[wahern]

Java's fibers actually work like the previous poster described, which may explain the confusion:

> The current prototype implements the mount/dismount operations by copying stack frames from the continuation stack – stored on the Java heap as two Java arrays, an Object array for the references on the stack and a primitive array for primitive values and metadata. Copying a frame from the thread stack (which we also call the vertical stack, or the v-stack) to the continuation stack (also, the horizontal stack, or the h-stack) is called freezing it, while copying a frame from the h-stack to the v-stack is called thawing. The prototype also optionally thaws just a small portion of the h-stack when mounting using an approach called lazy copy; see the JVMLS 2018 talk as well as the section on performance for more detail.

Source: https://wiki.openjdk.java.net/display/loom/Main

The video presentation describes it better. I think in their expected usage scenarios the stacks of fibers aren't particularly deep so the copying isn't that expensive. Also, IIRC, doing it this way was less intrusive to the existing JVM architecture; it's possible in time they'll rearchitect things to use a more traditional technique.