Event publishing with ApplicationEventPublisher in Spring Boot - Step-by-Step Execution

Concept Flow - Event publishing with ApplicationEventPublisher

Create ApplicationEventPublisher

↓

Create Custom Event Object

↓

Call publishEvent(event)

↓

Spring Framework Notifies Listeners

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Listeners Receive Event and React

This flow shows how an event is created and published using ApplicationEventPublisher, then Spring notifies all listeners to react.

Execution Sample

Spring Boot

public class MyEvent extends ApplicationEvent {
  public MyEvent(Object source) { super(source); }
}

@Component
public class Publisher {
  @Autowired
  private ApplicationEventPublisher publisher;

  public void publish() {
    publisher.publishEvent(new MyEvent(this));
  }
}

This code defines a custom event and publishes it using ApplicationEventPublisher.

Execution Table

Step	Action	Event Object	Publisher State	Listeners Notified	Output
1	Create MyEvent instance	MyEvent(source=Publisher)	Ready	None yet	Event object created
2	Call publishEvent(event)	MyEvent(source=Publisher)	Publishing	None yet	Event sent to Spring
3	Spring dispatches event	MyEvent(source=Publisher)	Publishing	Listener1, Listener2	Listeners receive event
4	Listeners handle event	MyEvent(source=Publisher)	Idle	Listener1 handled, Listener2 handled	Listeners react accordingly
5	Publish method ends	MyEvent(source=Publisher)	Idle	All notified	Event publishing complete

💡 All registered listeners have been notified and handled the event.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
event	null	MyEvent(source=Publisher)	MyEvent(source=Publisher)	MyEvent(source=Publisher)	MyEvent(source=Publisher)	MyEvent(source=Publisher)
publisher state	Idle	Idle	Publishing	Publishing	Idle	Idle
listeners notified	None	None	None	Listener1, Listener2	Listener1 handled, Listener2 handled	All notified

Key Moments - 3 Insights

Why does the event object stay the same through all steps?

The event object is created once (Step 1) and passed unchanged to the publisher and listeners (Steps 2-4), as shown in the execution_table rows.

When do listeners actually receive the event?

Listeners receive the event during Step 3 when Spring dispatches it, as indicated by 'Listeners Notified' column in the execution_table.

What happens if no listeners are registered?

If no listeners exist, the event is published but no reactions occur; the 'Listeners Notified' would remain 'None' and publishing ends immediately.

Visual Quiz - 3 Questions

Look at the execution table, what is the publisher state at Step 3?

A Publishing

B Idle

C Ready

D Not started

💡 Hint

Check the 'Publisher State' column at Step 3 in the execution_table.

At which step do listeners handle the event?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'Listeners Notified' and 'Output' columns in the execution_table for when listeners react.

If no listeners are registered, what changes in the execution table?

A Publisher state never changes to Publishing

B Listeners Notified remains 'None' and publishing ends quickly

C Event object is not created

D Listeners handle event anyway

💡 Hint

Refer to the key moment about no listeners and the 'Listeners Notified' column in the execution_table.

Concept Snapshot

Use ApplicationEventPublisher to send events.
Create a custom event by extending ApplicationEvent.
Call publishEvent(event) to send it.
Spring notifies all registered listeners.
Listeners react asynchronously or synchronously.
Useful for decoupling parts of your app.

Full Transcript

This visual execution trace shows how Spring Boot's ApplicationEventPublisher works. First, a custom event object is created. Then, the publisher calls publishEvent with this event. Spring framework receives the event and notifies all registered listeners. Listeners then handle the event and perform their logic. The event object remains the same throughout. The publisher state changes from idle to publishing during the event dispatch. If no listeners exist, the event is published but no reactions happen. This mechanism helps different parts of an application communicate without tight coupling.