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Spring ApplicationListener源码解析

陈汤姆 人气:0

正文

对于ApplicationListener使用Spring的应该也熟悉,因为这就是我们平时学习的观察者模式的实际代表。

Spring基于Java提供的EventListener实现了一套以Spring容器为基础的观察者模式的事件监听功能,

用于只要实现Spring提供的接口完成事件的定义和监听者的定义,那么就可以很快速的接入观察者模式的实现。

ApplicationListener介绍

说ApplicationListener之前先要知道EventListener。

EventListener本身是一个接口,它的作用跟前面讲到的Aware类似,都是只定义最顶级的接口,并没有实习对应的方法,并且该接口也是由JDK提供的,并不是直接由Spring提供,Spring只是基于该接口实现了自己的一套事件监听功能。

在Spring中实现事件监听的接口是ApplicationListener,该接口继承了EventListener,并做了对应的实现。

源码如下:

package java.util;
/**
 * A tagging interface that all event listener interfaces must extend.
 * @since JDK1.1
 */
public interface EventListener {
}
@FunctionalInterface
public interface ApplicationListener<E extends ApplicationEvent> extends EventListener {
	//监听者监听事件的逻辑处理
	void onApplicationEvent(E event);
	static <T> ApplicationListener<PayloadApplicationEvent<T>> forPayload(Consumer<T> consumer) {
		return event -> consumer.accept(event.getPayload());
	}
}

ApplicationListener使用

对于ApplicationListener的使用,因为Spring已经做了自己的封装,并且以Spring容器为基础做了实现,那么开发者使用时也可以很快的上手,只要简单的配置即可。

定义事件:

//事件继承Spring中的ApplicationEvent
public class MyEvent extends ApplicationEvent {
    private String name;
    public MyEvent(ApplicationContext source,String name) {
        super(source);
        this.name = name;
    }
    public String getName() {
        return name;
    }
}

定义事件的监听者

//定义监听者实现ApplicationListener,并通过泛型声明监听的事件
@Component
public class MyEventListener implements ApplicationListener<MyEvent> {
    @Override
    public void onApplicationEvent(MyEvent event) {
        System.out.println("监听MyEvent:收到消息时间:"+event.getTimestamp()+"【消息name:"+event.getName() + "】");
    }
}
@Component
public class MyEventProcessor implements ApplicationContextAware {
    private ApplicationContext applicationContext;
    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
        this.applicationContext = applicationContext;
    }
    public ApplicationContext getApplicationContext() {
        return applicationContext;
    }
}

发布事件

@SpringBootApplication
public class BootApplication {
    @Resource
    private DefaultListableBeanFactory defaultListableBeanFactory;
    @Resource
    private MySpringAware mySpringAware;
    @Resource
    private MyEventProcessor myEventProcessor;
    public static void main(String[] args) {
        SpringApplication.run(BootApplication.class,args);
    }
    @PostConstruct
    public void init() {
        ApplicationContext applicationContext = myEventProcessor.getApplicationContext();
        //发布事件,事件发布之后,前面订阅的监听者就会监听到该事件发布的消息
        applicationContext.publishEvent(new MyEvent(applicationContext,"陈汤姆"));
        applicationContext.publishEvent(new MyEvent(applicationContext,"陈汤姆2"));
    }
}

监听者收到发布的事件信息

ApplicationListener作用

从以上的例子中可以看到ApplicationListner的模式就是设计模式中的观察者模式。

观察者模式的作用很好的解决了同步交互的问题。

以发送者和接收者为例,接收者接收消息如果同步场景下需要与发送者实现同步调用,但是这样就导致两者之间无法解耦,而ApplicationListener就是解决同步的问题,ApplicationListener可以提供半解耦的方式实现两者之间的交互,即发送者发送消息不需要与接收者之间实现同步通知,只要订阅发送者的事件即可完成双发的交互。

这里为什么是半解耦,因为两者之间还是有一定交互的,交互的点就在于发送者的发送方需要维护一个接收者的集合,发送方在发送时需要将具体的接收者放在集合中,在发送时通过遍历集合发送给接收方,执行接收方的业务处理。

在ApplicationListener这个集合就是public final Set<ApplicationListener<?>> applicationListeners = new LinkedHashSet<>(); 这个集合中就存储了接收者的实例,最终会遍历该集合执行接收者的业务逻辑。

这里抛一个问题,其实ApplicationListener的功能通过MQ也可以实现,那么观察者模式发布订阅模式的区别是什么呢?欢迎评论区一起讨论!

