一、问题代码

任务：执行10轮，使用多线程list收集添加元素，检查每次执行的结果。

public static void main(String[] args) throws InterruptedException {         int loop = 0;         int threadNum = 10;         while (loop < 10) {             //创建计数器             CountDownLatch countDownLatch = new CountDownLatch(threadNum);             //创建集合             List<String> list = new ArrayList<>();             for (int i = 0; i < threadNum; i  ) {                 new Thread(() -> {                     //集合添加内容                     list.add(UUID.randomUUID().toString().substring(0, 8));                     ///线程操作完成                     countDownLatch.countDown();                 }, String.valueOf(i)).start();             }             //等待所有线程完成             countDownLatch.await();             System.out.println("---集合长度："   list.size()   "---loop:"   loop   "---");             System.out.println(list);             loop  ;         }     }

loop:6时，只加了9个。

---集合长度：10---loop:0--- [955419b4, cd30b81c, 8bee554b, ff91be9b, 841408f7, 8a132002, 4384a15c, df8c561e, 4897166a, 80714bf0] ---集合长度：10---loop:1--- [d29700a9, f512d3d8, c9bc5faf, 749e0e91, 25dc21d5, 570b7473, 49161838, a2c28a34, 8e138a7c, 77d887cb] ---集合长度：10---loop:2--- [1e9ca70b, a07560ad, cb4c76ba, b2d1893c, 45e0f262, 68e25ac2, 09d2fe4f, 281765b4, 9877edc4, 284b852c] ---集合长度：10---loop:3--- [b17ba08f, 68f289dc, 47182e97, 7bdb5210, 7917e4fe, 04a0b440, b733d289, e137c32b, 4e5c920c, 10a9acdb] ---集合长度：10---loop:4--- [6bd8be8e, e07b569f, e8b37d23, c724a1f4, 2fa0b4fa, a0359dd5, 441dec81, 47f55116, 7642ab44, 4619b854] ---集合长度：10---loop:5--- [345dbd60, cac7fbb0, 4e54acb8, 6f6a4a82, 941579a3, 9f7242db, 4c1aa6ee, 81eb7196, faa589b9, e35f2f3b] ---集合长度：9---loop:6--- [5fcaa369, a0ced8b5, a05d0c74, 8415e2d9, 72ab401b, 809fd7a0, 51442894, 52bcf2a3, d477d1c3] ---集合长度：10---loop:7--- [85c82ca3, 074e8881, fb798fb4, 99a9fabe, ec4b6b5a, 964f0b63, 87aa845e, bab5e35b, f40bc936, dc284c89] ---集合长度：10---loop:8--- [98e6c1f2, 37a4370b, 6b7360af, a51b0051, 6322a817, 055b62d2, 78201d93, bee8e60d, 537ac6c7, 876b8428] ---集合长度：10---loop:9--- [e706c947, cc068833, 01c34aea, 870a294a, effd91ea, 8cf00cab, 8ae2aa4c, b0441ffa, 1ccf31bc, 91b143f8]

loop:4.只加9个；

---集合长度：10---loop:0--- [627e8354, ce0eaff6, 7097d1d7, 9e5b07d5, 8b4f2e0c, e7ddf456, 59364b06, 3b2f5370, b9f2ea33, 8174b138] ---集合长度：10---loop:1--- [06701acb, 8af28cc8, 8ca4f803, 707722cb, cb277df8, 72a63097, e6b33f7e, 71bae0f6, d51ebeae, 779ef7fc] ---集合长度：10---loop:2--- [95f05da1, 5c068fd5, 1493ad15, 91561550, 17af73fd, c99c6616, 3e1ee3c2, 09c2ede0, ef9367fa, 65808dcd] ---集合长度：10---loop:3--- [ffc77c44, 36ef14d8, 7d0a36a4, 12bb2f90, 065b3dc5, 891a45f6, 8a6a4a1f, 4264090b, d585f760, 2c282e1b] ---集合长度：9---loop:4--- [17dda97c, 7aa517b3, 96984409, ad74eb02, d8e5c893, dff7a694, f7a72f69, 49457871, 791a0695] ---集合长度：10---loop:5--- [5346072d, d3050d5d, e87a942f, 0e8e8061, 3189c7f7, 4ee541cd, de0a0100, 370f0465, e03d71a7, 3f2572fe] ---集合长度：10---loop:6--- [f1ae5275, f62ec85f, b6c564c3, 04e999c0, d54d0863, 5e5c4e69, c12cc9e9, 74f6dc15, 87632932, 79c98321] ---集合长度：10---loop:7--- [c55d1196, b9e18d20, 24f23a0e, 8c54c083, 61cdf212, e60cc532, a5fc1d27, 79dc4bf2, ea6be1fe, 9df13359] ---集合长度：10---loop:8--- [6a40ef06, 22a69e75, 0d448a08, 6af1c353, c091190e, ef670d5b, ad790da0, 52b9f59d, 2c287738, f96176ce] ---集合长度：10---loop:9--- [13a0bdb1, fe98ce3f, 45aac986, b7b940bf, f41f8c8f, 62bf82ea, 8a123b4b, 85bb41a1, 776cfdd1, 149a8212]

