A pattern for structuring JUnit tests using @Nested classes and @RegisterExtension to create declarative scenario trees with scoped fixture lifecycle management. Each nested class represents a 'Given' clause, and fixtures set up once per scope using ExtensionContext.Store.computeIfAbsent with 'this' as the key. This ensures
Table of contents
The Typical ProblemThe patternThe piece that makes it work: Store + computeIfAbsentFixtures as factoriesCompatibilityWhen to useWhen not to useSummarySort: