# Why your Spring Boot app takes 8 seconds to start

Companion project for **[Why Your Spring Boot App Takes 8 Seconds to Start: A Bean-by-Bean
Diagnosis](post/post.md)**.

Every millisecond figure in the article came out of this project. The transcripts are in
[`docs/output/`](docs/output) and are regenerated by one command.

---

## Versions

Resolved by the build, not read from documentation — see
[`docs/output/00-versions.txt`](docs/output/00-versions.txt).

| Component | Version | Notes |
|---|---|---|
| Spring Boot | 4.1.1 | GA 20 August 2026 |
| Spring Framework | 7.0.9 | via `spring-boot-starter-parent` |
| Spring Data JPA | 4.1.1 | |
| Hibernate ORM | 7.4.5.Final | |
| Tomcat (embedded) | 11.0.24 | |
| Micrometer | 1.17.1 | |
| H2 | 2.4.240 | |
| JDK | Temurin 25.0.4.1+1 LTS | AOT cache needs 24+ |
| Maven | 3.9.11 | |

---

## Quickstart

```bash
export JAVA_HOME=/path/to/jdk-25
mvn -DskipTests package

# recording, and serving the self-time view at /diag/startup
java -Dstartup.tracking=buffering -jar target/startup-diagnosis-1.0.0.jar

curl -s 'localhost:8080/diag/startup?top=12'          # self time -- the useful list
curl -s  localhost:8080/actuator/startup              # GET peeks
curl -sX POST localhost:8080/actuator/startup         # POST drains. Only once.
```

Regenerate every transcript (~20 minutes):

```bash
JAVA_HOME=/path/to/jdk-25 ./scripts/run-all.sh
```

---

## Switches

Tracking is a **system property**, not a Spring property, because `ApplicationStartup` must
be set before `run()`. See [`docs/02`](docs/02-turning-instrumentation-on.md).

| Switch | Effect |
|---|---|
| `-Dstartup.tracking=buffering` | `BufferingApplicationStartup` (default in this project) |
| `-Dstartup.tracking=jfr` | `FlightRecorderApplicationStartup`; pair with `-XX:StartFlightRecording` |
| `-Dstartup.tracking=none` | no tracking — the uninstrumented baseline |
| `-Dstartup.buffer=2048` | buffer capacity in **steps**; too small truncates silently |
| `--spring.profiles.active=lazy` | `spring.main.lazy-initialization=true` |

---

## Endpoints

| Endpoint | What it gives you |
|---|---|
| `/diag/startup?top=N` | **this project's own** — self time, phase totals, both orderings |
| `/actuator/startup` | Boot's flat timeline. `GET` peeks, `POST` drains |
| `/actuator/health`, `/actuator/beans`, `/actuator/conditions` | exposed for context |
| `/orders/summary` | the application's actual job |

`/diag/startup` exposes bean names and wiring. **Delete it before shipping.**

---

## Documentation

| # | Chapter |
|---|---|
| 01 | [The number Boot logs, and what it hides](docs/01-the-number-boot-logs.md) |
| 02 | [Turning instrumentation on](docs/02-turning-instrumentation-on.md) |
| 03 | [The four phases hiding inside one number](docs/03-the-four-phases.md) |
| 04 | [Reading the step tree: self time versus total time](docs/04-reading-the-step-tree.md) |
| 05 | [JFR instead of a buffer](docs/05-jfr-instead-of-a-buffer.md) |
| 06 | [The classpath-scan tax, measured](docs/06-the-classpath-scan-tax.md) |
| 07 | [Failure modes](docs/07-failure-modes.md) |
| 08 | [What actually helps](docs/08-what-actually-helps.md) |

---

## Captured output

| File | Scenario |
|---|---|
| [`00-versions.txt`](docs/output/00-versions.txt) | resolved versions from the build |
| [`01-api-corrections.txt`](docs/output/01-api-corrections.txt) | `javap` and the compiler errors that corrected the code |
| [`02-startup-tree.txt`](docs/output/02-startup-tree.txt) | the step tree by self time and by total time |
| [`03-jfr.txt`](docs/output/03-jfr.txt) | the same startup as JFR events, read with `jfr` |
| [`04-scan-tax.txt`](docs/output/04-scan-tax.txt) | +5000 classes, with and without `@Component` |
| [`05-what-helps.txt`](docs/output/05-what-helps.txt) | tracking overhead, lazy init, JDK 25 AOT cache |
| [`06-buffer-overflow.txt`](docs/output/06-buffer-overflow.txt) | what a 2048-step buffer silently loses |

Step counts and orderings are reproducible. **Timings are indicative** and drift by a few
hundred milliseconds between runs on the same machine — the ratios are the result, not the
absolute numbers.

---

## Layout

```
pom.xml
scripts/
  run-all.sh                 regenerate everything under docs/output/
  run.sh / stop.sh           start and stop; stop.sh matches java+jar, never the main class
  gen-bulk.sh                generate N classes into the scanned package
  demo-*.sh                  one script per captured scenario
src/main/java/com/ankurm/startup/
  slow/                      four beans that do real work on the way up
  domain/                    one entity, one repository with derived queries
  web/                       StartupDiagnosticsEndpoint (self time) + the business controller
src/test/java/               contract tests for drain, truncation and double-counting
docs/                        numbered chapters
docs/output/                 captured real output
```

## Licence

MIT — see [LICENSE](../LICENSE).