# 05 — JFR instead of a buffer

← prev [04 — Reading the step tree](04-reading-the-step-tree.md) · next → [06 — The classpath-scan tax](06-the-classpath-scan-tax.md)

---

`FlightRecorderApplicationStartup` lives in `spring-core`, needs no dependency, and emits
each `StartupStep` as a JFR event. Swap the tracker and start a recording:

```bash
java -XX:StartFlightRecording=filename=startup.jfr,settings=profile,dumponexit=true \
     -Dstartup.tracking=jfr -jar app.jar
```

Everything below is from [`docs/output/03-jfr.txt`](output/03-jfr.txt).

## The event type

```
@Name("org.springframework.core.metrics.jfr.FlightRecorderStartupEvent")
@Category("Spring Application")
@Label("Startup Step")
@Description("Spring Application Startup")
class FlightRecorderStartupEvent extends jdk.jfr.Event { ... }
```

In JDK Mission Control it appears under the **Spring Application** category. On the command
line, the name matters more than you would expect:

```
  --events StartupEvent                     -> 0 events
  --events 'org.springframework.core.metrics.jfr.FlightRecorderStartupEvent' -> 398 events
```

`jfr print --events` matches the `@Name`, which is the fully qualified class name. The short
form silently returns nothing — no error, no warning, just an empty result that reads like
"Spring did not record anything".

## Reading it without Mission Control

```bash
jfr summary startup.jfr
jfr print --events 'org.springframework.core.metrics.jfr.FlightRecorderStartupEvent' \
          --json startup.jfr
```

The eight slowest, sorted by the recording's own `duration` field:

```
      duration  name / tags
PT6.544849841S  spring.context.refresh
PT2.263845435S  spring.beans.instantiate beanName=&entityManagerFactory,...
PT1.500763631S  spring.context.beans.post-process
PT1.202591188S  spring.context.beandef-registry.post-process postProcessor=...ConfigurationClassPostProcessor@76b224cd
PT1.176668418S  spring.context.config-classes.parse classCount=130
PT0.538651275S  spring.beans.instantiate beanName=reportTemplateRegistry
PT0.535484510S  spring.beans.instantiate beanName=tariffCacheWarmer
PT0.515340537S  spring.boot.webserver.create factory=...TomcatServletWebServerFactory
```

(That run started in 7.441 s; the buffered run in [chapter 04](04-reading-the-step-tree.md)
started in 6.6 s. Same jar, different runs — see [chapter 07](07-failure-modes.md) on why
a single measurement is not a result.)

Note that this is the *total time* list from
[chapter 04](04-reading-the-step-tree.md), with `spring.context.refresh` on top and
`reportTemplateRegistry` above `tariffCacheWarmer`. JFR gives you durations and parent ids;
it does not compute self time either. The same subtraction applies.

## What JFR buys you that the buffer does not

Correlation. The recording holds the JVM's own view of the same seconds:

```
 jdk.ExecutionSample            345
 jdk.GCPhasePauseLevel1         132
 jdk.GCPhasePause                39     (289.8 ms of pause in total)
 jdk.Compilation                 16
 jdk.ClassLoaderStatistics       10
```

If a bean's constructor is slow because a young collection landed in the middle of it, the
buffer shows you a slow bean and JFR shows you why. That is the case for using it.

## What it costs you

- The recording is written to a file on the machine, not served over HTTP. In a container
  that means a volume or a copy out.
- The `tags` are serialised into one flat `String` attribute, because JFR events only carry
  base types. Parsing `beanName=x,beanType=y,` is on you.
- `settings=profile` records a great deal more than startup steps. For a 7-second startup
  that is fine; left on in production it is not.

## Buffer or JFR?

Use the buffer when the question is "which bean", and you can reach the application over
HTTP. Use JFR when the question is "why is that bean slow", when startup fails before the
web server is up (the buffer is unreachable then; the JFR file is not), or when you need
the same recording to answer a GC question.

---

← prev [04 — Reading the step tree](04-reading-the-step-tree.md) · next → [06 — The classpath-scan tax](06-the-classpath-scan-tax.md)