Emperor.java

public class Emperor {
    private Emperor() {
    }
    
    private static List<Emperor> emperors;
    private static final Integer MAX_NUM_OF_EMPEROR = Integer.valueOf(3);
    private static AtomicInteger lastUsed = new AtomicInteger(-1);
    
    static {
        emperors = new ArrayList<>();
        for (int i = 0; i < MAX_NUM_OF_EMPEROR; ++i) {
            emperors.add(new Emperor());
        }
    }
   
    //todo 不显式加锁，如何实现对 emperors 集合中元素的轮询访问
    public static Emperor getInstance() {
        // lastUsed 增加到 Integer.MAX_VALUE 之后的行为和预期不符
        Integer index = lastUsed.incrementAndGet();
        return emperors.get(index % MAX_NUM_OF_EMPEROR);
    }
}