spring-test-slices: Spring Boot 4.1.2 test slices lab (@WebMvcTest, @JsonTest, @RestClientTest, @DataJpaTest vs @SpringBootTest)

Co-Authored-By: Claude Sonnet 5.5 <[email protected]> Claude-Session: https://claude.ai/code/session_017ic4b7Jbw5CxUQ13D7gn25
2026-11-14 15:26:55 +05:30
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+# spring-test-slices
+
+Companion code for the ankurm.com post *Spring Boot 4 Test Slices: @WebMvcTest, @JsonTest, @RestClientTest and @DataJpaTest Compared*.
+
+A small book-catalogue app (controller, service, JPA repository, a `RestClient` client for an ISBN service, a Jackson 3
+`@JacksonComponent`) and one test class per slice, plus the same app under a full `@SpringBootTest` as the baseline.
+Every test writes the real output to `output/` and asserts the same facts, so the numbers in the post fail the build if
+they stop being true.
+
+| Tool | Version | Source |
+|---|---|---|
+| Spring Boot | 4.1.2 | `maven-metadata.xml` (latest GA when this was written; 4.2.0 is not GA yet) |
+| Spring Framework | 7.0.10 | resolved by the Boot BOM |
+| Hibernate ORM | 7.4.13.Final | resolved by the Boot BOM |
+| JDK | 25 | |
+
+## Run
+
+```bash
+./scripts/run-all.sh      # about a minute; regenerates everything in output/
+mvn test                  # just the suite
+```
+
+Startup times (`output/30-startup-times.txt`) vary by machine and by run: treat them as indicative.
+
+## Files
+
+- `src/main/java/lab/slices/` the app: `BookController`, `ApiExceptionHandler`, `BookService`, `BookRepository`,
+  `IsbnLookupClient`, `PriceJson` (`@JacksonComponent`), `AuditLogger` (a plain `@Component`, the canary)
+- `src/test/java/lab/slices/` `WebMvcSliceTest`, `JsonSliceTest`, `RestClientSliceTest`, `DataJpaSliceTest`, `FullContextTest`,
+  `PackageMovesTest`, `OldImportsCompileTest`, `FailureCaptureTest`, `ZMatrixTest`
+- `src/test/java/lab/slices/failing/` test classes that fail on purpose (tag `failure-demo`, excluded from the normal run;
+  `FailureCaptureTest` runs them through the JUnit Launcher and records the real exception)
+- `src/broken/` Boot 3 style tests that no longer compile (not part of the Maven build; `OldImportsCompileTest` hands them to javac)
+- `output/` captured real output of the last run
+
+| Output | What it shows |
+|---|---|
+| `01` to `06` | one transcript per slice, plus the full context: requests, responses, and which beans exist |
+| `07`, `08` | the exceptions a missing mock, a missing scan and `@EnableJpaAuditing` produce |
+| `09` | where every test annotation lives in Boot 4.1.2, asked of the classpath |
+| `10` | javac's own errors for Boot 3 imports |
+| `20` | the bean-presence table, one column per slice |
+| `11`, `12` | which Jackson each JSON tester wraps; which test jars `spring-boot-starter-test` alone brings in |
+| `30` | context startup time per slice (indicative) |