Java / Java 21 Virtual Threads Interview questions
Explain the execution flow when a virtual thread makes a blocking call?
Consider a virtual thread issuing a network read. The sequence below shows how the JVM keeps the carrier thread productive instead of letting it sit idle.
sequenceDiagram
participant VT as Virtual Thread
participant Sched as Scheduler
participant Carrier as Carrier Thread
participant IO as OS / NIO Selector
VT->>Carrier: mounted, executing code
VT->>IO: issue blocking read()
Carrier-->>Sched: unmount VT, carrier freed
Sched->>Carrier: run a different virtual thread
IO-->>Sched: data ready, VT marked runnable
Sched->>Carrier: mount VT on an available carrier
Carrier->>VT: resume continuation after read()
The virtual thread never "knows" it was unmounted - its continuation resumes exactly where the blocking call left off, with the returned data available as if the call had simply blocked in place.
While a virtual thread waits on I/O, the freed carrier thread:
Once the awaited I/O completes, the virtual thread is:
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