Platform threads map 1:1 to OS threads. When a thread blocks on I/O, the OS scheduler parks a core. With Virtual Threads (Java 21+), the JVM manages millions of lightweight threads. Blocking on I/O yields the virtual thread without yielding the OS thread.

Subtitle: A Professional Developer’s Guide to JVM Deep Dives, Performance Tuning, and Enterprise Frameworks PDF Description (Back Cover / Blurb) "You’ve mastered the syntax. You’ve built your first Spring Boot API. Now it’s time to stop writing code that just ‘works’—and start writing code that scales, survives, and performs."