This answer was deleted by a moderator, probably because of link rot. Copy paste for those who can't see it:
Yes, see os::javaTimeMillis in
/src/os/solaris/vm/os_solaris.cpp
to see the Solaris implementation for instance. There are a few layers of wrappers to get there though, see JVM_CurrentTimeMillis in/src/share/vm/prims/jvm.cpp
Although it is true that the links were an important part of the answer (they point to the online JDK source browser), it also contained other useful information, and anyone who saw the rotten URL would be able to find the updated link themselves.
The result is that users with <10k reputation lose useful information.
Should we delete those answers or what else should be done?