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It's a lot easier to build custom allocators in C++ though.

For one, Java has a maximum mmap-size of 2 Gb, and as a cherry on top of that turd, you have no control over their lifecycle. The language is very clearly not designed for this type of work, and if you try to make it do it anyway, it fights you every step of the way.

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It has nothing to do with the language but with the API / VM capabilities.

Both those restrictions are fixed, granted that this is not yet in the offical API, it's an incubation module (the equivalent of from __future__ of Python).

https://docs.oracle.com/en/java/javase/17/docs/api/jdk.incub...

The foreign memory API which is currently incubating should help with most of these limitations: https://openjdk.java.net/jeps/419

Right - specifically they invented a way to make closing an mmapped segment safe and fast. The reason you can't officially (without private APIs) unmap something in current Java is because if you did then other threads would segfault, and "no segfaults" is kind a defining characteristic of Java. The new API fixes this using more VM magic, so closing a mapping from one thread will cause other threads to throw exceptions, but this is done without sacrificing performance (it doesn't require checking a status every time you read from memory).

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