发生原因：ArrayList的add该方法不是同步方法。n线程可能同时进入并获得size如果是相同的值，则n线同时只添加一个元素。

二、解决方案

1、使用List线程安全子类Vector

 public static void main(String[] args) throws InterruptedException {         int loop = 0;         int threadNum = 10;         while (loop < 10) {             //创建计数器             CountDownLatch countDownLatch = new CountDownLatch(threadNum);             //创建集合             List<String> list = new Vector<>();             for (int i = 0; i < threadNum; i  ) {                 new Thread(() -> {                     //集合添加内容                     list.add(UUID.randomUUID().toString().substring(0, 8));                     ///线程操作完成                     countDownLatch.countDown();                 }, String.valueOf(i)).start();             }             //等待所有线程完成             countDownLatch.await();             System.out.println("---集合长度："   list.size()   "---loop:"   loop   "---");             System.out.println(list);             loop+;
        }

    }

---集合长度：10---loop:0---
[391b7947, cc171749, 17326cbe, 696340e8, 6932ede4, 57769aeb, 445a93c6, 314bee8a, ee14af4c, 3b886645]
---集合长度：10---loop:1---
[4806f7e2, eccfa620, e19ee85e, 3a0bd870, c9f0f4bc, 6ed89d16, 3ea92668, 7237ddeb, eedea3e1, d13753f0]
---集合长度：10---loop:2---
[558e3ee2, 592d8534, ffbd8e1e, 2bd467e8, 3fc60170, 382a2460, 32ae670b, 5dd26920, 1acaf2a3, 7261d7e3]
---集合长度：10---loop:3---
[a520df0f, abe540ed, 223feeab, ccb285a5, 1283fce8, e2bd426a, d23c4b82, dd425f23, 03c683fd, 9650489a]
---集合长度：10---loop:4---
[fe667d22, f726c6ee, 88ad82e0, 0104cf48, 4c6dd4da, 9c50cd5e, 28a22a43, 20038d20, fd6a12c2, b4a2b869]
---集合长度：10---loop:5---
[653dfdef, 31818490, 56291645, c9bf42c6, f49cf28b, b8612671, 9ef9956c, af4c048f, 612a2aa8, 00488f64]
---集合长度：10---loop:6---
[54892a2c, 63eff31d, 50d67c43, c4984687, 89f86418, 5cd079f3, 93b8cdc7, 7f6e857e, 5eeec1a8, ef024516]
---集合长度：10---loop:7---
[a1c94d7b, f304ec2a, d9c32237, 2dc75056, 5df3701d, 34e1b122, ac2b3b41, a9d44e50, bbf95afa, 914b8d33]
---集合长度：10---loop:8---
[92350948, 3f691746, d7a64cdd, d31bef58, d23fb841, b594b477, 4c9494f8, 71bc27b8, 8293f055, fb0331b1]
---集合长度：10---loop:9---
[54b8d072, 9abbdbeb, daeca0a6, 0566c89f, cc9c4b56, 25fd1787, 7b3cb590, 292cff8a, 3295be2b, 7fea15ba]

2、使用 Collections.synchronizedList()

 使用同步方法操作 

 public static void main(String[] args) throws InterruptedException {
        int loop = 0;
        int threadNum = 10;
        while (loop < 10) {
            //创建一个计数器
            CountDownLatch countDownLatch = new CountDownLatch(threadNum);
            //创建集合
            List<String> list =Collections.synchronizedList(new ArrayList<>());
            for (int i = 0; i < threadNum; i++) {
                new Thread(() -> {
                    //集合添加内容
                    list.add(UUID.randomUUID().toString().substring(0, 8));
                    //线程操作完成一个减一
                    countDownLatch.countDown();
                }, String.valueOf(i)).start();
            }
            //等待所有线程执行完成
            countDownLatch.await();
            System.out.println("---集合长度：" + list.size() + "---loop:" + loop + "---");
            System.out.println(list);
            loop++;
        }
    }

