JUnit 5 splits into three sub-projects to address JUnit 4's monolithic design: Jupiter (the new test-writing API and its engine), Vintage (backward-compatible engine for JUnit 3/4 tests), and Platform (the launcher and engine API that tools integrate with). This separation lets IDEs and build tools depend on stable, published APIs instead of internal implementation details. The architecture also introduces a formal API lifecycle with statuses like Internal, Experimental, Maintained, and Stable. A key benefit is that third-party testing frameworks (TestNG, Spock, Cucumber, etc.) can implement a Platform engine and immediately gain first-class tool support across Maven, Gradle, Eclipse, and IntelliJ without each tool needing framework-specific integrations. The Open Test Alliance for the JVM is also introduced as a companion effort to standardize test failure exceptions across frameworks.
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