借鉴：https://blog.csdn.net/zxfhahaha/article/details/112558429

https://blog.csdn.net/qq_46940224/article/details/131043987

https://blog.csdn.net/weixin_37672801/article/details/128803430

https://www.cnblogs.com/shamo89/p/15715091.html

https://blog.csdn.net/laohuangaa/article/details/119772453

Spring事件发布原理(ApplicationEventPublisher接口)

1.事件发布的使用：

事件驱动机制其实是观察者模式（又称发布订阅）具体实现，事件对象（HtwBikeRoadSignEvent）相当于被观察对象, 事件监听器(EventListener) 相当于观察者。@EventListener（）括号里面是哪个对象就是监听哪个对象。

HtwBikeRoadSignAction.java: 自己定义的类，在方法中发布事件

@Service
public class HtwBikeRoadSignAction implements AssetInBoundAction {
    @Autowired
    private HtwBikeRepository htwBikeRepository;
    @Resource
    private ApplicationContext applicationContext;
    @Resource
    private CityExtInfoIntegrationService cityExtInfoIntegrationService;
    @Autowired
    private AssetAddressAndStatusConvertManager convertManager;
    @Override
    public AssetsInBoundResult apply(AssetContext context, AssetsInBoundParam param) {
        AssetsInBoundResult assetsResult = new AssetsInBoundResult();
        List bikeIds = Collections.emptyList();
        bikeIds.add(12245L);
        List htwBikeEntityList = htwBikeRepository.getByIds(bikeIds);
        HtwBikeRoadSignEvent htwBikeRoadSignEvent = HtwBikeRoadSignEvent.builder()
                .context(context)
                .htwBikeEntities(htwBikeEntityList)
                .build();
        //更新车辆信息
        modifyHtwBikeInfo(context, htwBikeEntityList);
         */
        // 5.发送车辆绑定事件
        applicationContext.publishEvent(htwBikeRoadSignEvent);
        assetsResult.setInBoundResult(true);
        return assetsResult;
    }
}

HtwBikeRoadSignEvent.java:

/**
 * 路面签收事件
 */
@Data
@Builder
public class HtwBikeRoadSignEvent extends AssetSopEvent {
    /**
     * 操作上下文
     */
    private AssetContext context;
    /**
     * 车辆信息
     */
    private List htwBikeEntities;
    /**
     * 车辆更新字段
     */
    private Map bikeUpdateMap;
    /**
     * 锁更新字段
     */
    private Map lockUpdateMap;
}

HtwBikeRoadSignSrmHandler.java:

/**
 * 车辆路面签收通知
 */
@Component
public class HtwBikeRoadSignSrmHandler {
    @Value("${srm.producer.topic}")
    private String                  srmGatewayTopic;
    /*@Resource(name = "exporterBlocMessageProducer")
    private BlocMessageProducerImpl producer;*/
    @Async
    @EventListener(HtwBikeRoadSignEvent.class)
    public void onRoadSign(HtwBikeRoadSignEvent event) {
        AssetContext context = event.getContext();
        List htwBikeEntities = event.getHtwBikeEntities();
        AssetChangeOrderMessageDTO assetChangeOrderMessageDTO = new AssetChangeOrderMessageDTO();
        assetChangeOrderMessageDTO.setChangeType(1);
        //producer.sendMessage(srmGatewayTopic, assetChangeOrderMessageDTO);
    }
}
@Async 添加注解表示这个方法要异步执行

2.Spring的两种监听器的实现有两种方式：

实现ApplicationListener接口 ：方法是onApplicationEvent(E event);

注解@EventListener

3.原理： 也可以看这个里面的有源码贴图： https://www.cnblogs.com/shamo89/p/15715091.html 或者 https://blog.csdn.net/laohuangaa/article/details/119772453

第一部分原理：发布事件 applicationContext.publishEvent(htwBikeRoadSignEvent)方法：

---

ApplicationContext接口： 继承了ApplicationEventPublisher接口。ApplicationContext接口的对象在SpringBoot启动的时候自动装配有初始化。

public interface ApplicationContext extends EnvironmentCapable, ListableBeanFactory, HierarchicalBeanFactory,
		MessageSource, ApplicationEventPublisher, ResourcePatternResolver {
	@Nullable
	String getId();
	String getApplicationName();
	String getDisplayName();
	long getStartupDate();
	@Nullable
	ApplicationContext getParent();
	
	AutowireCapableBeanFactory getAutowireCapableBeanFactory() throws IllegalStateException;
}

ApplicationEventPublisher接口：

public interface ApplicationEventPublisher {
	default void publishEvent(ApplicationEvent event) {
		publishEvent((Object) event);
	}
	void publishEvent(Object event);  //本文以看这个方法源码分析
}

AbstractApplicationContext.java:

public abstract class AbstractApplicationContext extends DefaultResourceLoader
		implements ConfigurableApplicationContext {
		
