The Java specification allows for green threads, N:1 mapping of threads, or system threads, 1:1 mapping to system threads.
It is up to the Java VM or compiler to offer one implementation or even both.
Tasks in java.util.concurrent make use of thread pooling to also achieve N:1 mapping of tasks to threads.
The Squawk and the Cacao VMs are two examples of a Java VM using green threads,
http://labs.oracle.com/projects/squawk/
http://www.cacaojvm.org/
The Java specification allows for green threads, N:1 mapping of threads, or system threads, 1:1 mapping to system threads.
It is up to the Java VM or compiler to offer one implementation or even both.
Tasks in java.util.concurrent make use of thread pooling to also achieve N:1 mapping of tasks to threads.
The Squawk and the Cacao VMs are two examples of a Java VM using green threads,
http://labs.oracle.com/projects/squawk/
http://www.cacaojvm.org/