依赖注入

依赖注入（Dependency Injection，DI）是IoC的具体实现方式，容器自动将依赖对象注入到目标对象。

三种注入方式

构造器注入（推荐）

@Service
public class OrderService {

    private final UserService userService;
    private final PaymentService paymentService;

    // 构造器注入，依赖明确且不可变
    @Autowired
    public OrderService(UserService userService, PaymentService paymentService) {
        this.userService = userService;
        this.paymentService = paymentService;
    }
}

Setter注入

@Service
public class OrderService {

    private UserService userService;

    // Setter注入，适合可选依赖
    @Autowired
    public void setUserService(UserService userService) {
        this.userService = userService;
    }
}

字段注入（不推荐）

@Service
public class OrderService {

    @Autowired
    private UserService userService; // 字段注入，依赖隐藏
}

注入方式对比

注解详解

@Autowired

@Service
public class OrderService {

    // 按类型注入
    @Autowired
    private UserService userService;

    // 可选依赖
    @Autowired(required = false)
    private OptionalService optionalService;

    // 构造器注入（Spring 4.3+ 单构造器可省略@Autowired）
    public OrderService(UserService userService) {
        this.userService = userService;
    }
}

@Resource

@Service
public class OrderService {

    // 按名称注入
    @Resource
    private UserService userService;

    // 指定Bean名称
    @Resource(name = "primaryService")
    private PaymentService paymentService;
}

@Autowired vs @Resource

多候选Bean处理

@Qualifier

@Service
public class OrderService {

    @Autowired
    @Qualifier("primaryPaymentService")
    private PaymentService paymentService;
}

@Primary

@Component
@Primary  // 默认优先注入
public class PrimaryPaymentService implements PaymentService {
}

@Component
public class BackupPaymentService implements PaymentService {
}

集合注入

@Service
public class OrderService {

    // 注入所有同类型Bean
    @Autowired
    private List<PaymentStrategy> strategies;

    // 注入Map，key为Bean名称
    @Autowired
    private Map<String, PaymentStrategy> strategyMap;
}

XML配置注入

<bean id="orderService" class="com.example.OrderService">
    <!-- 构造器注入 -->
    <constructor-arg ref="userService"/>
    <constructor-arg ref="paymentService"/>

    <!-- Setter注入 -->
    <property name="userService" ref="userService"/>
    <property name="timeout" value="30"/>
</bean>

要点总结

1. DI是IoC的具体实现，容器负责注入依赖
2. 三种注入方式：构造器、Setter、字段
3. 推荐构造器注入，依赖明确且便于测试
4. @Autowired按类型，@Resource按名称
5. 多候选Bean使用@Qualifier或@Primary指定