@Qualifier使用及详细源码展示

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

    /**
     * 限定符名称（用于匹配 Bean 的名称或自定义限定符）
     */
    String value() default "";
}

public interface PaymentService {
    void pay(Double amount);
}

@Service("alipayService") // 指定 Bean 名称为 "alipayService"
public class AlipayService implements PaymentService {
    @Override
    public void pay(Double amount) {
        System.out.println("支付宝支付：" + amount);
    }
}

@Service("wechatPayService") // 指定 Bean 名称为 "wechatPayService"
public class WechatPayService implements PaymentService {
    @Override
    public void pay(Double amount) {
        System.out.println("微信支付：" + amount);
    }
}

@Service
public class OrderService {

    private final PaymentService paymentService;

    // 通过 @Qualifier 指定注入 "alipayService"
    @Autowired
    public OrderService(@Qualifier("alipayService") PaymentService paymentService) {
        this.paymentService = paymentService;
    }

    public void createOrder(Double amount) {
        paymentService.pay(amount); // 输出：支付宝支付：100.0
    }
}

// 方式 1：通过 @Qualifier 注解标记（需配合 @Bean）
@Qualifier("domesticPay") // 自定义限定符名称
@Service
public class DomesticPaymentService implements PaymentService {
    @Override
    public void pay(Double amount) {
        System.out.println("国内支付：" + amount);
    }
}

// 方式 2：在 @Bean 中直接指定 value（更常见）
@Configuration
public class PaymentConfig {

    @Bean("overseasPay") // 自定义限定符名称
    public PaymentService overseasPaymentService() {
        return new PaymentService() {
            @Override
            public void pay(Double amount) {
                System.out.println("海外支付：" + amount);
            }
        };
    }
}

@Service
public class InternationalOrderService {

    private final PaymentService overseasPaymentService;

    @Autowired
    public InternationalOrderService(@Qualifier("overseasPay") PaymentService paymentService) {
        this.overseasPaymentService = paymentService;
    }

    public void createInternationalOrder(Double amount) {
        overseasPaymentService.pay(amount); // 输出：海外支付：200.0
    }
}

@Service
public class OptionalPaymentService {

    private final PaymentService optionalPaymentService;

    // required = false，无匹配 Bean 时注入 null
    @Autowired(required = false)
    public OptionalPaymentService(@Qualifier("optionalPay") PaymentService optionalPaymentService) {
        this.optionalPaymentService = optionalPaymentService;
    }

    public void optionalPay(Double amount) {
        if (optionalPaymentService != null) {
            optionalPaymentService.pay(amount);
        } else {
            System.out.println("无可用支付服务");
        }
    }
}