Working with Virtual Threads (Java 21+)

1. Creating Virtual Threads (Thread.startVirtualThread)

API	Use
`Thread.startVirtualThread(r)`	Quick start
`Thread.ofVirtual().start(r)`	Builder, named
`Thread.ofVirtual().unstarted(r)`	Manual start

Aspect	Detail
Per task	One virtual thread per submitted task
No pooling	Virtual threads are cheap
AutoCloseable	Use try-with-resources

try (var exec = Executors.newVirtualThreadPerTaskExecutor()) {
    List<Future<String>> results = exec.invokeAll(tasks);
}

Builder Method	Use
`name(prefix, counter)`	Diagnostic naming
`uncaughtExceptionHandler(h)`	Error handling
`inheritInheritableThreadLocals(b)`	TL inheritance
`factory()`	Get ThreadFactory

Virtual Thread (cheap, 100Ks possible)
       ↕ mount/unmount
Carrier Thread (platform, ForkJoinPool)
       ↕
Operating System

Cause	Mitigation
`synchronized` blocking	Use ReentrantLock instead
JNI/native call	Unavoidable — carrier blocked
Detect	`-Djdk.tracePinnedThreads=full`

API	Use
`ScopedValue.newInstance()`	Define
`ScopedValue.where(sv, val).run(r)`	Set in scope
`sv.get()`	Read in scope
vs ThreadLocal	Immutable, scope-bound, no leak

Replace	With
Fixed thread pool	Virtual-thread-per-task executor
`synchronized`	`ReentrantLock`
ThreadLocal heavy use	ScopedValue
Caching expensive objects	No need — VTs are cheap

Aspect	Detail
Type	Platform thread in dedicated FJP
Count	Default = available cores
Tune	`-Djdk.virtualThreadScheduler.parallelism=N`

Combo	Detail
VT + StructuredTaskScope	Idiomatic concurrent code
Cancellation	Scope shutdown propagates
Error handling	ShutdownOnFailure aggregates