I'm not asking where I can find it on my own computer, I'm asking where I can get it.
I've done my google homework, and I didn't find the headers in those paths. There wasn't even a Header folder in my /System/Library/Java/JavaVirtualMachines/... and stuff and I don't have a Developer folder under root.
Now I'm pretty sure I don't have those headers installed, I checked the MacOS developer website but I only found some broken links.
Shouldn't OpenJDK or SunJDK provide the jni.h header for MacOS somewhere?