# 01 — The number Boot logs, and what it hides

next → [02 — Turning instrumentation on](02-turning-instrumentation-on.md)

---

Every Spring Boot application ends its startup with one line:

```
Started StartupDiagnosisApplication in 6.6 seconds (process running for 7.4)
```

Two numbers. The first is measured from the `SpringApplication.run()` call to the
`ApplicationReadyEvent`. The second is `ManagementFactory.getRuntimeMXBean().getUptime()`,
so the gap between them is JVM bootstrap: opening the jar, verifying classes, starting the
JIT — work that happens before your code runs at all.

Neither number tells you where the 6.6 seconds went, and the usual next move — reading the
log timestamps — is worse than it looks. The log only shows you components that chose to
log. This application spends about half a second inside `tariffCacheWarmer` and logs
nothing while doing it. On the timeline it is a silent gap between two Hibernate lines,
and you would reasonably conclude that Hibernate was slow.

## What is actually available

Spring Framework has carried an `ApplicationStartup` SPI since 5.3. It is a tracing
interface with one method that matters:

```java
public interface ApplicationStartup {
    StartupStep start(String name);
}
```

Framework and Boot call it at about two dozen named points — `spring.beans.instantiate`,
`spring.context.config-classes.parse`, `spring.data.repository.proxy`, and so on — tagging
each step with the bean name or class count involved. Steps nest, so what you get is a
tree, not a list.

The default implementation, `DefaultApplicationStartup`, does nothing. That is the whole
reason startup profiling feels unavailable: the instrumentation is already in your
application and is switched off.

Two implementations record it:

| Implementation | Where it lives | Output |
|---|---|---|
| `BufferingApplicationStartup` | `spring-boot` | in-memory buffer, read over HTTP |
| `FlightRecorderApplicationStartup` | `spring-core` | JFR events in a `.jfr` file |

There is no third option and no third-party agent needed.

## What this repository measures

The application here is deliberately ordinary: web, JPA over H2, three repositories, and
four beans that do real work on the way up. It starts in about 6.6 seconds on the machine
that produced [`docs/output/`](output). Every figure in the article and in these chapters
came from a file in that directory.

---

next → [02 — Turning instrumentation on](02-turning-instrumentation-on.md)