user1 + user2

    @Bean
    @Qualifier // 进行逻辑分组
    public User user1() {
        return createUser(7L);
    }

    @Bean
    @Qualifier // 进行逻辑分组
    public static User user2() {
        return createUser(8L);

    }

基于@Qualifier扩展限定

• 自定义注解 - 如 Spring Cloud @LoadBalanced
• 自定义扩展限定分组具有继承性

@Target({ElementType.FIELD, ElementType.METHOD, ElementType.PARAMETER, ElementType.TYPE, ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Inherited
@Documented
@Qualifier
public @interface UserGroup {
}

	@Autowired
    private Collection<User> allUsers; // 2 Beans = user + superUser

    @Autowired
    @Qualifier
    private Collection<User> qualifiedUsers; // 2 Beans = user1 + user2 -> 4 Beans = user1 + user2 + user3 + user4

    @Autowired
    @UserGroup
    private Collection<User> groupedUsers; // 2 Beans = user3 + user4

    @Bean
    @Qualifier // 进行逻辑分组
    public User user1() {
        return createUser(7L);
    }

    @Bean
    @Qualifier // 进行逻辑分组
    public static User user2() {
        return createUser(8L);

    }

    @Bean
    @UserGroup
    public static User user3() {
        return createUser(9L);
    }

    @Bean
    @UserGroup
    public static User user4() {
        return createUser(10L);
    }

    private static User createUser(Long id) {
        User user = new User();
        user.setId(id);
        return user;
    }

延迟依赖注入

public class LazyAnnotationDependencyInjectionDemo {

    @Autowired
    @Qualifier("user")
    private User user; // 实时注入

    @Autowired
    private ObjectProvider<User> userObjectProvider; // 延迟注入

    @Autowired
    private ObjectFactory<Set<User>> usersObjectFactory;

    public static void main(String[] args) {

        // 创建 BeanFactory 容器
        AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext();
        // 注册 Configuration Class（配置类） -> Spring Bean
        applicationContext.register(LazyAnnotationDependencyInjectionDemo.class);

        XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(applicationContext);

        String xmlResourcePath = "classpath:/META-INF/dependency-lookup-context.xml";
        // 加载 XML 资源，解析并且生成 BeanDefinition
        beanDefinitionReader.loadBeanDefinitions(xmlResourcePath);

        // 启动 Spring 应用上下文
        applicationContext.refresh();

        // 依赖查找 QualifierAnnotationDependencyInjectionDemo Bean
        LazyAnnotationDependencyInjectionDemo demo = applicationContext.getBean(LazyAnnotationDependencyInjectionDemo.class);

        // 期待输出 superUser Bean
        System.out.println("demo.user = " + demo.user);
        // 期待输出 superUser Bean
        System.out.println("demo.userObjectProvider = " + demo.userObjectProvider.getObject()); // 继承 ObjectFactory
        // 期待输出 superUser user Beans
        System.out.println("demo.usersObjectFactory = " + demo.usersObjectFactory.getObject());

        demo.userObjectProvider.forEach(System.out::println);


        // 显示地关闭 Spring 应用上下文
        applicationContext.close();
    }

}