Preface

Event mechanism in Spring is actually the observer pattern in design pattern, which is mainly composed of the following roles:

1. Event
2. Event listener (listening for and handling events)
3. Event Publisher (Publishing Events)

First, look at the interface definitions for listeners and publishers

public interface ApplicationListener<E extends ApplicationEvent> extends EventListener {
      void onApplicationEvent(E event);
  }
  
  public interface ApplicationEventPublisher {
    default void publishEvent(ApplicationEvent event) {
        publishEvent((Object) event);
    }
    void publishEvent(Object event);

}

Event flow process

Initialize Event Broadcaster

Take a look at the method AbstractApplicationContext.refresh, which has been analyzed in the IOC Source Parsing article, so focus directly on the logic associated with event broadcasters and event publishing.

 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 Broadcaster
                initApplicationEventMulticaster();
 
                // Initialize other special beans in specific context subclasses.
                onRefresh();
 
                // Check for listener beans and register them.
                registerListeners();
 
                // Instantiate all remaining (non-lazy-init) singletons.
                finishBeanFactoryInitialization(beanFactory);
 
                // Release events
                finishRefresh();
            }
 
            catch (BeansException ex) {
                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;
            }
        }
}

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.isDebugEnabled()) {
                logger.debug("Using ApplicationEventMulticaster [" + this.applicationEventMulticaster + "]");
            }
        }
        else {
            this.applicationEventMulticaster = new SimpleApplicationEventMulticaster(beanFactory);
            beanFactory.registerSingleton(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, this.applicationEventMulticaster);
            if (logger.isDebugEnabled()) {
                logger.debug("Unable to locate ApplicationEventMulticaster with name '" +
                        APPLICATION_EVENT_MULTICASTER_BEAN_NAME +
                        "': using default [" + this.applicationEventMulticaster + "]");
            }
        }
    }

You can see that if there is no custom event broadcaster, the default is to use Simple Application Event Multicaster

Release events

Publishing events are after bean initialization

    protected void finishRefresh() {
        // Initialize lifecycle processor for this context.
        initLifecycleProcessor();
 
        // Propagate refresh to lifecycle processor first.
        getLifecycleProcessor().onRefresh();
 
        // Release events
        publishEvent(new ContextRefreshedEvent(this));
 
        // Participate in LiveBeansView MBean, if active.
        LiveBeansView.registerApplicationContext(this);
    }


public void publishEvent(ApplicationEvent event) {
        Assert.notNull(event, "Event must not be null");
        if (logger.isTraceEnabled()) {
            logger.trace("Publishing event in " + getDisplayName() + ": " + event);
        }
        //1. Get event broadcasters and publish events
        getApplicationEventMulticaster().multicastEvent(event);
        //2. If there is a parent container, the parent container will also publish events
        if (this.parent != null) {
            this.parent.publishEvent(event);
        }
    }

Specific Publishing Logic in MultcastEvent Method

public void multicastEvent(final ApplicationEvent event) {
        //Traversing through the listener execution, getApplicationListeners calls the AbstractApplicationEventMulticaster parent class method
        for (final ApplicationListener listener : getApplicationListeners(event)) {
            Executor executor = getTaskExecutor();
            if (executor != null) {
                executor.execute(new Runnable() {
                    @Override
                    public void run() {
                        listener.onApplicationEvent(event);
                    }
                });
            }
            else {
                listener.onApplicationEvent(event);
            }
        }
    }

As you can see, there's nothing special. It's just a thread pool to call the methods of these listeners.

The processing of listeners depends on the specific implementation of each listener.