Spring Boot @Enable注解与@Import原理

@Enable* 注解是 Spring 模块化配置的核心机制。

@Enable注解示例

@EnableScheduling      // 开启定时任务
@EnableAsync           // 开启异步
@EnableCaching         // 开启缓存
@EnableTransactionManagement  // 开启事务

@Import三种使用方式

1. 导入普通类

@Import(UserService.class)
@Configuration
public class AppConfig {
    // UserService被注册为Bean
}

2. 导入ImportSelector

public class MyImportSelector implements ImportSelector {

    @Override
    public String[] selectImports(AnnotationMetadata metadata) {
        // 根据条件返回要导入的类名
        return new String[]{
            "com.example.UserService",
            "com.example.OrderService"
        };
    }
}

@Import(MyImportSelector.class)
@Configuration
public class AppConfig {
}

3. 导入ImportBeanDefinitionRegistrar

public class MyRegistrar implements ImportBeanDefinitionRegistrar {

    @Override
    public void registerBeanDefinitions(
            AnnotationMetadata metadata,
            BeanDefinitionRegistry registry) {

        // 动态注册Bean
        BeanDefinitionBuilder builder = BeanDefinitionBuilder
            .genericBeanDefinition(UserService.class);
        builder.addPropertyValue("name", "custom");

        registry.registerBeanDefinition("userService",
            builder.getBeanDefinition());
    }
}

@Import(MyRegistrar.class)
@Configuration
public class AppConfig {
}

自定义@Enable注解

定义注解

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Import(EnableMyFeatureRegistrar.class)
public @interface EnableMyFeature {
    String[] basePackages() default {};
    boolean proxyTargetClass() default false;
}

实现Registrar

public class EnableMyFeatureRegistrar
        implements ImportBeanDefinitionRegistrar {

    @Override
    public void registerBeanDefinitions(
            AnnotationMetadata metadata,
            BeanDefinitionRegistry registry) {

        // 获取注解属性
        Map<String, Object> attrs = metadata.getAnnotationAttributes(
            EnableMyFeature.class.getName());

        String[] packages = (String[]) attrs.get("basePackages");

        // 注册扫描器
        ClassPathBeanDefinitionScanner scanner =
            new ClassPathBeanDefinitionScanner(registry);
        scanner.scan(packages);
    }
}

使用注解

@SpringBootApplication
@EnableMyFeature(basePackages = "com.example.service")
public class Application {
    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }
}

@Import源码解析

// ImportSelector调用链
ConfigurationClassParser#processImports
  → ImportSelector#selectImports
  → 返回类名数组
  → 递归处理返回的类

// ImportBeanDefinitionRegistrar调用链
ConfigurationClassParser#processImports
  → 存储Registrar实例
  → ConfigurationClassPostProcessor#processConfigBeanDefinitions
  → Registrar#registerBeanDefinitions

ImportSelector高级用法

public class ConditionalImportSelector implements ImportSelector {

    @Override
    public String[] selectImports(AnnotationMetadata metadata) {
        // 判断环境
        String env = System.getProperty("spring.profiles.active");

        if ("prod".equals(env)) {
            return new String[]{"com.example.ProdConfig"};
        } else {
            return new String[]{"com.example.DevConfig"};
        }
    }

    @Override
    public Predicate<String> getExclusionFilter() {
        // 排除特定类
        return className -> className.contains("internal");
    }
}

对比总结

@Import优先级高于@ComponentScan，在配置类解析阶段执行。

要点总结

• @Enable*注解本质是@Import的封装
• ImportSelector适合条件性导入
• Registrar适合动态注册Bean定义
• 掌握原理可实现模块化Starter