MySql悲观锁和乐观锁总结

末蓝、 2022-05-15 12:13 267阅读 0赞

现在我有一个购买商品的需求，我们知道当我们购买商品时，后台会进行减库存和增加购买记录的操作。我们分别在无锁和乐观锁和悲观锁进行相应的代码演示来说明问题。

建表语句如下：

CREATE TABLE `stock` (
      `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
      `name` varchar(50) NOT NULL DEFAULT '' COMMENT '名称',
      `count` int(11) NOT NULL COMMENT '库存',
      `sale` int(11) NOT NULL COMMENT '已售',
      `version` int(11) NOT NULL COMMENT '乐观锁，版本号',
      PRIMARY KEY (`id`)
    ) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8
    CREATE TABLE `stock_order` (
      `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
      `sid` int(11) NOT NULL COMMENT '库存ID',
      `name` varchar(30) NOT NULL DEFAULT '' COMMENT '商品名称',
      `create_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP COMMENT '创建时间',
      PRIMARY KEY (`id`)
    ) ENGINE=InnoDB AUTO_INCREMENT=981 DEFAULT CHARSET=utf8

一、无锁的Mysql：

先看代码：

public class Test2 {
        public static void main(String[] args) throws SQLException, InterruptedException {
            Test2 tests = new Test2();
            Thread[] threads = new Thread[100];
            for (int i=0;i<100;i++){
                threads[i] = new Thread(){
                    @Override
                    public void run() {
                        try {
                            tests.service();
                        }catch (Exception e){
                            e.printStackTrace();
                        }
                    }
                };
            }
            for (int i=0;i<100;i++){
                threads[i].start();
            }
        }
        public void service() throws Exception {
            Connection connection = dbUtils.getConnection();
            String selectSql = "select count from stock where id = 1";
            PreparedStatement statement1 = connection.prepareStatement(selectSql);
            ResultSet resultSet = statement1.executeQuery();
            resultSet.next();
            String count = resultSet.getString("count");
            System.out.println(count);
            int c = Integer.parseInt(count);
            Thread.sleep(10);
            if (c<1)
                throw new Exception();
            String updateSql = "update stock set count = count - 1 where count > 0";
            PreparedStatement preparedStatement = connection.prepareStatement(updateSql);
            int update = preparedStatement.executeUpdate();
            String insertSql = "insert into stock_order(sid,name) VALUES (1,'aaa')";
            PreparedStatement statement = connection.prepareStatement(insertSql);
            int insert = statement.executeUpdate();
        }
    }

从上述代码可以看到，有一百个线程去模拟一百个用户购买商品，数据库中只有10个商品，所以当商品卖完时，应该增加10条购买记录。为了让大家看个清楚，我在代码中加入了线程的睡眠。

![70][]

我们看到，增加了11条记录，也就是所谓的超卖现象，商家绝不可能允许这种情况的发生。

MySql的乐观锁：

我们在使用乐观锁时会假设在极大多数情况下不会形成冲突，只有在数据提交的时候，才会对数据是否产生冲突进行检验。如果数据产生冲突了，则返回错误信息，进行相应的处理。

实现：MySql最经常使用的乐观锁时进行版本控制，也就是在数据库表中增加一列，记为version，当我们将数据读出时，将版本号一并读出，当数据进行更新时，会对这个版本号进行加1，当我们提交数据时，会判断数据库表中当前的version列值和当时读出的version是否相同，若相同说明没有进行更新的操作，不然，则取消这次的操作。

public class Test {
        public static void main(String[] args) {
            Test test = new Test();
            Thread[] threads = new Thread[200];
            for (int i=0;i<200;i++){
                int finalI = i;
                threads[i] = new Thread(){
                    @Override
                    public void run() {
                       test.service();
                    }
                };
            }
            for (int i=0;i<200;i++){
                threads[i].start();
            }
        }
        public void service(){
            try {
                Connection connection = dbUtils.getConnection();
                Stock stock1 = checkStock(connection);
                updateCountByOpti(connection,stock1);
                createOrder(connection);
            }catch (Exception e){
                System.out.println(e.getMessage());
            }
        }
     
