Event-driven architecture pattern in Spring Boot - Step-by-Step Execution

Concept Flow - Event-driven architecture pattern

Event Occurs

↓

Event Published

↓

Event Broker/Bus

↓

Event Listeners/Subscribers

↓

Event Handlers Execute

↓

System Reacts to Event

This flow shows how an event is created, published to a broker, and then handled by listeners that react to it.

Execution Sample

Spring Boot

public class OrderCreatedEvent {
  private final String orderId;
  public OrderCreatedEvent(String orderId) { this.orderId = orderId; }
  public String getOrderId() { return orderId; }
}

@Component
public class OrderEventListener {
  @EventListener
  public void handleOrderCreated(OrderCreatedEvent event) {
    System.out.println("Order created: " + event.getOrderId());
  }
}

This code defines an event and a listener that reacts when the event is published.

Execution Table

Step	Action	Event State	Listeners Invoked	Output
1	OrderCreatedEvent instance created with orderId='1234'	OrderCreatedEvent(orderId='1234')	None yet	No output
2	Event published to Spring event bus	Event queued for delivery	None yet	No output
3	Spring event bus dispatches event	Event dispatched	OrderEventListener.handleOrderCreated	Prints: 'Order created: 1234'
4	Listener processes event	Event handled	OrderEventListener.handleOrderCreated	Listener logic executed
5	Event processing complete	No pending events	None	No further output

💡 All listeners have processed the event; event handling cycle ends.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
orderId	null	'1234'	'1234'	'1234'	'1234'
event	null	OrderCreatedEvent(orderId='1234')	OrderCreatedEvent(orderId='1234')	OrderCreatedEvent(orderId='1234')	null (handled)
listenersInvoked	[]	[]	[]	[OrderEventListener.handleOrderCreated]	[OrderEventListener.handleOrderCreated]

Key Moments - 3 Insights

Why doesn't the event listener run immediately when the event object is created?

Creating the event object (Step 1) just prepares data. The listener runs only after the event is published to the event bus (Step 2) and dispatched (Step 3), as shown in the execution_table.

Can multiple listeners handle the same event?

Yes. The event bus dispatches the event to all registered listeners. The execution_table shows one listener, but others would be invoked similarly.

What happens if no listener is registered for an event?

The event is published and dispatched but no listeners run. The event handling cycle ends with no output, as no handlers process it.

Visual Quiz - 3 Questions

Look at the execution_table, at which step is the event listener first invoked?

A Step 1

B Step 3

C Step 2

D Step 5

💡 Hint

Check the 'Listeners Invoked' column in the execution_table.

According to variable_tracker, what is the value of 'event' after Step 3?

A OrderCreatedEvent(orderId='1234')

B null

C Event handled

D Event queued for delivery

💡 Hint

Look at the 'event' row under 'After Step 3' in variable_tracker.

If we add another listener, how would the 'Listeners Invoked' column change at Step 3?

A It would remain empty

B It would list only the first listener

C It would list both listeners

D It would show an error

💡 Hint

Refer to the explanation in key_moments about multiple listeners.

Concept Snapshot

Event-driven architecture uses events to trigger actions.
Events are objects representing something that happened.
Publish events to an event bus or broker.
Listeners subscribe to events and react when events occur.
Spring Boot uses @EventListener to handle events synchronously by default.
This decouples components and improves scalability.

Full Transcript

In event-driven architecture, components communicate by sending and receiving events. When something important happens, an event object is created. This event is then published to an event bus, which delivers it to all listeners registered for that event type. Listeners react by running their event handling code. In Spring Boot, you define event classes and listener methods annotated with @EventListener. The flow starts with event creation, then publishing, dispatching, listener invocation, and finally handling completion. Variables like the event object and listeners invoked change state as the event moves through the system. This pattern helps keep parts of the system loosely connected and responsive to changes.