Understanding Synchronous vs Asynchronous Event Execution in Spring's SimpleApplicationEventMulticaster

After upgrading a core component, a domain event that was originally executed synchronously in a Spring application started executing asynchronously, leading to business runtime errors.

The execution mode is decided by the SimpleApplicationEventMulticaster implementation. When an event is published, the multicastEvent method checks whether a task executor (thread pool) is configured; if it exists, the listener is invoked via the executor (asynchronously), otherwise it is invoked directly (synchronously).

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

@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)) { if (executor != null) { executor.execute(() -> invokeListener(listener, event)); } else { invokeListener(listener, event); } } }

The task executor field itself can be null:

@Nullable private Executor taskExecutor;

If the application context does not define a custom ApplicationEventMulticaster , Spring creates a default SimpleApplicationEventMulticaster without a thread pool, which means events are processed synchronously.

org.springframework.context.support.AbstractApplicationContext#initApplicationEventMulticaster

In summary, Spring events are synchronous by default; configuring a thread pool either on the default multicaster or on a custom implementation makes the event handling asynchronous.

Source: spring-cloud-context-3.1.1