---集合长度：10---loop:0---
[2a59eb58, c04f64de, 5d2f6924, c3772c5f, 440b529c, 997ac389, f95717d7, da233e53, 05894c56, 26da6f22]
---集合长度：10---loop:1---
[19dc29cc, 7eb82b00, d8715162, f7b82a61, f8106ccf, 81b4a4d1, cc07a96f, 6b405055, 2074db7e, 87b2a8b8]
---集合长度：10---loop:2---
[eea3f5f0, 2f555a94, 7ce5bcc2, 99547f75, 7b2b3c41, 64718afe, 3d1e3350, 86420d89, e1b7c452, 353a8843]
---集合长度：10---loop:3---
[40621d07, 200f11f9, 815e6613, d84e2943, 84760222, fd8385d6, 5c928382, 6dc9c863, a1c49665, f0e0472a]
---集合长度：10---loop:4---
[cb800a5c, 5f81fc14, a2d64da5, f5a67780, ee845c8d, b631e9b1, 5961daf6, 7ba11e93, 24f0513c, 786eb7bf]
---集合长度：10---loop:5---
[0db1a50e, 43b374ca, 768c363f, 4baf5e9f, 65ee9ebc, c0d2c24b, 532a29fd, ff5682d6, d3250085, 590d3eb2]
---集合长度：10---loop:6---
[9223a887, ce7f3d69, 551c3199, 5c092523, 2af16ab2, e401f80b, b2db6e9d, dbb48564, fa6076a9, 9bfcb824]
---集合长度：10---loop:7---
[134a9b7c, 3e9ba896, d01847c1, d8263425, d63002ff, 5d19ed22, 60704521, 42065456, 8c9501d8, 36224a1f]
---集合长度：10---loop:8---
[b14957b4, 33a51813, 0c5a992c, 9a5a8fe6, 1df1187c, a6cd0e73, 3dd89d6a, 3d0be681, 071e0d5d, 20d8a7d5]
---集合长度：10---loop:9---
[57f4bf4a, 8ef7282b, 5fbee16c, 4ceb41d3, a6afd3d7, 08c3a07c, 4d17d47e, 48616ce7, dbe45214, 76b332d5]

3、使用CopyOnWriteArrayList（推荐）

Java CopyOnWriteArrayList详解 - 简书 (jianshu.com)

public static void main(String[] args) throws InterruptedException {
        int loop = 0;
        int threadNum = 10;
        while (loop < 10) {
            //创建一个计数器
            CountDownLatch countDownLatch = new CountDownLatch(threadNum);
            //创建集合
            List<String> list = new CopyOnWriteArrayList<>();
            for (int i = 0; i < threadNum; i++) {
                new Thread(() -> {
                    //集合添加内容
                    list.add(UUID.randomUUID().toString().substring(0, 8));
                    //线程操作完成一个减一
                    countDownLatch.countDown();
                }, String.valueOf(i)).start();
            }
            //等待所有线程执行完成
            countDownLatch.await();
            System.out.println("---集合长度：" + list.size() + "---loop:" + loop + "---");
            System.out.println(list);
            loop++;
        }

    }

---集合长度：10---loop:0---
[89e6a3e4, b24efa55, 26dfde63, 344fdd63, f17d9ff4, 00dcc8ec, 3550b167, de14127a, cad5aa6e, 81c025db]
---集合长度：10---loop:1---
[b785dc56, edf42136, 46722ecf, 93d37d13, d0de78df, d4e8d3ba, 481c40f1, 57bec525, a68d51c4, 006c8406]
---集合长度：10---loop:2---
[651cdab5, a1d6a5d2, f90052ec, 1594f2e6, c1cc4d69, 46bf3d47, a0050804, bb7e7068, ec850dad, 9da3c35d]
---集合长度：10---loop:3---
[2bcfc2fc, 3b16ae9f, 10666ded, 096d7904, f65702be, 5af12eb4, 53a4edb8, 465a7907, 2497082a, 66c9db76]
---集合长度：10---loop:4---
[646ce345, f4a8ca8e, cfded69f, 185d394f, f42cadeb, 3e926b34, 729f71f6, 5916a8d7, 806f331b, 0566775a]
---集合长度：10---loop:5---
[df47784e, 8f7dbe4e, ba5e3f00, 72520d47, 86af1ddf, edba78eb, 2670bdd3, 3663b234, ea8930d4, c00b2f7e]
---集合长度：10---loop:6---
[e4399e5b, 412405c7, b547bebe, 70da2c66, d85fb42a, 32fe82b9, ed1ffea9, 5a1e0f8b, e77683c9, b1ed414c]
---集合长度：10---loop:7---
[75b2ff0a, e22f6eef, 249fe313, a1771cb3, cf8d33c2, a2aeb0ad, 4562bbf2, c09abd16, 92489b03, e68e2ced]
---集合长度：10---loop:8---
[687a916d, 6acea36d, 27bfad3c, 94ba873c, 39626ce0, bb2836fc, 8f7f88e7, 42ea11cc, c82bd4c7, 6de280ff]
---集合长度：10---loop:9---
[43d88928, adc1476c, 465f9e1c, ade2bf6c, c90c2e16, 2deb25ec, b59c62c7, 13abb545, 098bac48, 1c6213b5]