Add resilience: Spring Framework 7 @Retryable and @ConcurrencyLimit

Companion code for "Spring Framework 7's Built-in Resilience: @Retryable,
@ConcurrencyLimit, and What's Left for Resilience4j". Every retry counted
by recording real invocations: defaults, backoff and jitter, timeout,
reactive and CompletableFuture returns, the concurrency limit's BLOCK and
REJECT policies, retries around transactions, composition with
Resilience4j 2.4.0, and the annotation API across 7.0.0-7.0.9.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01C3TETMrqVUWeFkNtz3Jbo3

resilience/README.md

@@ -0,0 +1,65 @@
+# Spring Framework 7's built-in resilience - and what is left for Resilience4j
+
+Companion project for [**Spring Framework 7's Built-in Resilience: @Retryable, @ConcurrencyLimit, and What's Left for Resilience4j**](https://ankurm.com/spring-framework-7-retryable-concurrencylimit-resilience4j/)
+on ankurm.com.
+
+Every behaviour the article states was counted here - by recording each real invocation of the
+guarded method, not by trusting the retry machinery's own view. `./scripts/run-all.sh`
+regenerates every transcript in [`docs/output/`](docs/output).
+
+## Versions
+
+| | |
+|---|---|
+| Spring Boot | 4.1.1 |
+| Spring Framework | 7.0.9 (`org.springframework.resilience`, `org.springframework.core.retry`) |
+| Resilience4j | 2.4.0 (`resilience4j-spring-boot4`, not managed by Boot - pin it) |
+| JDK | Eclipse Temurin 25.0.4.1 (LTS) |
+
+## Quickstart
+
+```bash
+export JAVA_HOME=/path/to/jdk-25
+mvn -DskipTests package
+./scripts/run.sh                                  # port 8081
+curl -s localhost:8081/demo/retry/exhausted | python3 -m json.tool
+./scripts/run-all.sh                              # about 90 s: some scenarios sleep through 1 s delays
+mvn test                                          # 16 contract tests
+```
+
+## Scenarios
+
+| Endpoint | Shows |
+|---|---|
+| `/demo/retry/{defaults,exhausted,exponential,jitter,includes,cause,future,mono,timeout,hang,self-invocation}` | what `@Retryable` does, counted |
+| `/demo/tx/{standalone,joined}` | `@Retryable` + `@Transactional`, alone and inside a caller's transaction |
+| `/demo/limit/{none,block,reject,limit-and-retry,r4j-bulkhead}` | `@ConcurrencyLimit` policies, and Resilience4j's bulkhead for comparison |
+| `/demo/r4j/{retry,breaker,combo,timelimiter}` | what only Resilience4j does, and both libraries on one method |
+| `/demo/proxy/{payments,stock-writer,reports}` | the advisor chain on each proxy, outermost first |
+
+All of these are diagnostics for the article. Delete [`DemoController`](src/main/java/com/ankurm/resilience/web/DemoController.java) before shipping anything.
+
+## Documentation
+
+1. [What Spring Framework 7 ships, and in which 7.0.x release](docs/01-whats-in-framework-7.md)
+2. [Switching it on - and the three ways it stays off](docs/02-enabling.md)
+3. [`@Retryable`, measured](docs/03-retryable-measured.md)
+4. [`@ConcurrencyLimit`: BLOCK, REJECT, and nesting with retry](docs/04-concurrency-limit.md)
+5. [Retry and transactions](docs/05-retry-and-transactions.md)
+6. [What is left for Resilience4j](docs/06-what-is-left-for-resilience4j.md)
+7. [Migrating from spring-retry](docs/07-migrating-from-spring-retry.md)
+
+## Findings worth the trip
+
+- **`maxRetries = 3` is four invocations.** Resilience4j's `maxAttempts: 3` is three, and so was
+  spring-retry's `maxAttempts`. A mechanical migration adds one call.
+- **`timeout` is a budget checked between attempts, not a call timeout.** A 1.5 s attempt under a
+  500 ms timeout runs its full 1.5 s.
+- **Jitter only ever adds delay** (for the first delay, and for every delay when `multiplier` is 1):
+  `delay=200, jitter=100` produced gaps of 211-296 ms, never below 200.
+- **A `CompletableFuture` that completes exceptionally is not retried**; a `Mono` is.
+- **`@Retryable` is placed outside `@Transactional` and outside Resilience4j** regardless of any
+  `order` - both post-processors call `setBeforeExistingAdvisors(true)`. Good for transactions, bad
+  for a circuit breaker: an open circuit is retried three times.
+- **`timeout` and `ConcurrencyLimit.ThrottlePolicy.REJECT` did not exist in 7.0.0.** `timeout`
+  arrived in 7.0.2, `policy` in 7.0.3.