perf: Let HostnameCache worker thread idle out (JAVA-653)

[runningcode]

📜 Description

HostnameCache created its worker with Executors.newSingleThreadExecutor, whose thread stays alive for the entire process lifetime — even though the cache only refreshes once every 5 hours (and the hostname lookup runs for at most 1 second). It now uses a ThreadPoolExecutor (core = max = 1) with a 30s keep-alive and allowCoreThreadTimeOut(true), so the worker thread self-terminates when idle and is recreated on the next refresh.

The cache keeps its own dedicated executor rather than sharing the SDK-wide one on purpose: updateCache() blocks on future.get(1s), and running that on a shared executor could spuriously time out when the executor is busy with other work.

💡 Motivation and Context

Part of reducing the number of threads created by the SDK: JAVA-653.

On the JVM, any app that sends events with attachServerName enabled (the default off Android) kept this thread alive 24/7 for what is effectively one DNS lookup every 5 hours.

💚 How did you test it?

New HostnameCacheTest asserting the executor is configured to time out its core thread, plus the existing MainEventProcessorTest hostname coverage.

📝 Checklist

• I added GH Issue ID & Linear ID
• I added tests to verify the changes.
• No new PII added or SDK only sends newly added PII if sendDefaultPII is enabled.
• I updated the docs if needed.
• I updated the wizard if needed.
• Review from the native team if needed.
• No breaking change or entry added to the changelog.
• No breaking change for hybrid SDKs or communicated to hybrid SDKs.

🔮 Next steps

Related PRs in this effort: RateLimiter (#5814), LifecycleWatcher (#5819), performance collector (#5816), batch processors (#5818).

🤖 Generated with Claude Code

[linear-code]

JAVA-653

[sentry]

📲 Install Builds

Android

🔗 App Name	App ID	Version	Configuration
SDK Size	io.sentry.tests.size	8.50.0 (1)	release

⚙️ sentry-android Build Distribution Settings

[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	313.40 ms	375.26 ms	61.86 ms
Size	0 B	0 B	0 B

Baseline results on branch: main

Startup times

Revision	Plain	With Sentry	Diff
eb95ded	317.51 ms	369.08 ms	51.57 ms
2124a46	319.19 ms	415.04 ms	95.85 ms
c3ee041	310.64 ms	361.90 ms	51.26 ms
dcc6bbf	382.58 ms	462.13 ms	79.54 ms
7a19fee	315.46 ms	368.62 ms	53.16 ms
9054d65	330.94 ms	403.24 ms	72.30 ms
d500866	326.13 ms	378.70 ms	52.58 ms
ad8da22	365.86 ms	427.00 ms	61.14 ms
ed33deb	337.52 ms	484.06 ms	146.54 ms
d15471f	310.26 ms	377.04 ms	66.78 ms

App size

Revision	Plain	With Sentry	Diff
eb95ded	0 B	0 B	0 B
2124a46	1.58 MiB	2.12 MiB	551.51 KiB
c3ee041	0 B	0 B	0 B
dcc6bbf	1.58 MiB	2.12 MiB	553.10 KiB
7a19fee	0 B	0 B	0 B
9054d65	1.58 MiB	2.29 MiB	723.38 KiB
d500866	0 B	0 B	0 B
ad8da22	1.58 MiB	2.29 MiB	719.83 KiB
ed33deb	1.58 MiB	2.13 MiB	559.52 KiB
d15471f	1.58 MiB	2.13 MiB	559.54 KiB