BeanDefinitionRegistryPostProcessor

BeanDefinitionRegistryPostProcessor扩展了BeanFactoryPostProcessor，提供Bean定义注册能力。

接口定义

public interface BeanDefinitionRegistryPostProcessor extends BeanFactoryPostProcessor {
    // 注册BeanDefinition
    void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry)
            throws BeansException;
}

执行顺序

┌─────────────────────────────────────┐
│  BeanDefinitionRegistryPostProcessor │
│  .postProcessBeanDefinitionRegistry  │ ← 最先执行
├─────────────────────────────────────┤
│  BeanFactoryPostProcessor            │
│  .postProcessBeanFactory             │ ← 后执行
├─────────────────────────────────────┤
│  Bean实例化                          │
└─────────────────────────────────────┘

动态注册BeanDefinition

@Component
public class DynamicBeanRegistrar implements BeanDefinitionRegistryPostProcessor {

    @Override
    public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) {
        // 注册多个BeanDefinition
        for (int i = 0; i < 5; i++) {
            GenericBeanDefinition bd = new GenericBeanDefinition();
            bd.setBeanClass(Worker.class);
            bd.setScope("singleton");
            bd.getConstructorArgumentValues()
                .addGenericArgumentValue("worker-" + i);

            registry.registerBeanDefinition("worker-" + i, bd);
        }
        System.out.println("注册5个Worker Bean");
    }

    @Override
    public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {
        // 可在此修改已注册的BeanDefinition
    }
}

条件性注册

@Component
public class ConditionalBeanRegistrar implements BeanDefinitionRegistryPostProcessor {

    @Override
    public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) {
        // 检查环境变量决定是否注册
        if (System.getenv("ENABLE_CACHE") != null) {
            registerCacheBeans(registry);
        }

        // 检查配置决定注册策略
        String strategy = System.getProperty("db.strategy", "mysql");
        registerDatabaseStrategy(registry, strategy);
    }

    private void registerCacheBeans(BeanDefinitionRegistry registry) {
        GenericBeanDefinition bd = new GenericBeanDefinition();
        bd.setBeanClassName("com.example.RedisCacheService");
        registry.registerBeanDefinition("cacheService", bd);
    }

    private void registerDatabaseStrategy(BeanDefinitionRegistry registry,
                                          String strategy) {
        String className = "com.example." + strategy + "DataSource";
        GenericBeanDefinition bd = new GenericBeanDefinition();
        bd.setBeanClassName(className);
        registry.registerBeanDefinition("dataSource", bd);
    }
}

BeanDefinitionRegistry API

// 注册BeanDefinition
void registerBeanDefinition(String beanName, BeanDefinition beanDefinition);

// 移除BeanDefinition
void removeBeanDefinition(String beanName);

// 获取BeanDefinition
BeanDefinition getBeanDefinition(String beanName);

// 判断是否包含
boolean containsBeanDefinition(String beanName);

// 获取BeanDefinition数量
int getBeanDefinitionCount();

// 获取所有Bean名称
String[] getBeanDefinitionNames();

扫描并注册

@Component
public class ComponentScannerRegistrar implements BeanDefinitionRegistryPostProcessor {

    @Override
    public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) {
        // 自定义扫描逻辑
        ClassPathBeanDefinitionScanner scanner =
            new ClassPathBeanDefinitionScanner(registry);

        // 添加过滤器
        scanner.addIncludeFilter(
            new AnnotationTypeFilter(MyComponent.class));

        // 扫描指定包
        scanner.scan("com.example.plugins");
    }
}

ImportBeanDefinitionRegistrar对比

// 通过@Import方式注册
@Import(MyRegistrar.class)
@Configuration
public class AppConfig { }

public class MyRegistrar implements ImportBeanDefinitionRegistrar {

    @Override
    public void registerBeanDefinitions(AnnotationMetadata metadata,
            BeanDefinitionRegistry registry) {
        // 注册BeanDefinition
        GenericBeanDefinition bd = new GenericBeanDefinition();
        bd.setBeanClass(MyService.class);
        registry.registerBeanDefinition("myService", bd);
    }
}

动态代理Bean注册

@Component
public class RpcServiceRegistrar implements BeanDefinitionRegistryPostProcessor {

    @Override
    public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) {
        // 扫描RPC接口
        ClassPathScanningCandidateComponentProvider scanner =
            new ClassPathScanningCandidateComponentProvider(false);
        scanner.addIncludeFilter(new AnnotationTypeFilter(RpcService.class));

        for (BeanDefinition bd : scanner.findCandidateComponents("com.example.rpc")) {
            String className = bd.getBeanClassName();

            // 创建FactoryBean定义
            GenericBeanDefinition factoryBd = new GenericBeanDefinition();
            factoryBd.setBeanClass(RpcServiceFactoryBean.class);
            factoryBd.getConstructorArgumentValues()
                .addGenericArgumentValue(className);

            String beanName = generateBeanName(className);
            registry.registerBeanDefinition(beanName, factoryBd);
        }
    }
}

与BeanFactoryPostProcessor对比

BeanDefinitionRegistryPostProcessor优先级更高，可注册新Bean。

要点总结

• 继承BeanFactoryPostProcessor，优先执行
• postProcessBeanDefinitionRegistry用于注册Bean
• postProcessBeanFactory用于修改已注册Bean
• 执行顺序在标准BeanFactoryPostProcessor之前
• 适合动态注册大量Bean的场景