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Java / Java 21 Virtual Threads Interview questions

What is the difference between virtual threads and platform threads?

The core difference is who does the scheduling and how expensive creation is. Platform threads map 1:1 to OS threads and are scheduled by the operating system; virtual threads are scheduled by the JVM and share a small pool of carrier threads.

AspectPlatform ThreadVirtual Thread
Mapping to OS thread1:1Many:few (via carrier threads)
Creation costRelatively highVery low
Stack storageFixed native stackSmall, resizable, on heap
Typical countHundreds to low thousandsMillions

Both expose the identical Thread API, so code written for one generally runs unmodified on the other.

Which is scheduled by the JVM rather than the OS?
Roughly how many virtual threads can a typical JVM support versus platform threads?

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