Stateless objects are always thread-safe?

Question

In Java Concurrency In Practice, Section 2.1, it states:

Stateless objects are always thread-safe.

And gives the following class as an example:

@ThreadSafe
public class StatelessFactorization implements Servlet {
    public void service(ServletRequest req, ServletResponse resp){
        BigInteger i = extractFromRequest(req);
        BigInteger[] factors = factor(i);
        encodeIntoResponse(resp, factors);       // <-- isn't it possible for resp to be
                                                 //     modified by mult. threads at once?
    }
}

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Question:

As indicated in the code above, what happens if multiple threads try to modify the same ServletResponse variable.

From my understanding of memory allocation, the above class does not seem completely thread safe.

While the reference to the ServletRequest and ServletResponse are put on the local stack for the calling thread, the actual objects are stored on the heap --which is shared between all threads.

Answer 1

Here the stateless object is StatelessFactorization class. It's stateless because it has no own state, namely it has no instance fields. Thus this object is thread-safe. The resp is another object implementing the ServletResponse interface and it may or may not be thread-safe. Here the thread-safety of resp is not discussed.