在使用spring时,有时候有会有一些自定义annotation的需求,比如一些Listener的回调函数。
比如:
@Service public class MyService { @MyListener public void onMessage(Message msg){ } }
一开始的时候,我是在Spring的 ContextRefreshedEvent
事件里,通过 context.getBeansWithAnnotation(Component.class)
来获取到所有的bean,然后再检查method是否有 @MyListener
的annotation。
后来发现这个方法有缺陷,当有一些spring bean的 @Scope
设置为session/request时,创建bean会失败。
参考:
http://docs.spring.io/spring/docs/current/spring-framework-reference/htmlsingle/#beans-factory-scopes
在网上搜索了一些资料,发现不少人都是用 context.getBeansWithAnnotation(Component.class)
,这样子来做的,但是这个方法并不对。
后来看了下spring jms里的 @JmsListener
的实现,发现实现 BeanPostProcessor
接口才是最合理的办法。
public interface BeanPostProcessor { /** * Apply this BeanPostProcessor to the given new bean instance <i>before</i> any bean * initialization callbacks (like InitializingBean's {@code afterPropertiesSet} * or a custom init-method). The bean will already be populated with property values. * The returned bean instance may be a wrapper around the original. * @param bean the new bean instance * @param beanName the name of the bean * @return the bean instance to use, either the original or a wrapped one; * if {@code null}, no subsequent BeanPostProcessors will be invoked * @throws org.springframework.beans.BeansException in case of errors * @see org.springframework.beans.factory.InitializingBean#afterPropertiesSet */ Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException; /** * Apply this BeanPostProcessor to the given new bean instance <i>after</i> any bean * initialization callbacks (like InitializingBean's {@code afterPropertiesSet} * or a custom init-method). The bean will already be populated with property values. * The returned bean instance may be a wrapper around the original. * <p>In case of a FactoryBean, this callback will be invoked for both the FactoryBean * instance and the objects created by the FactoryBean (as of Spring 2.0). The * post-processor can decide whether to apply to either the FactoryBean or created * objects or both through corresponding {@code bean instanceof FactoryBean} checks. * <p>This callback will also be invoked after a short-circuiting triggered by a * {@link InstantiationAwareBeanPostProcessor#postProcessBeforeInstantiation} method, * in contrast to all other BeanPostProcessor callbacks. * @param bean the new bean instance * @param beanName the name of the bean * @return the bean instance to use, either the original or a wrapped one; * if {@code null}, no subsequent BeanPostProcessors will be invoked * @throws org.springframework.beans.BeansException in case of errors * @see org.springframework.beans.factory.InitializingBean#afterPropertiesSet * @see org.springframework.beans.factory.FactoryBean */ Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException; }
所有的bean在创建完之后,都会回调 postProcessAfterInitialization
函数,这时就可以确定bean是已经创建好的了。
所以扫描自定义的annotation的代码大概是这个样子的:
public class MyListenerProcessor implements BeanPostProcessor { @Override public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException { return bean; } @Override public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException { Method[] methods = ReflectionUtils.getAllDeclaredMethods(bean.getClass()); if (methods != null) { for (Method method : methods) { MyListener myListener = AnnotationUtils.findAnnotation(method, MyListener.class); // process } } return bean; } }
看spring jms的代码时,发现SmartInitializingSingleton 这个接口也比较有意思。
就是当所有的singleton的bean都初始化完了之后才会回调这个接口。不过要注意是 4.1 之后才出现的接口。
public interface SmartInitializingSingleton { /** * Invoked right at the end of the singleton pre-instantiation phase, * with a guarantee that all regular singleton beans have been created * already. {@link ListableBeanFactory#getBeansOfType} calls within * this method won't trigger accidental side effects during bootstrap. * <p><b>NOTE:</b> This callback won't be triggered for singleton beans * lazily initialized on demand after {@link BeanFactory} bootstrap, * and not for any other bean scope either. Carefully use it for beans * with the intended bootstrap semantics only. */ void afterSingletonsInstantiated(); }
https://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/beans/factory/SmartInitializingSingleton.html