The Java specification which can be found here [1] makes clear that with respect to its memory model the following is true:

    1. Per 17.4.5 your example can lead to a data race.

       "When a program contains two conflicting accesses (§17.4.1) that are not ordered by a happens-before relationship, it is said to contain a data race."

    2. Per 17.7 the variable s is accessed atomically.

       "Writes to and reads of references are always atomic, regardless of whether they are implemented as 32-bit or 64-bit values."

However, atomic reads and writes are not sufficient to protect against data races. What atomic reads and writes will protect against is word tearing (outlined in 17.6 where two threads simultaneously write to overlapping parts of the same object with the result being bits from both writes mixed together in memory). However, a data race involving atomic objects can still result in future reads from that object to result in inconsistent values, and this can last indefinitely into the future. This does not mean that reading from a reference will produce a garbage value, but it does mean that two different threads reading from the same reference may end up reading two entirely different objects. So, you can have thread A in an infinite loop repeatedly reading the value "uninitialized" and thread B in another infinite loop repeatedly reading the value args[0]. This can happen because both threads have their own local copy of the reference which will never be updated to reflect a consistent shared state.

As per 17.4.3, a data-race free program will not have this kind of behavior where two threads are in a perpetually inconsistent state, as the spec says "If a program has no data races, then all executions of the program will appear to be sequentially consistent."

So while atomicity protects against certain types of data corruption, it does not protect against data races.

[1] https://docs.oracle.com/javase/specs/jls/se24/html/jls-17.ht...