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Java / JVM Architecture (Java21) Interview questions

What is the difference between platform threads and virtual threads at the JVM level?

The two thread types differ in how the JVM represents and schedules them, not in the public API a developer writes against.

Platform ThreadVirtual Thread
1:1 wrapper around an OS threadJVM-managed continuation, not tied 1:1 to an OS thread
Fixed, relatively large stack, often around 1MB, reserved upfrontSmall stack stored on the heap that grows and shrinks as needed
Scheduled by the operating systemScheduled by the JVM onto a small pool of carrier platform threads
Practical limit: thousands per processPractical limit: millions per process

Because a blocked platform thread ties up a full OS thread until it unblocks, thread-per-request servers built on platform threads traditionally had to cap concurrency with bounded thread pools; virtual threads remove that specific constraint by making a "blocked" thread cheap to have millions of.

A platform thread's stack, compared to a virtual thread's, is:
The practical concurrency limit differs mainly because:

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