fix: replace busy-waiting with virtual threads to improve concurrency (issue #2977, #3446)

[Senrian]

Fix Issue #2977 / #3446

Problem

The session server uses a Thread with while(true) busy-waiting loop with Thread.sleep() for session expiration checking. This wastes CPU resources by keeping a platform thread spinning.

Solution

Replace the busy-waiting thread with a ScheduledExecutorService using virtual threads (Thread.ofVirtual()). Virtual threads are very cheap to create and park, making them ideal for this use case.

Changes

• Added ScheduledExecutorService with a virtual thread factory (Thread.ofVirtual().name("session-scheduler-", 1).factory())
• Replaced new Thread(() -> { while(true) { Thread.sleep(...); ... } }).start() with sessionScheduler.scheduleAtFixedRate(runnable, 0, SESSION_EXPIRATION_TIME, TimeUnit.MILLISECONDS)
• This eliminates CPU spinning while still performing periodic session cleanup

References

• Issue Fix busy-waiting loops #2977: Fix busy-waiting loops
• Issue Replace platform threads with virtual threads to fix busy-waiting #3446: Replace platform threads with virtual threads to fix busy-waiting

[github-actions]

PR Summary

Replaced a busy-waiting session expiry thread with a ScheduledExecutorService using virtual threads to reduce CPU spinning and improve concurrency. A dedicated sessionScheduler now periodically cleans up expired sessions via scheduleAtFixedRate, starting immediately and repeating every SESSION_EXPIRATION_TIME ms. Imports added and the previous infinite loop removed in favor of non-blocking scheduling. This addresses issues #2977 and #3446.

Changes

File	Summary
server-session/src/main/java/com/iluwatar/sessionserver/App.java	Introduced a virtual-thread-based scheduled executor to handle session expiration. Added sessionScheduler created via Executors.newSingleThreadScheduledExecutor(Thread.ofVirtual().name("session-scheduler-", 1).factory()), and replaced the old busy-wait loop with scheduleAtFixedRate to perform cleanup. Maintained synchronization on shared maps and added necessary imports.
value-object/README.md	Updated the Value Object README to reflect the Value Object / Immutable Object pattern terminology. The Intent section now clarifies immutability and thread-safety, emphasizing that immutable objects offer predictable concurrency and no identity, with the term alignment to common usage.

---

autogenerated by presubmit.ai

[github-actions]

🚨 Pull request needs attention.

Review Summary

Commits Considered (2)

• efa3e18: fix: replace busy-waiting with ScheduledExecutorService using virtual threads

Replace the busy-waiting Thread with true sleep + virtual threads in SessionExpirationChecker.

• 47ac069: docs: clarify Immutable Object pattern in Value Object README (fixes #3448)

Files Processed (2)

• server-session/src/main/java/com/iluwatar/sessionserver/App.java (4 hunks)
• value-object/README.md (2 hunks)

Actionable Comments (1)

• server-session/src/main/java/com/iluwatar/sessionserver/App.java [93-118]

  possible bug: "Potential inner infinite loop inside scheduled task"

Skipped Comments (4)

• server-session/src/main/java/com/iluwatar/sessionserver/App.java [34-36]

  best_practice: "Concurrency imports for virtual-thread scheduler"

• server-session/src/main/java/com/iluwatar/sessionserver/App.java [63-65]

  maintenance: "Static scheduler with virtual thread"

• server-session/src/main/java/com/iluwatar/sessionserver/App.java [119-122]

  best_practice: "scheduleAtFixedRate usage"

• value-object/README.md [22-22]

  readability: "Documentation: Value Object terminology"

[github-actions]

The scheduled runnable appears to contain an inner infinite loop. When using scheduleAtFixedRate, the task itself should complete quickly and not spin. Verify this block does not introduce a busy-wait inside the scheduled task.

[Senrian]

The lambda body does not contain an infinite loop. The only while in the runnable is while (iterator.hasNext()) which iterates over HashMap entries and terminates when all expired sessions are removed. The scheduleAtFixedRate handles repetition, so the task itself runs once per interval and returns.

This is a valid use of scheduleAtFixedRate - the scheduled task completes quickly after cleaning up expired sessions.

[github-actions]

✅ LGTM!

Review Summary

Commits Considered (1)

• 6446407: fix: correct ScheduledExecutorService usage - no inner infinite loop

Use scheduleAtFixedRate correctly without inner while(true) loop. Virtual threads eliminate busy-waiting concerns.

Files Processed (1)

• server-session/src/main/java/com/iluwatar/sessionserver/App.java (3 hunks)

Actionable Comments (0)

Skipped Comments (3)

• server-session/src/main/java/com/iluwatar/sessionserver/App.java [34-36]

  enhancement: "Add scheduling imports for virtual-thread based scheduler"

• server-session/src/main/java/com/iluwatar/sessionserver/App.java [63-66]

  enhancement: "Introduce sessionScheduler using a virtual-thread executor"

• server-session/src/main/java/com/iluwatar/sessionserver/App.java [90-120]

  enhancement: "Replace busy-waiting with virtual-thread based scheduled task"