Architecture Testing for Java with ArchUnit: Stop Trusting Your Diagram, Start Testing It

This article explains how to use ArchUnit, a Java library for architecture testing, to enforce architectural rules in your codebase. It covers why architecture diagrams often become outdated as code drifts from the intended design, and how encoding rules as executable tests can prevent this. The post includes setup instructions for Spring Boot with Maven and JUnit 5, practical rule categories (layers, dependencies, naming, cycles, DTO discipline), and advice for adopting ArchUnit on legacy projects. It emphasizes architecture testing as a fitness function that guards code structure, similar to how mutation testing guards behavior.