Project loom: Why are virtual threads not the default?

Question

According to the project loom documentation virtual threads behave like normal threads while having almost zero cost and the ability to turn blocking calls into non-blocking ones.

If this is true, then why are they separate things? Why not just make them the default? Is there any reason to not use them?

Answer 1

There are really two questions here: 1. Why are virtual threads not the default? and 2. Is there ever a reason not to use them.

Regarding the default, Java really has not concept of a "default" thread. Once virtual threads arrive, when you create a thread, you must specify whether you want a platform thread or a virtual thread. The question then becomes why have we decided not to automatically replace today's threads with virtual threads (i.e. make new Thread() create a virtual thread). The answer to that is quite simple: it would not be helpful at all and might well be quite harmful. It would not be helpful because the advantages of virtual threads come from the ability to create a great many of them. If your application creates N threads today, nothing would be gained by turning those N threads into virtual threads. The scaling advantage of virtual threads would only kick in when your application creates, say, 1000N threads, which means it would need to be changed, anyway (e.g. by replacing Executors.newFixedThreadPool with Executors.newVirtualThreadPerTaskExector). It might be harmful because while virtual threads' semantics are almost the same as platform threads, they are not perfectly backward compatible (see JEP 425 for details).

As to the question about when not to use virtual threads, there are some obvious cases. E.g. when your threads heavily interact with native code, which knows nothing about virtual threads, or when you depend on some detail that has changed for virtual threads, like the ability to subclass Thread. Other cases are not so clear. For example, CPU-bound operations do not benefit from having more threads than CPU cores, so they do not benefit from the multitude of virtual threads, but that doesn't mean that they would be harmed. We're still not ready to say that users should pick virtual threads by default, but we might well get there, as we learn more about how people use them.