How the example from Java Concurrency in practice is thread safe

Question

Below is an example from book Java Concurrency in Practice(Listing 2.8) of a thread safe class.

My question is how the below class is thread safe?

For example if two threads Thread A and Thread B enter into service method of CachedFactorizer. Thread B followed by Thread A. Now if Thread A is executing first synchronized block and Thread B obviously waits for the intrinsic lock of the object. And if Thread B makes it to the first synchronized block before the Thread A makes it to the second synchronized block, it will be viewing a stale value and potentially this condition is known as Race Condition.

So, is my understanding right here? Or am I lacking some basic understanding of concurrency?

@ThreadSafe
public class CachedFactorizer implements Servlet {

    @GuardedBy("this") private BigInteger lastNumber;

    @GuardedBy("this") private BigInteger[] lastFactors;

    @GuardedBy("this") private long hits;

    @GuardedBy("this") private long cacheHits;

    public synchronized long getHits() { return hits; }

    public synchronized double getCacheHitRatio() {

        return (double) cacheHits / (double) hits;

    }


    public void service(ServletRequest req, ServletResponse resp) {

        BigInteger i = extractFromRequest(req);
        BigInteger[] factors = null;
        synchronized (this) {
            ++hits;
            if (i.equals(lastNumber)) {
                ++cacheHits;
                factors = lastFactors.clone();
            }
        }
        if (factors == null) {
            factors = factor(i);
            synchronized (this) {
                lastNumber = i;
                lastFactors = factors.clone();
            }
        }
        encodeIntoResponse(resp, factors);
    }
}

Answer 1

This class is thread safe because all shared variables are accessed in sychronized blocks, and in such situation there is no 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.https://docs.oracle.com/javase/specs/jls/se9/html/jls-17.html"

The question is if this behaviour is valid from business point of view, eg. if two threads meet after first synchronized blok, they can execute second block in different order.