A deep dive into the non-obvious behaviors of Spring's @TestConfiguration annotation across four different usage patterns: top-level class, nested class, nested class inherited from a parent, and top-level class imported via a parent. Each scenario produces different results, with key gotchas including the need for different bean names when overriding, the requirement for @Primary in some cases, and the necessity of @ContextConfiguration when using nested @TestConfiguration in a parent class. Practical code examples demonstrate each case and the workarounds needed to get the expected test bean to take effect.
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