SpringBoot Bean加载顺序控制

Bean初始化时机

Bean初始化的两种方式

在Spring容器中,Bean的初始化有两个时机:

方式一:容器主动初始化

Spring容器启动时扫描到Bean定义,主动创建Bean实例。

@Component
public class LogService {
    public LogService() {
        System.out.println("LogService初始化");
    }
}

方式二:依赖触发初始化

其他Bean依赖该Bean时,容器会先初始化被依赖的Bean。

@Component
public class OrderService {
    
    @Autowired
    private InventoryService inventoryService;  // 触发InventoryService初始化
    
    public OrderService() {
        System.out.println("OrderService初始化");
    }
}

初始化顺序:

InventoryService初始化
OrderService初始化

方式一:直接依赖控制

通过构造器依赖

最直接的方式是在构造器中注入依赖:

@Component
public class PaymentService {
    
    private final NotificationService notificationService;
    
    // NotificationService会先于PaymentService初始化
    public PaymentService(NotificationService notificationService) {
        this.notificationService = notificationService;
        System.out.println("PaymentService初始化");
    }
}

@Component
public class NotificationService {
    public NotificationService() {
        System.out.println("NotificationService初始化");
    }
}

输出顺序:

NotificationService初始化
PaymentService初始化

优点:

• 依赖关系明确
• 支持final字段
• 便于单元测试

通过字段依赖

使用@Autowired注解字段:

@Component
public class OrderService {
    
    @Autowired
    private ProductService productService;  // ProductService先初始化
    
    @Autowired
    private PriceService priceService;  // PriceService先初始化
}

注意:多个字段依赖的初始化顺序是不确定的。

方式二:@DependsOn注解

适用场景

当无法修改外部库代码,或需要显式声明依赖关系时,使用@DependsOn。

@Bean方法上使用

@Configuration
public class CacheConfiguration {
    
    @Bean
    public RedisTemplate<String, Object> redisTemplate(RedisConnectionFactory factory) {
        System.out.println("创建RedisTemplate");
        RedisTemplate<String, Object> template = new RedisTemplate<>();
        template.setConnectionFactory(factory);
        return template;
    }
    
    @Bean
    @DependsOn("redisTemplate")  // 确保RedisTemplate先初始化
    public CacheManager cacheManager(RedisConnectionFactory factory) {
        System.out.println("创建CacheManager");
        RedisCacheManager cacheManager = RedisCacheManager.builder(factory)
            .cacheDefaults(RedisCacheConfiguration.defaultCacheConfig())
            .build();
        return cacheManager;
    }
}

输出顺序:

创建RedisTemplate
创建CacheManager

@Component类上使用

@Component
public class DataSourceInitializer {
    public DataSourceInitializer() {
        System.out.println("初始化数据源配置");
    }
}

@Component
@DependsOn("dataSourceInitializer")  // 指定依赖的Bean名称
public class DatabaseMigration {
    public DatabaseMigration() {
        System.out.println("执行数据库迁移");
    }
}

多个依赖

@Component
@DependsOn({"configLoader", "licenseValidator", "systemChecker"})
public class ApplicationBootstrap {
    public ApplicationBootstrap() {
        System.out.println("应用启动完成");
    }
}

初始化顺序:

configLoader初始化
licenseValidator初始化  
systemChecker初始化
ApplicationBootstrap初始化

注意事项

循环依赖会报错:

@Component
@DependsOn("serviceB")
public class ServiceA {
}

@Component
@DependsOn("serviceA")  // 循环依赖!
public class ServiceB {
}

错误信息:

Circular depends-on relationship between 'serviceA' and 'serviceB'

方式三:BeanFactoryPostProcessor

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