Ankur Mhatre
958b401f0f
Adds spring-boot-startup-time/, the companion project for BLOG-618: a runnable Spring Boot 4.1.1 application on JDK 25 that installs BufferingApplicationStartup and FlightRecorderApplicationStartup behind a system property, and a /diag/startup endpoint that computes step self time -- the number /actuator/startup does not give you and the one that names the actual culprits. Captured under docs/output/: the step tree sorted both ways, the same startup as JFR events, a +5000-class experiment putting 0.11 ms per scanned class on the classpath scan tax, the silent truncation a 2048-step buffer performs, and JDK 25 AOT cache timings (6.93 s to 4.82 s). Post body and metadata live in post/. Moves the existing Actuator project into actuator-in-production/ so the repository holds one directory per article; the root README is now an index.
83 lines
3.1 KiB
Markdown
83 lines
3.1 KiB
Markdown
# 02 — Turning instrumentation on
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← prev [01 — The number Boot logs](01-the-number-boot-logs.md) · next → [03 — The four phases](03-the-four-phases.md)
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---
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## There is no property for this
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The single most common wasted hour: looking for `spring.application.startup=buffering` in
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`application.yaml`. It does not exist. `ApplicationStartup` has to be set on the
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`SpringApplication` **before** `run()`, because the steps you care about are recorded
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before any configuration file has been read.
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```java
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public static void main(String[] args) {
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SpringApplication app = new SpringApplication(StartupDiagnosisApplication.class);
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int capacity = Integer.getInteger("startup.buffer", 16384);
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switch (System.getProperty("startup.tracking", "buffering")) {
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case "buffering" -> app.setApplicationStartup(new BufferingApplicationStartup(capacity));
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case "jfr" -> app.setApplicationStartup(new FlightRecorderApplicationStartup());
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default -> { }
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}
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app.run(args);
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}
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```
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That means changing which tracker you use is a **redeploy**, not a config change — worth
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knowing before an incident, not during one. See
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[`StartupDiagnosisApplication.java`](../src/main/java/com/ankurm/startup/StartupDiagnosisApplication.java).
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## Exposing the endpoint
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`startup` is not web-exposed by default:
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```yaml
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management:
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endpoints:
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web:
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exposure:
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include: health,info,startup,beans,conditions
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```
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Exposing it without installing a tracker gives you an endpoint that returns an empty
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timeline rather than an error, which is a confusing way to lose twenty minutes.
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## GET peeks, POST drains
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This is the trap that costs people their only recording. From
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[`docs/output/02-startup-tree.txt`](output/02-startup-tree.txt):
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```
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GET /actuator/startup -> events in response: 400
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POST /actuator/startup -> events in response: 400
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POST /actuator/startup -> events in response: 0
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GET /actuator/startup -> events in response: 0
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```
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`POST` calls `drainBufferedTimeline()`, which empties the buffer so the memory can be
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reclaimed. Every guide shows the `POST`. If you pipe it to a file and the file is wrong, the
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data is gone — the application has to be restarted to record it again. Use `GET` while you
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are still working out what you want.
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## You do not need a `@Bean` for it
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Boot registers the instance as a singleton named `applicationStartup` before the context
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refreshes, so it can simply be injected. Declaring your own `@Bean` produces:
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```
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Parameter 0 of constructor in ...StartupDiagnosticsEndpoint required a single bean,
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but 2 were found:
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- bufferingApplicationStartup: defined by method 'bufferingApplicationStartup' ...
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- applicationStartup: a programmatically registered singleton
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```
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Inject the `ApplicationStartup` interface and narrow with `instanceof`, not the concrete
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`BufferingApplicationStartup` — otherwise the application refuses to start whenever
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somebody runs it without tracking. Full transcript in
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[`docs/output/01-api-corrections.txt`](output/01-api-corrections.txt).
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---
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← prev [01 — The number Boot logs](01-the-number-boot-logs.md) · next → [03 — The four phases](03-the-four-phases.md)
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