ApplicationListener注册

对于ApplicationListener的注册比较好梳理的,只要找到存储ApplicationListener的集合就可以知道怎么add集合的。

在Spring中ApplicationListener的注册也是在Spring中实现的。

具体的梳理逻辑如下:

org.springframework.context.support.AbstractApplicationContext#refresh

org.springframework.context.support.AbstractApplicationContext#registerListeners

org.springframework.context.event.AbstractApplicationEventMulticaster#addApplicationListener

源码梳理如下:

public abstract class AbstractApplicationContext extends DefaultResourceLoader
		implements ConfigurableApplicationContext {
    @Override
	public void refresh() throws BeansException, IllegalStateException {
		synchronized (this.startupShutdownMonitor) {
			// Prepare this context for refreshing.
			prepareRefresh();
			// Tell the subclass to refresh the internal bean factory.
			ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
			// Prepare the bean factory for use in this context.
			prepareBeanFactory(beanFactory);
			try {
				// Allows post-processing of the bean factory in context subclasses.
				postProcessBeanFactory(beanFactory);
				// Invoke factory processors registered as beans in the context.
				invokeBeanFactoryPostProcessors(beanFactory);
				// Register bean processors that intercept bean creation.
				registerBeanPostProcessors(beanFactory);
				// Initialize message source for this context.
				initMessageSource();
				// Initialize event multicaster for this context.
				initApplicationEventMulticaster();
				// Initialize other special beans in specific context subclasses.
				onRefresh();
				//注册观察者
				registerListeners();
				// Instantiate all remaining (non-lazy-init) singletons.
				finishBeanFactoryInitialization(beanFactory);
				// Last step: publish corresponding event.
				finishRefresh();
			}
			catch (BeansException ex) {
				if (logger.isWarnEnabled()) {
					logger.warn("Exception encountered during context initialization - " +
							"cancelling refresh attempt: " + ex);
				}
				// Destroy already created singletons to avoid dangling resources.
				destroyBeans();
				// Reset 'active' flag.
				cancelRefresh(ex);
				// Propagate exception to caller.
				throw ex;
			}
			finally {
				// Reset common introspection caches in Spring's core, since we
				// might not ever need metadata for singleton beans anymore...
				resetCommonCaches();
			}
		}
	}
    //将观察者注入到集合中
	protected void registerListeners() {
		// Register statically specified listeners first.
		for (ApplicationListener<?> listener : getApplicationListeners()) {
			getApplicationEventMulticaster().addApplicationListener(listener);
		}
		// Do not initialize FactoryBeans here: We need to leave all regular beans
		// uninitialized to let post-processors apply to them!
		String[] listenerBeanNames = getBeanNamesForType(ApplicationListener.class, true, false);
		for (String listenerBeanName : listenerBeanNames) {
            //调用集合的add操作
			getApplicationEventMulticaster().addApplicationListenerBean(listenerBeanName);
		}
		// Publish early application events now that we finally have a multicaster...
		Set<ApplicationEvent> earlyEventsToProcess = this.earlyApplicationEvents;
		this.earlyApplicationEvents = null;
		if (earlyEventsToProcess != null) {
			for (ApplicationEvent earlyEvent : earlyEventsToProcess) {
                //获取观察者的集合
				getApplicationEventMulticaster().multicastEvent(earlyEvent);
			}
		}
	}
}
public abstract class AbstractApplicationEventMulticaster
		implements ApplicationEventMulticaster, BeanClassLoaderAware, BeanFactoryAware {
	private class ListenerRetriever {
        //存储观察者的集合
		public final Set<ApplicationListener<?>> applicationListeners = new LinkedHashSet<>();
		public final Set<String> applicationListenerBeans = new LinkedHashSet<>();
		private final boolean preFiltered;
		public ListenerRetriever(boolean preFiltered) {
			this.preFiltered = preFiltered;
		}
        //获取观察者的集合
		public Collection<ApplicationListener<?>> getApplicationListeners() {
			List<ApplicationListener<?>> allListeners = new ArrayList<>(
					this.applicationListeners.size() + this.applicationListenerBeans.size());
			allListeners.addAll(this.applicationListeners);
			if (!this.applicationListenerBeans.isEmpty()) {
				BeanFactory beanFactory = getBeanFactory();
				for (String listenerBeanName : this.applicationListenerBeans) {
					try {
						ApplicationListener<?> listener = beanFactory.getBean(listenerBeanName, ApplicationListener.class);
						if (this.preFiltered || !allListeners.contains(listener)) {
							allListeners.add(listener);
						}
					}
					catch (NoSuchBeanDefinitionException ex) {
						// Singleton listener instance (without backing bean definition) disappeared -
						// probably in the middle of the destruction phase
					}
				}
			}
			if (!this.preFiltered || !this.applicationListenerBeans.isEmpty()) {
				AnnotationAwareOrderComparator.sort(allListeners);
			}
			return allListeners;
		}
	}
}

