Spring全家桶笔记八：依赖注入与Bean管理

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Spring全家桶笔记八：依赖注入与Bean管理

依赖注入方式

构造器注入

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@Service
public class OrderService {
    private final UserService userService;
    private final OrderDao orderDao;
    
    public OrderService(UserService userService, OrderDao orderDao) {
        this.userService = userService;
        this.orderDao = orderDao;
    }
}

Setter注入

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@Service
public class PaymentService {
    private PaymentGateway paymentGateway;
    
    @Autowired
    public void setPaymentGateway(PaymentGateway paymentGateway) {
        this.paymentGateway = paymentGateway;
    }
}

字段注入

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@Service
public class OrderService {
    @Autowired
    private UserService userService;
}

Bean作用域

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@Service
@Scope("prototype")  // 多例
public class MyService {}

@Service
@Scope("request")   // 请求作用域
public class RequestService {}

@Conditional条件装配

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@ConditionalOnProperty(name = "feature.enabled", havingValue = "true")
@Service
public class FeatureService {}

@ConditionalOnMissingBean(UserService.class)
@Bean
public UserService userService() {
    return new UserService();
}

@ConditionalOnClass(RedisTemplate.class)
@Configuration
public class RedisConfig {}

Bean生命周期

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@Component
public class MyBean implements InitializingBean, DisposableBean {
    
    @PostConstruct
    public void init() {
        System.out.println("初始化");
    }
    
    @PreDestroy
    public void destroy() {
        System.out.println("销毁");
    }
    
    @Override
    public void afterPropertiesSet() {}
    
    @Override
    public void destroy() {}
}

外部化配置

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@Component
public class MyComponent {
    @Value("${custom.property}")
    private String property;
    
    @Value("${db.url:localhost}")
    private String dbUrl;
    
    @Value("#{systemProperties['user.dir']}")
    private String userDir;
}

总结

依赖注入是Spring核心特性，合理选择注入方式、配置Bean作用域、条件装配等是构建健壮应用的关键。