[altfredd]

> it's difficult to change while keeping compatibility

Actually, it is trivial to change. Just embed a ReentrantLock into every object and rewrite all calls to "synchronized"/"Object.wait" to use that lock.

Unfortunately, this would result in a bit of performance regression (increasing per-object memory footprint). To solve that would require turning ReentrantLock into a magical intrinsic, fully integrated with lock bytes in the object header. Which is actually not that hard either — other runtimes like Golang or Android VM solve problems like this on daily basis. Oracle, however…

[samus]

... was taking years to land Project Loom. So long that people started calling it vaporware. Project Valhalla is still regarded as such by many. It had to be shipped as soon as it was usable, even though a few rough edges remain that really ought to be deburred.

As you indicate, the complexity lies in not burning too many bridges with existing users and use cases. This is something that Android regularly does and which Go never really had to do due to its shorter history and up-front design.