从以上的源码可以看到核心的集合就是applicationListeners。可以根据该集合梳理注册和执行流程。

ApplicationListener执行

注册梳理清晰,那么执行自然也很好梳理了, 毕竟使用的都是同一个集合。

ApplicationListener的执行其实就是观察者的执行,也就是在使用篇章中的MyEventListener,在MyEventListener中重写了onApplicationEvent,其中实现了自己的逻辑,那么执行就是将MyEventListener中重写的方式如何在没有同步调用的情况下执行。

执行的实现就是依赖观察者的集合,在注册中我们已经将所有的观察者添加到了ApplicationListener集合中,只要将该集合中的观察者取出执行,即可完成半解耦的执行。

梳理流程如下:

org.springframework.context.ApplicationEventPublisher#publishEvent(org.springframework.context.ApplicationEvent)

org.springframework.context.support.AbstractApplicationContext#publishEvent(java.lang.Object, org.springframework.core.ResolvableType)

org.springframework.context.event.SimpleApplicationEventMulticaster#multicastEvent(org.springframework.context.ApplicationEvent, org.springframework.core.ResolvableType)

org.springframework.context.event.SimpleApplicationEventMulticaster#invokeListener

org.springframework.context.event.SimpleApplicationEventMulticaster#doInvokeListener

源码如下:

public class SimpleApplicationEventMulticaster extends AbstractApplicationEventMulticaster {
	@Override
	public void multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) {
		ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
        //getApplicationListeners就是获取ApplicationListener的集合
		for (final ApplicationListener<?> listener : getApplicationListeners(event, type)) {
			Executor executor = getTaskExecutor();
			if (executor != null) {
                //执行监听者的
				executor.execute(() -> invokeListener(listener, event));
			}
			else {
				invokeListener(listener, event);
			}
		}
	}
    //执行监听者逻辑
    protected void invokeListener(ApplicationListener<?> listener, ApplicationEvent event) {
		ErrorHandler errorHandler = getErrorHandler();
		if (errorHandler != null) {
			try {
				doInvokeListener(listener, event);
			}
			catch (Throwable err) {
				errorHandler.handleError(err);
			}
		}
		else {
			doInvokeListener(listener, event);
		}
	}
	//最终执行逻辑
    private void doInvokeListener(ApplicationListener listener, ApplicationEvent event) {
		try {
            //调用监听者重写的onApplicationEvent方法
			listener.onApplicationEvent(event);
		}
		catch (ClassCastException ex) {
			String msg = ex.getMessage();
			if (msg == null || matchesClassCastMessage(msg, event.getClass())) {
				// Possibly a lambda-defined listener which we could not resolve the generic event type for
				// -> let's suppress the exception and just log a debug message.
				Log logger = LogFactory.getLog(getClass());
				if (logger.isDebugEnabled()) {
					logger.debug("Non-matching event type for listener: " + listener, ex);
				}
			}
			else {
				throw ex;
			}
		}
	}
}

总结

从以上的梳理中,对ApplicationListener的逻辑做一个总结,对于ApplicationListener整体逻辑梳理如下:

以上就是自己关于Spring中ApplicationListener的理解,更多关于Spring ApplicationListener的资料请关注其它相关文章!

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