利用MySQL中的乐观锁和悲观锁实现分布式锁

背景

对于一些并发量不是很高的场景，使用MySQL的乐观锁实现会比较精简且巧妙。
下面就一个小例子，针对不加锁、乐观锁以及悲观锁这三种方式来实现。
主要是一个用户表，它有一个年龄的字段，然后并发地对其加一，看看结果是否正确。
一些基础实现类

1. @Data
2. @Builder
3. @AllArgsConstructor
4. @NoArgsConstructor
5. public class User {
6.     private Integer age;
7.     private String name;
8.     private Long id;
9.     private Long version;
10. }
12. public interface UserMapper {
13.     @Results(value = {
14.             @Result(property = "id", column = "id", javaType = Long.class, jdbcType = JdbcType.BIGINT),
15.             @Result(property = "age", column = "age", javaType = Integer.class, jdbcType = JdbcType.INTEGER),
16.             @Result(property = "name", column = "name", javaType = String.class, jdbcType = JdbcType.VARCHAR),
17.             @Result(property = "version", column = "version", javaType = Long.class, jdbcType = JdbcType.BIGINT),
18.     })
19.     @Select("SELECT id, age, name, version FROM user WHERE id = #{id}")
20.     User getUser(Long id);
22.     @Update("UPDATE user SET age = #{age}, version=version+1 WHERE id = #{id} AND version = #{version}")
23.     Boolean compareAndSetAgeById(Long id, Long version, Integer age);
25.     @Update("UPDATE user SET age = #{age} WHERE id = #{id}")
26.     Boolean setAgeById(Long id, Integer age);
28.     @Select("SELECT id, age, name, version FROM user WHERE id = #{id} for update")
29.     User getUserForUpdate(Long id);
30. }
32. private static void exe(CountDownLatch countDownLatch, int threads, Runnable runnable) {
33.     ExecutorService executorService = Executors.newCachedThreadPool();
34.     for (int i = 0; i < threads; i++) {
35.         executorService.execute(() -> {
36.             runnable.run();
37.             countDownLatch.countDown();
38.         });
39.     }
40.     executorService.shutdown();
41. }
43. private static User getUser(long id) {
44.     try (SqlSession session = openSession(getDataSource1())) {
45.         UserMapper userMapper = session.getMapper(UserMapper.class);
46.         return userMapper.getUser(id);
47.     }
48. }
50. public static SqlSession openSession(DataSource dataSource) {
51.     TransactionFactory transactionFactory = new JdbcTransactionFactory();
52.     Environment environment = new Environment("development", transactionFactory, dataSource);
53.     Configuration configuration = new Configuration(environment);
54.     configuration.addMapper(UserMapper.class);
55.     configuration.setCacheEnabled(false);
56.     SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(configuration);
57.     return sqlSessionFactory.openSession();
58. }
60. private static DataSource getDataSource1() {
61.     MysqlConnectionPoolDataSource dataSource = new MysqlConnectionPoolDataSource();
62.     dataSource.setUser("root");
63.     dataSource.setPassword("root");
64.     dataSource.setUrl("jdbc:mysql://localhost:3306/test?useUnicode=true&characterEncoding=utf-8");
65.     return dataSource;
66. }

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不加锁

1. private static Boolean addAge(long id, int value) {
2.     Boolean result;
3.     try (SqlSession session = openSession(getDataSource1())) {
4.         UserMapper userMapper = session.getMapper(UserMapper.class);
5.         User user = userMapper.getUser(id);
6.         result = userMapper.setAgeById(user.getId(), user.getAge() + value);
7.         session.commit();
8.     }
9.     return result;
10. }
12. public static void main(String[] args) throws Exception {
13.     long id = 1L;
14.     int threads = 50;
15.     CountDownLatch countDownLatch = new CountDownLatch(threads);
16.     log.info("user:{}", getUser(id));
17.     long start = System.currentTimeMillis();
18.     exe(countDownLatch, threads, () -> addAge(1, 1));
19.     countDownLatch.await();
20.     long end = System.currentTimeMillis();
21.     log.info("end - start : {}", end - start);
22.     log.info("user:{}", getUser(id));
23. }

