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by Jtsummers
694 days ago
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The "not at all obvious" gotcha is described in the documentation near the top, under the heading "What is a Virtual Thread?": https://docs.oracle.com/en/java/javase/21/core/virtual-threa... > Like a platform thread, a virtual thread is also an instance of java.lang.Thread. However, a virtual thread isn't tied to a specific OS thread. A virtual thread still runs code on an OS thread. However, when code running in a virtual thread calls a blocking I/O operation, the Java runtime suspends the virtual thread until it can be resumed. The OS thread associated with the suspended virtual thread is now free to perform operations for other virtual threads. It's not been hidden at all in their presentation on virtual threads. The OS thread that the virtual thread is mounted to can still be preempted, but that won't free up the OS thread for another virtual thread. However, if you use them for what they're intended for this shouldn't be a problem. In practice, it will be because no one can be bothered to RTFM. |
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> what they're intended for
Java prides itself on careful and deliberate changes to eliminate foot guns, but this seems like a pretty major restriction. Usually these kinds of cooperative threads are called fibers or something else to distinguish them from truly preempt-able threads.
Expecting developers to read the minutiae of documentation (there's another restriction around synchronized blocks) is a fool's errand TBH. Principle of least surprise, etc.