书接上文,前面在 [Spring 应用合并之路(一):摸石头过河]介绍了几种不成功的经验,下面继续折腾…
在经历了上面的尝试,在同事为啥不搞两个独立的容器提醒下,决定抛开 Spring Boot 内置的父子容器方案,完全自己实现父子容器。
web
项目?现在的难题只有一个:如何加载 web
项目?加载完成后,如何持续持有 web
项目?经过思考后,可以创建一个 boot
项目的 Spring Bean,在该 Bean 中加载并持有 web
项目的容器。由于 Spring Bean 默认是单例的,并且会伴随 Spring 容器长期存活,就可以保证 web
容器持久存活。结合 Spring 扩展点概览及实践 中介绍的 Spring 扩展点,有两个地方可以利用:
1.可以利用 ApplicationContextAware 获取 boot 容器的 ApplicationContext 实例,这样就可以实现自己实现的父子容器;
2.可以利用 ApplicationListener 获取 ContextRefreshedEvent 事件,该事件表示容器已经完成初始化,可以提供服务。在监听到该事件后,来进行 web 容器的加载。
思路确定后,代码实现就很简单了:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 | package com.diguage.demo.boot.config; import org.slf4j.Logger; import org.slf4j.LoggerFactory; import org.springframework.beans.BeansException; import org.springframework.context.ApplicationContext; import org.springframework.context.ApplicationContextAware; import org.springframework.context.ApplicationEvent; import org.springframework.context.ApplicationListener; import org.springframework.context.event.ContextRefreshedEvent; import org.springframework.context.support.ClassPathXmlApplicationContext; import org.springframework.stereotype.Component; / * * * @author D瓜哥 · https: / / www.diguage.com * / @Component public class WebLoaderListener implements ApplicationContextAware, ApplicationListener<ApplicationEvent> { private static final Logger logger = LoggerFactory.getLogger(WebLoaderListener. class ); / * * * 父容器,加载 boot 项目 * / private static ApplicationContext parentContext; / * * * 子容器,加载 web 项目 * / private static ApplicationContext childContext; @Override public void setApplicationContext(ApplicationContext ctx) throws BeansException { WebLoaderListener.parentContext = ctx; } @Override public void onApplicationEvent(ApplicationEvent event) { logger.info( "receive application event: {}" , event); if (event instanceof ContextRefreshedEvent) { WebLoaderListener.childContext = new ClassPathXmlApplicationContext( new String[]{ "classpath:web/spring-cfg.xml" }, WebLoaderListener.parentContext); } } } |
这次自己实现的父子容器,如同设想的那样,没有同名 Bean 的检查,省去了很多麻烦。但是,观察日志,会发现 com.diguage.demo.boot.config.WebLoaderListener#onApplicationEvent
方法被两次执行,也就是监听到了两次 ContextRefreshedEvent
事件,导致 web
容器会被加载两次。由于项目的 RPC 服务不能重复注册,第二次加载抛出异常,导致启动失败。
最初,怀疑是 web
容器,加载了 WebLoaderListener
,但是跟踪代码,没有发现 childContext
容器中有 WebLoaderListener
的相关 Bean。
昨天做了个小实验,又调试了一下 Spring 的源代码,发现了其中的奥秘。直接贴代码吧:
SPRING/spring-context/src/main/java/org/springframework/context/support/AbstractApplicationContext.java
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 | / * * * Publish the given event to all listeners. * <p>This is the internal delegate that all other {@code publishEvent} * methods refer to. It is not meant to be called directly but rather serves * as a propagation mechanism between application contexts in a hierarchy, * potentially overridden in subclasses for a custom propagation arrangement. * @param event the event to publish (may be an {@link ApplicationEvent} * or a payload object to be turned into a {@link PayloadApplicationEvent}) * @param typeHint the resolved event type , if known. * The implementation of this method also tolerates a payload type hint for * a payload object to be turned into a {@link PayloadApplicationEvent}. * However, the recommended way is to construct an actual event object via * {@link PayloadApplicationEvent #PayloadApplicationEvent(Object, Object, ResolvableType)} * instead for such scenarios. * @since 4.2 * @see ApplicationEventMulticaster #multicastEvent(ApplicationEvent, ResolvableType) * / protected void publishEvent( Object event, @Nullable ResolvableType typeHint) { Assert.notNull(event, "Event must not be null" ); ResolvableType eventType = null; / / Decorate event as an ApplicationEvent if necessary ApplicationEvent applicationEvent; if (event instanceof ApplicationEvent applEvent) { applicationEvent = applEvent; eventType = typeHint; } else { ResolvableType payloadType = null; if (typeHint ! = null && ApplicationEvent. class .isAssignableFrom(typeHint.toClass())) { eventType = typeHint; } else { payloadType = typeHint; } applicationEvent = new PayloadApplicationEvent<>(this, event, payloadType); } / / Determine event type only once ( for multicast and parent publish) if (eventType = = null) { eventType = ResolvableType.forInstance(applicationEvent); if (typeHint = = null) { typeHint = eventType; } } / / Multicast right now if possible - or lazily once the multicaster is initialized if (this.earlyApplicationEvents ! = null) { this.earlyApplicationEvents.add(applicationEvent); } else if (this.applicationEventMulticaster ! = null) { this.applicationEventMulticaster.multicastEvent(applicationEvent, eventType); } / / Publish event via parent context as well... / / 如果有父容器,则也将事件发布给父容器。 if (this.parent ! = null) { if (this.parent instanceof AbstractApplicationContext abstractApplicationContext) { abstractApplicationContext.publishEvent(event, typeHint); } else { this.parent.publishEvent(event); } } } |
在 publishEvent
方法的最后,如果父容器不为 null
的情况下,则也会向父容器广播容器的相关事件。
看到这里就清楚了,不是 web
容器持有了 WebLoaderListener
这个 Bean,而是 web
容器主动向父容器广播了 ContextRefreshedEvent
事件。
除了上述问题,还有一个问题需要思考:如何销毁 web
容器?如果不能销毁容器,会有一些意想不到的问题。比如,注册中心的 RPC 提供方不能及时销毁等等。
这里的解决方案也比较简单:同样基于事件监听,Spring 容器销毁会有 ContextClosedEvent
事件,在 WebLoaderListener
中监听该事件,然后调用 AbstractApplicationContext#close
方法就可以完成 Spring 容器的销毁工作。
结合上面的所有论述,完整的代码如下:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 | package com.diguage.demo.boot.config; import org.slf4j.Logger; import org.slf4j.LoggerFactory; import org.springframework.beans.BeansException; import org.springframework.context.ApplicationContext; import org.springframework.context.ApplicationContextAware; import org.springframework.context.ApplicationEvent; import org.springframework.context.ApplicationListener; import org.springframework.context.event.ContextClosedEvent; import org.springframework.context.event.ContextRefreshedEvent; import org.springframework.context.support.AbstractApplicationContext; import org.springframework.context.support.ClassPathXmlApplicationContext; import org.springframework.stereotype.Component; import java.util.Objects; / * * * 基于事件监听的 web 项目加载器 * * @author D瓜哥 · https: / / www.diguage.com * / @Component public class WebLoaderListener implements ApplicationContextAware, ApplicationListener<ApplicationEvent> { private static final Logger logger = LoggerFactory.getLogger(WebLoaderListener. class ); / * * * 父容器,加载 boot 项目 * / private static ApplicationContext parentContext; / * * * 子容器,加载 web 项目 * / private static ClassPathXmlApplicationContext childContext; @Override public void setApplicationContext(ApplicationContext ctx) throws BeansException { WebLoaderListener.parentContext = ctx; } / * * * 事件监听 * * @author D瓜哥 · https: / / www.diguage.com * / @Override public void onApplicationEvent(ApplicationEvent event) { logger.info( "receive application event: {}" , event); if (event instanceof ContextRefreshedEvent refreshedEvent) { ApplicationContext context = refreshedEvent.getApplicationContext(); if (Objects.equals(WebLoaderListener.parentContext, context)) { / / 加载 web 容器 WebLoaderListener.childContext = new ClassPathXmlApplicationContext( new String[]{ "classpath:web/spring-cfg.xml" }, WebLoaderListener.parentContext); } } else if (event instanceof ContextClosedEvent) { / / 处理容器销毁事件 if (Objects.nonNull(WebLoaderListener.childContext)) { synchronized (WebLoaderListener. class ) { if (Objects.nonNull(WebLoaderListener.childContext)) { AbstractApplicationContext ctx = WebLoaderListener.childContext; WebLoaderListener.childContext = null; ctx.close(); } } } } } } |
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作者:京东科技 李君
来源:京东云开发者社区 转载请注明来源