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
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Table of contents
The Typical ProblemThe patternThe piece that makes it work: Store + computeIfAbsentFixtures as factoriesCompatibilityWhen to useWhen not to useSummarySort: