[pron]

Yes, it was in Java 6 (except for direct access to fences, which has been added in Java 8).

The sun.misc.Unsafe class is used extensively by JDK classes, and is meant for internal use. It provides intrinsics that are translated to a single machine instruction. Normally, you don't use the class directly. For example, you use, say, AtomicInt for CAS operations (which, internally uses s.m.Unsaafe) or the ByteBuffer class (which internally uses s.m.Unsafe for direct pointer access). The JDK classes add all sorts of protection (like range checks) around s.m.Unsafe, but if you know what you're doing, using s.m.Unsafe directly and eschewing some of those protections (usually adding ones more pertinent to your domain), you get some performance gains which may be significant depending on your use case.