Mastering Spring Event: Synchronous & Asynchronous Patterns in Java

Preface

In real‑world business development, logic can become extremely complex with a core business plus many sub‑tasks, leading to long, tightly coupled code that is hard to maintain. Some scenarios, such as sending emails or SMS, do not need to be completed synchronously within a single request.

Message queues can solve this problem, but they add weight and complexity; when not necessary, we prefer a lighter solution. To address these cases, we explore Spring Event.

Spring Event Synchronous Usage

Spring Event (Application Event)

is essentially an observer pattern: after a bean finishes a task, it can notify other beans, or a bean can listen to another bean's behavior.

1. Custom Event

Define an event class that extends ApplicationEvent.

2. Define Listener

Implement the ApplicationListener interface or annotate a method with @EventListener to handle the event.

3. Define Publisher

Publish the event using ApplicationEventPublisher.

4. Unit Test Execution

Run the test and observe the results.

Spring Event Asynchronous Usage

Some business scenarios, like sending emails or SMS, do not require synchronous completion.

1. Custom Event

2. Define Listener

It is recommended to use the @EventListener annotation.

3. Define Publisher

4. Unit Test Execution (Synchronous)

5. Enable Asynchronous Execution

Add the @EnableAsync annotation to the main application class and annotate asynchronous listener methods with @Async.

6. Unit Test Execution (Asynchronous)

The SMS sending thread appears as task-1; after the main thread finishes (total time: 2017 ms), the console stops printing.