Add locking module: optimistic vs pessimistic locking on Hibernate 7.4.14.Final

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01YPgGH6aMbSLANL3E2HHmS6

locking/README.md

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+# locking
+
+Companion module for the ankurm.com article on **optimistic vs pessimistic locking in Hibernate 7**. Two threads, one
+in-memory H2 database, plain Hibernate (no Spring). Each test writes what it observed to `output/`.
+
+## Versions
+
+| Component | Version | Notes |
+|---|---|---|
+| Hibernate ORM | `7.4.14.Final` | latest 7.x GA on Maven Central on 2026-10-25 (`8.0.0.Beta3` is also listed; it is not GA) |
+| Jakarta Persistence | `3.2.0` | |
+| H2 | `2.4.240` | in memory, `OPTIMIZE_REUSE_RESULTS=0` (see `Db.java`) |
+| JUnit Jupiter / AssertJ | `6.0.3` / `3.27.7` | |
+| Java | `25` (Temurin 25.0.5+8) | |
+
+This module has its own `pom.xml` and does not use the Spring Boot parent of the main `hibernate-demo` project.
+All locking behaviour here is H2's. `SKIP LOCKED`, `NOWAIT` and deadlock handling differ between databases, so check
+your own.
+
+## Run
+
+```bash
+./scripts/run-all.sh
+```
+
+## Output
+
+| File | What it shows |
+|---|---|
+| `01-lost-update.txt` | two transactions read 100, both deposit, one deposit is lost (no `@Version`) |
+| `02-version-stops-it.txt` | the same race with `@Version`: the second commit fails with `OptimisticLockException` |
+| `03-retry-loop.txt` | six threads, thirty increments, retry on `OptimisticLockException`; the final balance is exact |
+| `04-pessimistic-write.txt` | `PESSIMISTIC_WRITE` makes the second transaction wait; SQL uses `for update` |
+| `05-no-wait.txt` | `Timeouts.NO_WAIT` fails at once (`for update nowait`) |
+| `06-skip-locked.txt` | two workers claim different tickets (`for update skip locked`) |
+| `07-deadlock.txt` | opposite lock order deadlocks; the same order for everyone does not |