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1. 865  [main] INFO  cn.eagle.li.mybatis.UpdateMain - user:User(age=0, name=3, id=1, version=0)
2. 1033 [main] INFO  cn.eagle.li.mybatis.UpdateMain - end - start : 164
3. 1046 [main] INFO  cn.eagle.li.mybatis.UpdateMain - user:User(age=9, name=3, id=1, version=0)

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从输出可以看出，50个并发，但是执行成功的只有9个，这种实现很明显是有问题的。
乐观锁

1. private static Boolean compareAndAddAge(long id, int value, int times) {
2.     int time = 0;
3.     Boolean result = false;
4.     while (time++ < times && BooleanUtils.isFalse(result)) {
5.         result = compareAndAddAge(id, value);
6.     }
7.     return result;
8. }
10. private static Boolean compareAndAddAge(long id, int value) {
11.     try (SqlSession session = openSession(getDataSource1())) {
12.         UserMapper userMapper = session.getMapper(UserMapper.class);
13.         User user = userMapper.getUser(id);
14.         Boolean result = userMapper.compareAndSetAgeById(id, user.getVersion(), user.getAge() + value);
15.         session.commit();
16.         return result;
17.     }
18. }
20. public static void main(String[] args) throws Exception {
21.     long id = 1L;
22.     int threads = 50;
23.     CountDownLatch countDownLatch = new CountDownLatch(threads);
24.     log.info("user:{}", getUser(id));
25.     long start = System.currentTimeMillis();
26.     exe(countDownLatch, threads, () -> addAge(1, 1, 20));
27.     countDownLatch.await();
28.     long end = System.currentTimeMillis();
29.     log.info("end - start : {}", end - start);
30.     log.info("user:{}", getUser(id));
31. }

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1. 758  [main] INFO  cn.eagle.li.mybatis.UpdateMain - user:User(age=0, name=3, id=1, version=0)
2. 1270 [main] INFO  cn.eagle.li.mybatis.UpdateMain - end - start : 509
3. 1277 [main] INFO  cn.eagle.li.mybatis.UpdateMain - user:User(age=50, name=3, id=1, version=50)

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从输出可以看出，并发的情况下，结果是没问题的。
悲观锁

1. private static Boolean addAgeForUpdate(long id, int value) {
2.     Boolean result;
3.     try (SqlSession session = openSession(getDataSource1())) {
4.         UserMapper userMapper = session.getMapper(UserMapper.class);
5.         User user = userMapper.getUserForUpdate(id);
6.         result = userMapper.setAgeById(id, user.getAge() + value);
7.         session.commit();
8.     }
9.     return result;
10. }
12. public static void main(String[] args) throws Exception {
13.     long id = 1L;
14.     int threads = 50;
15.     CountDownLatch countDownLatch = new CountDownLatch(threads);
16.     log.info("user:{}", getUser(id));
17.     long start = System.currentTimeMillis();
18.     exe(countDownLatch, threads, () -> addAgeForUpdate(1, 1));
19.     countDownLatch.await();
20.     long end = System.currentTimeMillis();
21.     log.info("end - start : {}", end - start);
22.     log.info("user:{}", getUser(id));
23. }

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1. 631  [main] INFO  cn.eagle.li.mybatis.UpdateMain - user:User(age=0, name=3, id=1, version=50)
2. 829  [main] INFO  cn.eagle.li.mybatis.UpdateMain - end - start : 196
3. 837  [main] INFO  cn.eagle.li.mybatis.UpdateMain - user:User(age=50, name=3, id=1, version=50)

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从输出可以看出，并发的情况下，结果是没问题的。
总结

从以上来看，乐观锁和悲观锁实现都是没有问题的，至于选哪一种，还是要看业务的场景，比如说并发量的多少，加锁时长等等。

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