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。
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。
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
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。