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Spring Boot 4 Complete Guide · Lesson

Internal Application Events and Listeners

Decouple modules using domain events published and consumed within the application context.

Why Decouple Modules with Events?

In a modular monolith, an Order module often needs to trigger work in other modules: send a confirmation email, update inventory, award loyalty points. The naive approach is to have OrderService call EmailService, InventoryService, and LoyaltyService directly.

  • This creates tight coupling — Order depends on every downstream module.
  • Adding a new reaction means editing OrderService again.
  • Modules can no longer evolve or be tested independently.

Spring's internal application events invert this: the Order module simply publishes a domain event, and interested modules listen for it. The publisher never knows who is listening.

The Domain Event Record

An internal event is just a plain object — in Spring Boot 4 (Java 21+) a record is the idiomatic choice. It should be immutable and carry only the data listeners need.

  • Name it in the past tense (OrderCompleted) — it describes something that already happened.
  • Keep it inside the owning module's package.
  • No Spring annotations are required on the event itself.
package com.shop.order;

import java.math.BigDecimal;
import java.util.UUID;

public record OrderCompleted(
        UUID orderId,
        String customerEmail,
        BigDecimal total) {
}

All lessons in this course

  1. Application Modules and Boundary Verification
  2. Internal Application Events and Listeners
  3. Transactional Event Publication and Outbox
  4. Module Integration Testing and Scenarios
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