spring-boot-starter-data-redis Cacheable(sync=true) sees not work?

Question

i want to use Cacheable(sync=true) to controll concurrent behaviors to access a method

@Service
@CacheConfig(cacheNames = "book")
public class BookService {

@Cacheable(key = "#isbn",sync = true)
public Book book(String isbn,int id) {
    System.out.println("wait 3s...");
    try {
        Thread.sleep(3000);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
    return new Book(isbn, "xxxx");
  }

}

write a test case:

ExecutorService executorService = Executors.newFixedThreadPool(10);
    for (int i = 0; i < 100; i++) {
        final int index = i;
        executorService.execute(new Runnable() {
            @Override
            public void run() {
                bookService.book("1100011", index);
            }
        });
    }

all keys are the same,so sync method may works, but i get :

wait 3s...
wait 3s...
wait 3s...
wait 3s...
wait 3s...

is it a wrong way to use redis cache like this?