# 02 — Turning instrumentation on

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## There is no property for this

The single most common wasted hour: looking for `spring.application.startup=buffering` in
`application.yaml`. It does not exist. `ApplicationStartup` has to be set on the
`SpringApplication` **before** `run()`, because the steps you care about are recorded
before any configuration file has been read.

```java
public static void main(String[] args) {
    SpringApplication app = new SpringApplication(StartupDiagnosisApplication.class);
    int capacity = Integer.getInteger("startup.buffer", 16384);
    switch (System.getProperty("startup.tracking", "buffering")) {
        case "buffering" -> app.setApplicationStartup(new BufferingApplicationStartup(capacity));
        case "jfr" -> app.setApplicationStartup(new FlightRecorderApplicationStartup());
        default -> { }
    }
    app.run(args);
}
```

That means changing which tracker you use is a **redeploy**, not a config change — worth
knowing before an incident, not during one. See
[`StartupDiagnosisApplication.java`](../src/main/java/com/ankurm/startup/StartupDiagnosisApplication.java).

## Exposing the endpoint

`startup` is not web-exposed by default:

```yaml
management:
  endpoints:
    web:
      exposure:
        include: health,info,startup,beans,conditions
```

Exposing it without installing a tracker gives you an endpoint that returns an empty
timeline rather than an error, which is a confusing way to lose twenty minutes.

## GET peeks, POST drains

This is the trap that costs people their only recording. From
[`docs/output/02-startup-tree.txt`](output/02-startup-tree.txt):

```
  GET   /actuator/startup -> events in response: 400
  POST  /actuator/startup -> events in response: 400
  POST  /actuator/startup -> events in response: 0
  GET   /actuator/startup -> events in response: 0
```

`POST` calls `drainBufferedTimeline()`, which empties the buffer so the memory can be
reclaimed. Every guide shows the `POST`. If you pipe it to a file and the file is wrong, the
data is gone — the application has to be restarted to record it again. Use `GET` while you
are still working out what you want.

## You do not need a `@Bean` for it

Boot registers the instance as a singleton named `applicationStartup` before the context
refreshes, so it can simply be injected. Declaring your own `@Bean` produces:

```
Parameter 0 of constructor in ...StartupDiagnosticsEndpoint required a single bean,
but 2 were found:
	- bufferingApplicationStartup: defined by method 'bufferingApplicationStartup' ...
	- applicationStartup: a programmatically registered singleton
```

Inject the `ApplicationStartup` interface and narrow with `instanceof`, not the concrete
`BufferingApplicationStartup` — otherwise the application refuses to start whenever
somebody runs it without tracking. Full transcript in
[`docs/output/01-api-corrections.txt`](output/01-api-corrections.txt).

---

← prev [01 — The number Boot logs](01-the-number-boot-logs.md) · next → [03 — The four phases](03-the-four-phases.md)