Java基础教程（247）IoC容器之定制Bean：掌控Bean的每一个细节，深度剖析Java IoC容器定制之道

在Spring框架中，IoC容器不仅是依赖注入的引擎，更是Bean生命周期的精密控制系统。本文将深入探讨如何定制Bean的各个阶段，并通过代码示例展示实际应用。

1. Bean生命周期定制

通过实现InitializingBean和DisposableBean接口，我们可以介入Bean的初始化和销毁阶段：

public class DatabaseService implements InitializingBean, DisposableBean {
    private Connection connection;
    
    @Override
    public void afterPropertiesSet() throws Exception {
        this.connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/mydb");
        System.out.println("数据库连接已建立");
    }
    
    @Override
    public void destroy() throws Exception {
        if (connection != null && !connection.isClosed()) {
            connection.close();
            System.out.println("数据库连接已关闭");
        }
    }
}

更现代的做法是使用注解：

@Component
public class CacheManager {
    @PostConstruct
    public void initCache() {
        System.out.println("缓存预热完成");
    }
    
    @PreDestroy
    public void clearCache() {
        System.out.println("缓存已清空");
    }
}

2. 作用域扩展与自定义

除了默认的单例和原型作用域，Spring支持自定义作用域：

@Configuration
public class ThreadScopeConfig implements BeanFactoryPostProcessor {
    @Override
    public void postProcessBeanFactory(ConfigurableListableBeanFactory factory) {
        factory.registerScope("thread", new SimpleThreadScope());
    }
}

@Component
@Scope("thread")
public class UserContext {
    private final ThreadLocal<User> currentUser = new ThreadLocal<>();
}

3. 条件化Bean注册

通过@Conditional注解实现基于条件的Bean创建：

@Configuration
public class ConditionConfig {
    @Bean
    @Conditional(DevEnvironmentCondition.class)
    public DataSource devDataSource() {
        return new EmbeddedDatabaseBuilder().build();
    }
    
    @Bean
    @Conditional(ProdEnvironmentCondition.class)
    public DataSource prodDataSource() {
        return new MysqlDataSource();
    }
}

public class DevEnvironmentCondition implements Condition {
    @Override
    public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
        return "dev".equals(context.getEnvironment().getProperty("app.env"));
    }
}

4. Bean后处理器深度定制

BeanPostProcessor提供了Bean初始化前后的钩子：

@Component
public class ValidationBeanPostProcessor implements BeanPostProcessor {
    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName) {
        if (bean instanceof Validatable) {
            ((Validatable) bean).validate();
        }
        return bean;
    }
    
    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName) {
        System.out.println("Bean " + beanName + " 初始化完成");
        return bean;
    }
}

5. 综合示例：定制缓存管理器

@Configuration
@ConditionalOnProperty(name = "cache.enabled", havingValue = "true")
public class CacheConfig {
    
    @Bean(initMethod = "init", destroyMethod = "shutdown")
    @Scope("singleton")
    public CacheManager cacheManager(Environment env) {
        return new GuavaCacheManager(env.getProperty("cache.spec"));
    }
}

public class GuavaCacheManager {
    private Cache<Object, Object> cache;
    
    public void init() {
        this.cache = CacheBuilder.from(spec).build();
    }
    
    public void shutdown() {
        cache.cleanUp();
    }
    
    @PostConstruct
    public void logInitialization() {
        System.out.println("缓存管理器初始化完成");
    }
}

总结

通过精细控制Bean的生命周期、作用域和创建条件，开发者可以构建高度灵活和可维护的Spring应用。掌握这些定制技术不仅能够解决复杂业务场景下的特定需求，更能提升应用程序的性能和稳定性。