错误的序列号生成器

public class IncorrectSerialGenerator {
    
    public static final int MAX_VAL = 999;
    public static final int R_NUM = getRNum(MAX_VAL);
    
    private static AtomicInteger val = new AtomicInteger(0);

/*    private static AtomicReference<AtomicInteger> ref = new AtomicReference<AtomicInteger>(new AtomicInteger(0));
    
    public static int getSerialNo() {
        AtomicInteger old = ref.get();
        int serialNo = old.getAndIncrement();
        while (serialNo > MAX_VAL) {
            AtomicInteger update = new AtomicInteger(0);
            ref.compareAndSet(old, update);
            old = ref.get();
            serialNo = old.getAndIncrement();
        }
        
        return serialNo;
    }*/
    
    public static int getSerialNo() {
        int serialNo = val.getAndIncrement();
        if (serialNo > MAX_VAL) {
            val.set(0); //原子操作
            serialNo = val.getAndIncrement();//原子操作，但是两者一起就不是原子操作，存在线程安全问题
        }
        
        return serialNo;
    }
    
    /**
     * 求整数有几位，如234是3位。
     * @param i
     * @return
     */
    private static final int getRNum(int i) {
        if (i < 0) {
            throw new RuntimeException("Illegal arg i, i=" + i);
        }
        
        int div = i;
        int num = 1;
        div /= 10;
        while (div > 0) {
            div /= 10;
            num++;
        }
        
        return num;
    }
    
    /**
     * 获取序列号，该序列号为字符串表示，长度固定为序列号的最大长度，
     * 如果长度不足，则在前面补0，比如"015"
     * @return
     */
    public static String getSerialNoInStr() {
        int serialNo = getSerialNo();
        int rnum = getRNum(serialNo);
        
        StringBuilder sb = new StringBuilder();
        for (int i=0; i<R_NUM-rnum; i++) {
            sb.append("0");
        }
        
        return sb.append(serialNo).toString();
    }
    
    
    public static void main(String[] args) throws InterruptedException, ExecutionException {
        IncorrectSerialGenerator gen = new IncorrectSerialGenerator();
        gen.test();
    }
    
    @Test
    public void test() throws InterruptedException, ExecutionException {
        //测10次
        for (int t=0; t<10; t++) {
            int taskNum = 1 + new Random().nextInt(19);
            @SuppressWarnings("unchecked")
            Future<Integer>[] fs = new Future[taskNum];
            ExecutorService executor = Executors.newFixedThreadPool(taskNum);
            for (int i=0; i<taskNum; i++) {
                fs[i] = executor.submit(new Callable<Integer>() {
                    int sum = 0;
                    @Override
                    public Integer call() throws Exception {
                        for (int i=0; i<=MAX_VAL; i++) {
                            sum += getSerialNo();
//                            System.out.println(Thread.currentThread().getName() + " : " + sum);
                        }
                        
                        return new Integer(sum);
                    } 
                });
            }
            
            int result = 0;
            for (Future<Integer> future : fs) {
                result += future.get();
            }
            
            int expect = 0;
            for (int i=0; i<taskNum; i++) {
                for (int j=0; j<=MAX_VAL; j++) {
                    expect += j;
                }
            }
            Assert.assertEquals(expect, result);
            System.out.println(taskNum + " : " + expect);
        }
        
        Random random = new Random();
        for (int i=0; i<10000; i++) {
            getSerialNoInStr();
            if (random.nextInt(1000) < 9) {
                System.out.println(getSerialNoInStr());
            } 
        } 
    }
}