unit-test-application-events

This skill provides patterns for unit testing Spring application events using JUnit 5 and Mockito, without booting a full Spring context. It solves a common testing gap: verifying that services publish the correct ApplicationEvent payloads and that @EventListener consumers behave correctly, including asynchronous handlers, in fast isolated tests.

The guidance covers mocking ApplicationEventPublisher on the service under test, capturing published events with ArgumentCaptor to inspect payload fields, and invoking listeners directly against mocked dependencies to verify side effects. It addresses async event handling (@Async plus @EventListener) using timed waits or Awaitility, and adds validation checkpoints such as confirming a captured event is non-null before asserting fields, and increasing wait time or checking executor saturation when async assertions fail. Worked examples include a custom UserCreatedEvent, a publishing service, publisher verification tests, direct listener tests, error-path tests ensuring listeners do not propagate exceptions, and an async listener test. A best-practices section stresses field-level assertions, isolating listeners, and keeping events immutable and serializable.

It targets Java and Spring Boot developers writing unit tests for event-driven code. It is well suited to teams practicing event choreography who want deterministic, context-free tests for their publishers and listeners rather than slow integration tests.