Distributed lock based on MySQL
d为db指的是数据库，或者"Distributed lock"中的d。
在实际业务开发中，并不一定所有的场景都需要高性能的分布式锁，比如基于redis，zookeeper。有点时候，也需要基于数据库的分布式锁。 对性能要求不高，但是因为项目本身就已经用到了mysql，没有必要再引用一个多余的中间件产品。
因为锁本身是必须在finally中unlock的，所以不需要和spring事务关联，并且就算业务失败，分布式锁也不应该回退到之前的数据。 绑定spring事务参考"DataSourceUtils.getConnection(dataSource);" 和 "spring jdbctemplate"
会占用mysql的链接，所以不使用， 比较适合等待排他锁，但是不适合不等待的排他锁
这种方法没有考虑到锁超时，如果出现锁超时，又需要检查数据。
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