正确实现用spring扫描自定义的annotation

背景

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。

BeanPostProcessor接口

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;
	}
}

SmartInitializingSingleton 接口

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