Most Spring Boot applications call external APIs — payment gateways, shipping providers, notification services, CRM systems. Testing these integrations is hard because external APIs have rate limits, authentication requirements, and a habit of being unavailable at exactly the wrong moment. WireMock runs a real HTTP server in a Docker container, accepts your requests, and returns whatever responses you configure. This article covers WireMock as a Testcontainer for testing Spring Boot REST client integrations.