Java 21: virtual threads for sampler execution + API migration

[olka]

Summary

• Virtual threads for sampler execution: ThreadGroup now uses Thread.ofVirtual() for sampler threads and thread starters, reducing OS thread creation by ~3.5x (65 vs 226 thread creates for 200 concurrent users)
• synchronized → ReentrantLock conversions in StandardJMeterEngine, ThreadGroup, JMeterContextService, TestCompiler, AbstractThreadGroup to avoid virtual thread carrier pinning
• Java 21 API migration: deprecated new URL() → URI.create().toURL() in test code, @SuppressWarnings for production code where URI is too strict, new Locale() → Locale.of(), Thread.getId() → threadId(), removed deprecated ThreadDeath / System.runFinalization() usage
• Build target: Java 21

Design decision

StandardJMeterEngine.EXECUTOR_SERVICE uses Thread.ofPlatform() (not virtual) — virtual threads are daemon threads and would cause immediate JVM exit in non-GUI mode. Sampler threads remain virtual, tracked via waitThreadsStopped().

A/B Test Results (200 threads, 180s, 30ms server delay)

Metric	Virtual Threads	Platform Threads (master)
Throughput	~2,450 req/s	~2,390 req/s
Avg latency	79ms	81ms
Max latency	6.5s	7.6s
OS thread creates (JFR)	65	226
Thread alloc stats (JFR)	146	470

Test plan

• ./gradlew check — all tests pass (only pre-existing batchFTP_TESTS external dependency failure)
• 180s load test with 200 threads: 440k+ requests, 0 JMeter errors
• JFR comparison: virtual threads vs platform threads
• Distributed mode testing (engine executor + remote engines)