为什么ConcurrentHashMap读取性能优于并发环境中的HashMap？

Question

在并发环境中，只从map中获取对象，没有put。 HashMap和ConcurrentHashMap，在jdk 1.6中表现更好。

根据我对这两张地图的有限经验，ConcurrentHashMap由细分组成。当从中获取对象时，它至少需要两个定位动作。首先是找到正确的segment，然后找到正确的table，而HashMap只需要找到一次。而且，在我看来，当我们从地图中获取对象时，没有锁定种族。所以我认为在jdk 1.6中，毫无疑问HashMap在并发环境中从ConcurrentHashMap获得对象时会比public static void main(String[] args) throws InterruptedException { final Map<Integer, Integer> hashMap = new HashMap<Integer, Integer>(); final Map<Integer, Integer> concurrentHashMap = new ConcurrentHashMap<Integer, Integer>(); init(hashMap); init(concurrentHashMap); final Random random = new Random(System.currentTimeMillis()); final CountDownLatch latch1 = new CountDownLatch(100); long start1 = System.nanoTime(); for (int i = 0; i < 100; i++) { new Thread(new Runnable() { @Override public void run() { for (int count = 0; count < 10000; count++) { hashMap.get(random.nextInt(100)); } latch1.countDown(); } }).start(); } latch1.await(); long end1 = System.nanoTime(); System.out.println("hashMap : " + (end1 - start1)); final CountDownLatch count2 = new CountDownLatch(100); long start2 = System.nanoTime(); for (int i = 0; i < 100; i++) { new Thread(new Runnable() { @Override public void run() { for (int count = 0; count < 10000; count++) { concurrentHashMap.get(random.nextInt(100)); } count2.countDown(); } }).start(); } count2.await(); long end2 = System.nanoTime(); System.out.println("concurrentHashMap : " + (end2 - start2)); } private static void init(Map<Integer, Integer> map) { for (int i = 0; i < 100; i++) { map.put(i, i); } } 更好。

但不幸的是，测试结果并不支持我的观点。

这是我的测试代码。

hashMap : 78448168
concurrentHashMap : 68820282

hashMap : 87819463
concurrentHashMap : 76098734

hashMap : 87901572
concurrentHashMap : 71385562

最后三个输出是：

HashMap

然后我在单线程环境中测试性能。（从地图重复随机获取对象10000次。）

这是我的测试结果。

ConcurrentHashMap的平均时间为4982047 ns，而ConcurrentHashMap的平均时间为3150367 ns。

结果表明HashMap性能优于from random import randint def print_scores(uscore, cscore): uscore = 0 cscore = 0 print(uscore, cscore) def roll_two_dice(): roll1 = randint(1, 6) roll2 = randint(1, 6) if roll1 or roll2 == 1: return 0 elif roll1 and roll2 == 1: return 25 else: return roll1 + roll2 def computerTurn(score): turn_score = 0 while score + turn_score < 100 and turn_score < 20: roll = roll_two_dice() if roll == 0: return 0 score = roll + turn_score return turn_score def userTurn(score): turn_score = 0 while score + turn_score < 100 or user == hold: roll = roll_two_dice() if roll == 0: return 0 score = roll + turn_score return turn_score def final_result(uscore, cscore): uscore = 0 cscore = 0 print('Final results:{}'.format(uscore, cscore)) if uscore >= 100: print('You win') else: print('Computer wins') def main(): c_score = 0 u_score = 0 if randint(0, 1) == 0: print('Computer starts') c_turn = computerTurn(c_score) + c_score while c_score < 100: print('Current scores') print_scores(u_score, c_score) u_turn = userTurn(u_score) + u_score if u_score >= 100: exit c_turn = computerTurn(c_score) print(c_turn) c_score = c_score + c_turn final_result(u_score, c_score) main() ，即使在单线程环境中也是如此。

我真的很困惑。我有什么理解错了吗？或者只是我的测试代码不严谨？