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Project Loom [1] is intended to provide green threads, or 'fibers', at the JVM level.

There have been other projects like Quasar from ParallelUniverse [2] that retrofits them by processing the compiled bytecode to enable saving/restoring the IP & stack.

[1] http://cr.openjdk.java.net/~rpressler/loom/Loom-Proposal.htm...

[2] http://docs.paralleluniverse.co/quasar/

Kilim [1] (afaik) was first in this space. (edit: just FYI)

[1]: http://www.malhar.net/sriram/kilim/kilim_ecoop08.pdf

There is a proposal to add fibers to Java: http://cr.openjdk.java.net/~rpressler/loom/Loom-Proposal.htm...

Neither the JVM nor the Java specifications impose restrictions on this, so theoretically I'd say it's possible.