        private void createOrder(Connection connection) throws SQLException {
            String insertSql = "insert into stock_order(sid,name) VALUES (1,'aaa')";
            PreparedStatement statement = connection.prepareStatement(insertSql);
            int insert = statement.executeUpdate();
        }
        private void updateCountByOpti(Connection connection,Stock stock) throws SQLException {
            String sql = "update stock set count = count -1,version = version + 1 where version = " + stock.getVersion();
            PreparedStatement preparedStatement = connection.prepareStatement(sql);
            int update = preparedStatement.executeUpdate();
            if (update==0)
                throw new RuntimeException("没抢到");
        }
        public Stock checkStock(Connection connection) throws SQLException {
            String sql = "select * from stock where id = 1";
            PreparedStatement preparedStatement = connection.prepareStatement(sql);
            ResultSet resultSet = preparedStatement.executeQuery();
            Stock stock = null;
            if (resultSet.next()){
                stock = new Stock();
                stock.setId(resultSet.getInt("id"));
                stock.setName(resultSet.getString("name"));
                stock.setCount(resultSet.getInt("count"));
                stock.setSale(resultSet.getInt("sale"));
                stock.setVersion(resultSet.getInt("version"));
            }
            if (stock.getCount()<1)
                throw new RuntimeException("没有库存了");
            return stock;
        }
    }

上述在提交时，对version字段进行了比较，当数据库中的version和之前读取的version一样才会进行提交，否则提交失败，接下来进行测试。

![70 1][]

可以看到，只有10条记录，乐观锁保证了数据的一致性。

三、悲观锁

MySql的悲观锁就是打开事务，当启动事务时，如果事务中的sql语句涉及到索引并用索引进行了条件判断，那么会使用行级锁锁定所要修改的行，否则使用表锁锁住整张表。

public class Test {
        public static void main(String[] args) {
            Test test = new Test();
            Thread[] threads = new Thread[200];
            for (int i=0;i<200;i++){
                threads[i] = new Thread(){
                    @Override
                    public void run() {
                        try {
                            test.service();
                        } catch (SQLException e) {
                            e.printStackTrace();
                        }
                    }
                };
            }
            for (int i=0;i<200;i++){
                threads[i].start();
            }
        }
            public void service() throws SQLException {
                Connection connection = null;
                try {
                    connection = dbUtils.getConnection();
                    connection.setAutoCommit(false);
                    Stock stock1 = checkStock(connection);
                    updateCountByOpti(connection,stock1);
                    createOrder(connection);
                    connection.commit();
                }catch (Exception e){
                    System.out.println(e.getMessage());
                    connection.rollback();
                }
            }
     
            private void createOrder(Connection connection) throws SQLException {
                String insertSql = "insert into stock_order(sid,name) VALUES (1,'aaa')";
                PreparedStatement statement = connection.prepareStatement(insertSql);
                int insert = statement.executeUpdate();
            }
            private void updateCountByOpti(Connection connection,Stock stock) throws SQLException {
                String sql = "update stock set count = count -1,version = version + 1 where version = " + stock.getVersion();
                PreparedStatement preparedStatement = connection.prepareStatement(sql);
                int update = preparedStatement.executeUpdate();
                if (update==0)
                    throw new RuntimeException("没抢到");
            }
     
            public Stock checkStock(Connection connection) throws SQLException, InterruptedException {
                String sql = "select * from stock where id = 1";
                PreparedStatement preparedStatement = connection.prepareStatement(sql);
                ResultSet resultSet = preparedStatement.executeQuery();
                Stock stock = null;
                if (resultSet.next()){
                    stock = new Stock();
                    stock.setId(resultSet.getInt("id"));
                    stock.setName(resultSet.getString("name"));
                    stock.setCount(resultSet.getInt("count"));
                    stock.setSale(resultSet.getInt("sale"));
                    stock.setVersion(resultSet.getInt("version"));
                }
                if (stock.getCount()<1)
                    throw new RuntimeException("没有库存了");
                return stock;
            }
    }

开启事务并不难，所以使用悲观锁很简单，让我们看一下结果

![70 2][]

结果还是10条记录

我们可以在不同的场合使用不同的处理方法，乐观锁并发高并且性能也很好，而悲观锁虽然并发不是很高，但是它不允许脏读，所以各有各的优点。

本文转载自：[https://blog.csdn.net/yanghan1222/article/details/80449528][https_blog.csdn.net_yanghan1222_article_details_80449528]

[70]: /images/20220515/4dbcbb2fa9874f35aca900660d7ded8d.png
[70 1]: /images/20220515/0cc0586f1ba84c2cb5db37cfd9e05399.png
[70 2]: /images/20220515/80bfc2f7b5f64d02905acf409f9c6f76.png
[https_blog.csdn.net_yanghan1222_article_details_80449528]: https://blog.csdn.net/yanghan1222/article/details/80449528