Explain the execution flow of a virtual thread when it performs a blocking I/O call?

sequenceDiagram
  participant VT as Virtual Thread
  participant Sched as JVM Scheduler
  participant Carrier as Carrier Thread
  participant IO as I/O Subsystem
  VT->>Carrier: Mounted, running normally
  VT->>IO: Issue blocking read()
  Carrier->>Sched: Unmount VT continuation, park it
  Sched->>Carrier: Assign a different ready Virtual Thread
  IO-->>Sched: I/O completes, VT marked ready
  Sched->>Carrier: Mount VT (same or different carrier) when free
  Carrier->>VT: Resume execution after read()

The virtual thread starts mounted on a carrier thread and issues what looks like an ordinary blocking read; the JDK's I/O layer recognizes this call and, instead of parking the OS thread, unmounts the virtual thread's continuation and hands the carrier to the scheduler.

The scheduler immediately assigns that now-free carrier to another ready virtual thread, so no CPU-bound OS thread sits idle waiting on the disk or network.

Once the underlying I/O completes, the virtual thread is marked ready and re-mounted onto whichever carrier thread becomes available next, resuming execution right after the original blocking call as if nothing unusual happened.