SpringEvent事件通知机制解读

// 定义一个事件
public class EventDemo extends ApplicationEvent {
    private String message;


    public EventDemo(Object source, String message) {
        super(source);
        this.message = message;
    }

    public String getMessage() {
        return message;
    }
}

// 定义一个事件监听者
@Component
public class EventDemoListener implements ApplicationListener<EventDemo> {
    @Override
    public void onApplicationEvent(EventDemo event) {
        System.out.println("receiver " + event.getMessage());
    }
}

// 事件发布
@Component
public class EventDemoPublish {
    @Autowired
    private ApplicationEventPublisher applicationEventPublisher;

    public void publish(String message) {
        EventDemo demo = new EventDemo(this, message);
        applicationEventPublisher.publishEvent(demo);
    }
}

protected void publishEvent(Object event, @Nullable ResolvableType eventType) {
    Assert.notNull(event, "Event must not be null");

    // Decorate event as an ApplicationEvent if necessary
    ApplicationEvent applicationEvent;
    if (event instanceof ApplicationEvent) {
        // 如果event是ApplicationEvent对象
        applicationEvent = (ApplicationEvent) event;
    }
    else {
        // 如果event不是ApplicationEvent对象，则将其包装成PayloadApplicationEvent事件，并获取对应的事件类型
        applicationEvent = new PayloadApplicationEvent<>(this, event);
        if (eventType == null) {
            eventType = ((PayloadApplicationEvent) applicationEvent).getResolvableType();
        }
    }

    // Multicast right now if possible - or lazily once the multicaster is initialized
    if (this.earlyApplicationEvents != null) {
        this.earlyApplicationEvents.add(applicationEvent);
    }
    else {
        // 获取ApplicationEventMulticaster，调用`multicastEvent`方法广播事件
        getApplicationEventMulticaster().multicastEvent(applicationEvent, eventType);
    }

    // 如果当前命名空间还有父亲节点，也需要给父亲推送该消息
    // Publish event via parent context as well...
    if (this.parent != null) {
        if (this.parent instanceof AbstractApplicationContext) {
            ((AbstractApplicationContext) this.parent).publishEvent(event, eventType);
        }
        else {
            this.parent.publishEvent(event);
        }
    }
}

// 获取ApplicationEventMulticaster
ApplicationEventMulticaster getApplicationEventMulticaster() throws IllegalStateException {
    if (this.applicationEventMulticaster == null) {
        throw new IllegalStateException("ApplicationEventMulticaster not initialized - " +
                "call 'refresh' before multicasting events via the context: " + this);
    }
    return this.applicationEventMulticaster;
}

// Initialize message source for this context.
initMessageSource();

// Initialize event multicaster for this context.
// 在Spring容器中初始化事件广播器，事件广播器用于事件的发布
initApplicationEventMulticaster();

// Initialize other special beans in specific context subclasses.
onRefresh();

// Check for listener beans and reGISter them.
// 把Spring容器内的事件监听器和BeanFactory中的事件监听器都添加的事件广播器中。
registerListeners();

// Instantiate all remaining (non-lazy-init) singletons.
finishBeanFactoryInitialization(beanFactory);

// Last step: publish corresponding event.
finishRefresh();

// 初始化事件广播器
protected void initApplicationEventMulticaster() {
    ConfigurableListableBeanFactory beanFactory = getBeanFactory();
    // 如果用户手动新建了一个名为applicationEventMulticaster类型为ApplicationEventMulticaster的bean，则将这个bean作为事件广播器
    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 {
        // 否则新建一个SimpleApplicationEventMulticaster作为默认的事件广播器
        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() + "]");
        }
    }
}

// 注册监听器
protected void registerListeners() {
    // Register statically specified listeners first.
    // 把提前存储好的监听器添加到监听器容器中
    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!
    // 获取类型是ApplicationListener的beanName集合，此处不会去实例化bean
    String[] listenerBeanNames = getBeanNamesForType(ApplicationListener.class, true, false);
    for (String listenerBeanName : listenerBeanNames) {
        getApplicationEventMulticaster().addApplicationListenerBean(listenerBeanName);
    }

    // Publish early application events now that we finally have a multicaster...
    Set<ApplicationEvent> earlyEventsToProcess = this.earlyApplicationEvents;
    this.earlyApplicationEvents = null;
    // 如果存在earlyEventsToProcess，提前处理这些事件
    if (earlyEventsToProcess != null) {
        for (ApplicationEvent earlyEvent : earlyEventsToProcess) {
            getApplicationEventMulticaster().multicastEvent(earlyEvent);
        }
    }
}