	@Override
	public void publishEvent(Object event) {
		publishEvent(event, null);
	}
	protected void publishEvent(Object event, @Nullable ResolvableType eventType) {
		Assert.notNull(event, "Event must not be null");
		ApplicationEvent applicationEvent;
		if (event instanceof ApplicationEvent) {
			applicationEvent = (ApplicationEvent) event;
		}
		else {
			applicationEvent = new PayloadApplicationEvent<>(this, event);
			if (eventType == null) {
				eventType = ((PayloadApplicationEvent) applicationEvent).getResolvableType();
			}
		}
		if (this.earlyApplicationEvents != null) {
			this.earlyApplicationEvents.add(applicationEvent);
		}
		else {
		//重点看这个：调用了SimpleApplicationEventMulticaster类的multicastEvent()
			getApplicationEventMulticaster().multicastEvent(applicationEvent, eventType);
		}
		if (this.parent != null) {
			if (this.parent instanceof AbstractApplicationContext) {
				((AbstractApplicationContext) this.parent).publishEvent(event, eventType);
			}
			else {
				this.parent.publishEvent(event);
			}
		}
	}
}

SimpleApplicationEventMulticaster.java: 事件广播器

public class SimpleApplicationEventMulticaster extends AbstractApplicationEventMulticaster {
	@Nullable
	private Executor taskExecutor;
	@Nullable
	private ErrorHandler errorHandler;
	
	public SimpleApplicationEventMulticaster() {
	}
	@Override
	public void multicastEvent(ApplicationEvent event) {
	//调了下一个multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) 方法
		multicastEvent(event, resolveDefaultEventType(event));
	}
	@Override
	public void multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) {
		ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
		Executor executor = getTaskExecutor();
		for (ApplicationListener listener : getApplicationListeners(event, type)) {//感兴趣可以继续看getApplicationListeners方法获取所有的监听器
		//根据监听对象类型返回所有的监听器，循环调用
			if (executor != null) {
			//线程池异步调用invokeListener（）
				executor.execute(() -> invokeListener(listener, event));
			}
			else {
				invokeListener(listener, event);
			}
		}
	}
	
	//执行invokeListener（）
	protected void invokeListener(ApplicationListener listener, ApplicationEvent event) {
		ErrorHandler errorHandler = getErrorHandler();
		if (errorHandler != null) {
			try {
			//调用doInvokeListener（）
				doInvokeListener(listener, event);
			}
			catch (Throwable err) {
				errorHandler.handleError(err);
			}
		}
		else {
			doInvokeListener(listener, event);
		}
	}
	
	//执行（）
	@SuppressWarnings({"rawtypes", "unchecked"})
	private void doInvokeListener(ApplicationListener listener, ApplicationEvent event) {
		try {
		//这个地方调了监听器所实现的接口ApplicationListener的onApplicationEvent（）。注解@EventListener其实就是生成了一个代理类，代理类也实现了接口ApplicationListener的onApplicationEvent（）方法，所以原理一样
			listener.onApplicationEvent(event);
		}
		catch (ClassCastException ex) {
			String msg = ex.getMessage();
			if (msg == null || matchesClassCastMessage(msg, event.getClass())) {
				Log logger = LogFactory.getLog(getClass());
				if (logger.isTraceEnabled()) {
					logger.trace("Non-matching event type for listener: " + listener, ex);
				}
			}
			else {
				throw ex;
			}
		}
	}
}

---

第二部分原理：事件广播器的初始化：

SpringApplication.java: SpringApplication.run(UsualServiceApplication.class, args)启动的入口

public class SpringApplication {
	public ConfigurableApplicationContext run(String... args) {
		StopWatch stopWatch = new StopWatch();
		stopWatch.start();
		ConfigurableApplicationContext context = null;
		Collection exceptionReporters = new ArrayList<>();
		configureHeadlessProperty();
		SpringApplicationRunListeners listeners = getRunListeners(args);
		listeners.starting();
		try {
			ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
			ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
			configureIgnoreBeanInfo(environment);
			Banner printedBanner = printBanner(environment);
			context = createApplicationContext();
			exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class,
					new Class[] { ConfigurableApplicationContext.class }, context);
			prepareContext(context, environment, listeners, applicationArguments, printedBanner);
			//重点在这个方法里面
			refreshContext(context);
			afterRefresh(context, applicationArguments);
			stopWatch.stop();
			if (this.logStartupInfo) {
				new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);
			}
			listeners.started(context);
			callRunners(context, applicationArguments);
		}
		catch (Throwable ex) {
			handleRunFailure(context, ex, exceptionReporters, listeners);
			throw new IllegalStateException(ex);
		}
		try {
			listeners.running(context);
		}
		catch (Throwable ex) {
			handleRunFailure(context, ex, exceptionReporters, null);
			throw new IllegalStateException(ex);
		}
		return context;
	}
	private void refreshContext(ConfigurableApplicationContext context) {
	//调用refresh（）
		refresh((ApplicationContext) context);
		if (this.registerShutdownHook) {
			try {
				context.registerShutdownHook();
			}
			catch (AccessControlException ex) {
				// Not allowed in some environments.
			}
		}
	}
	//执行refresh（）
	@Deprecated
	protected void refresh(ApplicationContext applicationContext) {
		Assert.isInstanceOf(ConfigurableApplicationContext.class, applicationContext);
		//调用refresh((ConfigurableApplicationContext) applicationContext)
		refresh((ConfigurableApplicationContext) applicationContext);
	}
	//执行refresh((ConfigurableApplicationContext) applicationContext)
	protected void refresh(ConfigurableApplicationContext applicationContext) {
	//调用的是AbstractApplicationContext.java抽象类的refresh（）
		applicationContext.refresh();
	}
}

