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Well because in java you wont be able to do low level stuff like managing individual memory blocks that is essential in a websockets networking lib.

This is just false with NIO buffers. There’s a large corpus of Netty-based servers and clients for all sorts of binary protocols.

I wonder how other websockets libraries for java do it if it is impossible /s

Look at Netty.io, that's the defecto java networking library. They have some optional JNI stuff