@Override
public void multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) {
    ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
    // 根据event类型获取适合的监听器
    for (final ApplicationListener<?> listener : getApplicationListeners(event, type)) {
        // 获取SimpleApplicationEventMulticaster中的线程执行器，如果存在线程执行器则在新线程中异步执行，否则直接同步执行监听器中的方法
        Executor executor = getTaskExecutor();
        if (executor != null) {
            executor.execute(() -> invokeListener(listener, event));
        }
        else {
            invokeListener(listener, event);
        }
    }
}

protected void invokeListener(ApplicationListener<?> listener, ApplicationEvent event) {
    // 如果存在ErrorHandler，调用监听器方法如果抛出异常则调用ErrorHandler来处理异常。否则直接调用监听器方法
    ErrorHandler errorHandler = getErrorHandler();
    if (errorHandler != null) {
        try {
            doInvokeListener(listener, event);
        }
        catch (Throwable err) {
            errorHandler.handleError(err);
        }
    }
    else {
        doInvokeListener(listener, event);
    }
}

@Component
public class EventDemoListener implements ApplicationListener<EventDemo> {
    Logger logger = LoggerFactory.getLogger(EventDemoListener.class);

    @Override
    public void onApplicationEvent(EventDemo event) {
        logger.info("receiver " + event.getMessage());
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

@Component
public class EventDemoListener2 implements ApplicationListener<EventDemo> {
    Logger logger = LoggerFactory.getLogger(EventDemoListener2.class);

    @Override
    public void onApplicationEvent(EventDemo event) {
        logger.info("receiver 2 " + event.getMessage());
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

@Configuration
@ComponentScan("love.wangqi")
public class Config {
    @Bean
    public SimpleAsyncTaskExecutor simpleAsyncTaskExecutor() {
        return new SimpleAsyncTaskExecutor();
    }

    @Bean
    public SimpleApplicationEventMulticaster applicationEventMulticaster() {
        SimpleApplicationEventMulticaster simpleApplicationEventMulticaster = new SimpleApplicationEventMulticaster();
        simpleApplicationEventMulticaster.setTaskExecutor(simpleAsyncTaskExecutor());
        return simpleApplicationEventMulticaster;
    }
}

@Component
public class EventDemoListener implements ApplicationListener<EventDemo> {
    Logger logger = LoggerFactory.getLogger(EventDemoListener.class);

    @Async
    @Override
    public void onApplicationEvent(EventDemo event) {
        logger.info("receiver " + event.getMessage());
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

@Configuration
@ComponentScan("love.wangqi")
@EnableAsync
public class Config {
    @Bean
    public SimpleAsyncTaskExecutor simpleAsyncTaskExecutor() {
        return new SimpleAsyncTaskExecutor();
    }
}

@Override
public Object postProcessAfterInitialization(Object bean, String beanName) {
    if (this.advisor == null || bean instanceof aopInfrastructureBean) {
        // Ignore AOP infrastructure such as scoped proxies.
        return bean;
    }

    if (bean instanceof Advised) {
        Advised advised = (Advised) bean;
        if (!advised.isFrozen() && isEligible(AopUtils.getTargetClass(bean))) {
            // Add our local Advisor to the existing proxy's Advisor chain...
            if (this.beforeExistingAdvisors) {
                advised.addAdvisor(0, this.advisor);
            }
            else {
                advised.addAdvisor(this.advisor);
            }
            return bean;
        }
    }

    if (isEligible(bean, beanName)) {
        ProxyFactory proxyFactory = prepareProxyFactory(bean, beanName);
        if (!proxyFactory.isProxyTargetClass()) {
            evaluateProxyInterfaces(bean.getClass(), proxyFactory);
        }
        proxyFactory.addAdvisor(this.advisor);
        customizeProxyFactory(proxyFactory);
        return proxyFactory.getProxy(getProxyClassLoader());
    }

    // No proxy needed.
    return bean;
}

@Override
@Nullable
public Object invoke(final MethodInvocation invocation) throws Throwable {
    Class<?> targetClass = (invocation.getThis() != null ? AopUtils.getTargetClass(invocation.getThis()) : null);
    Method specificMethod = ClassUtils.getMostSpecificMethod(invocation.getMethod(), targetClass);
    final Method userDeclaredMethod = BridgeMethodResolver.findBridgedMethod(specificMethod);

    // 获取executor
    AsyncTaskExecutor executor = determineAsyncExecutor(userDeclaredMethod);
    if (executor == null) {
        throw new IllegalStateException(
                "No executor specified and no default executor set on AsyncExecutionInterceptor either");
    }

    // 将我们真正的方法包装成一个`Callable`任务
    Callable<Object> task = () -> {
        try {
            Object result = invocation.proceed();
            if (result instanceof Future) {
                return ((Future<?>) result).get();
            }
        }
        catch (ExecutionException ex) {
            handleError(ex.getCause(), userDeclaredMethod, invocation.getArguments());
        }
        catch (Throwable ex) {
            handleError(ex, userDeclaredMethod, invocation.getArguments());
        }
        return null;
    };
    
    // 将任务提交到`executor`中执行
    return doSubmit(task, executor, invocation.getMethod().getReturnType());
}