谁帮忙看看这段 Java 并发代码

我开了 100 个线程用同一个 id 分别去请求这两个方法，把这 100 次方法执行时间的平均值取出来，并且相同的方法重复做了三次得到结果： a: 24907.9ms, b: 24831.1ms ，看结果方法 b 好一点，但这个差距是不是很小啊？

@Controller
@RequestMapping("/bench/")
public class BenchController {

    private static Object[] lockObj;
    private static AtomicReference<Integer>[] locks;
    static {
        lockObj = new Object[100];
        for (int i = 0; i < lockObj.length; i++) {
            lockObj[i] = new Object();
        }

        locks = new AtomicReference[100];
        for (int i = 0; i < locks.length; i++) {
            locks[i] = new AtomicReference<Integer>(null);
        }
    }

    @RequestMapping("a")
    @ResponseBody
    public long a(int id) throws Exception {
        long start = System.currentTimeMillis();
        int index = id % 100;
        synchronized (lockObj[index]) {
            Thread.sleep(500);
        }
        long result=System.currentTimeMillis() - start;
        System.out.println(result);
        return result;
    }

    @RequestMapping("b")
    @ResponseBody
    public long b(int id) throws Exception {
        long start = System.currentTimeMillis();
        int index = id % 100;
        while (!locks[index].compareAndSet(null, id)) {
        }
        Thread.sleep(500);
        locks[index].compareAndSet(id, null);
        long result=System.currentTimeMillis() - start;
        System.out.println(result);
        return result;
    }
}