AbstractApplicationContext.java抽象类:

//执行AbstractApplicationContext.java抽象类的refresh（）
public abstract class AbstractApplicationContext extends DefaultResourceLoader implements ConfigurableApplicationContext {
	@Override
	public void refresh() throws BeansException, IllegalStateException {
		synchronized (this.startupShutdownMonitor) {
			prepareRefresh();
			ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
			prepareBeanFactory(beanFactory);
			try {
				postProcessBeanFactory(beanFactory);
				invokeBeanFactoryPostProcessors(beanFactory);
				registerBeanPostProcessors(beanFactory);
				initMessageSource();
				//调用初始化事件广播器方法
				initApplicationEventMulticaster();
				onRefresh();
				registerListeners();
				finishBeanFactoryInitialization(beanFactory);
				finishRefresh();
			}
			catch (BeansException ex) {
				if (logger.isWarnEnabled()) {
					logger.warn("Exception encountered during context initialization - " +
							"cancelling refresh attempt: " + ex);
				}
				destroyBeans();
				cancelRefresh(ex);
				throw ex;
			}
			finally {
				resetCommonCaches();
			}
		}
	}
	//执行初始化事件广播器的方法
	protected void initApplicationEventMulticaster() {
		ConfigurableListableBeanFactory beanFactory = getBeanFactory();
		if (beanFactory.containsLocalBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME)) {
			this.applicationEventMulticaster =
					beanFactory.getBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, ApplicationEventMulticaster.class);
			if (logger.isTraceEnabled()) {
				logger.trace("Using ApplicationEventMulticaster [" + this.applicationEventMulticaster + "]");
			}
		}
		else {
			this.applicationEventMulticaster = new SimpleApplicationEventMulticaster(beanFactory);
			beanFactory.registerSingleton(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, this.applicationEventMulticaster);
			if (logger.isTraceEnabled()) {
				logger.trace("No '" + APPLICATION_EVENT_MULTICASTER_BEAN_NAME + "' bean, using " +
						"[" + this.applicationEventMulticaster.getClass().getSimpleName() + "]");
			}
		}
	}
}

ConfigurableApplicationContext接口： 继承了ApplicationContext接口

public interface ConfigurableApplicationContext extends ApplicationContext, Lifecycle, Closeable {
}

ApplicationContext接口：

public interface ApplicationContext extends EnvironmentCapable, ListableBeanFactory, HierarchicalBeanFactory,
		MessageSource, ApplicationEventPublisher, ResourcePatternResolver {
}

4.观察者模式：

包含观察者和目标(被观察者)两种对象。目标和观察者之间存在一对多的关系，通过建立目标与观察者之间的联系，当目标的某个状态发生变化时，所有与之关联的观察者都会得到通知并执行各自的任务。

也称 发布-订阅模式：

订阅：观察者向目标对象注册自己，告诉目标对象自己在观察它。

发布：当目标对象的状态发生改变时，向它所有的观察者发送通知，观察者收到通知后执行各自的操作。

优点：降低了目标与观察者之间的耦合关系。

缺点：目标与观察者之间没有完全解耦，还可能出现循环引用；当观察者对象很多时，通知的发布可能会很耗时。

5.Spring5种标准事件： Spring源码中也是用了事件发布，下面的是监听对象即事件

上下文更新事件（ContextRefreshedEvent）： ApplicationContext初始化或刷新，或者说在容器实例化(refresh())时发布事件；

上下文开始事件（ContextStartedEvent）：Spring容器启动时，即调用ConfigurableApplicationContext接口的start()时发布事件；

上下文停止事件（ContextStoppedEvent）：Spring容器停止时，即调用ConfigurableApplicationContext接口的stop()时发布事件，关闭的容器可以通过start()重启；

上下文关闭事件（ContextClosedEvent）：Spring容器关闭时，即调用ConfigurableApplicationContext接口的close()时发布事件，所有的Bean已被销毁，无法重启容器；

请求处理事件（RequestHandledEvent）：当一个请求被处理完成时发布事件。