Java Virtual Threads and Kotlin Coroutines Differ in How They Suspend, Shifting the Bottleneck

A DEV Community article compares Java virtual threads with Kotlin coroutines, focusing on how their suspension models affect code structure and runtime behavior. Traditional thread-per-request servers are limited by thread count, even though many threads sit idle waiting on databases or external APIs. Both technologies release the underlying OS thread during waits and resume at the suspension point, but virtual threads save and restore the full stack while coroutines rely on compiler-transformed, stackless functions. The author argues that virtual threads hide suspension points from the source code, whereas coroutines mark them explicitly. The article also notes that these approaches reduce the cost of waiting without adding total resources, so the bottleneck moves elsewhere, often out of sight in the virtual thread model.