Mysql并发保证数据一致性——实例

1. 背景

最近的项目中遇到一项问题，并发更新某一单据的时候，出现了更新失效的情况。比如：

@Transactional(rollbackFor = Exception.class)
public void update(Integer id){
    //1.按id查询
    //2.更新某一字段的值
}

生成的sql大概是这样的：

UPDATE  table 
SET field = #{field,jdbcType=INTEGER}
WHERE id= 1

那么以上代码产生的问题就是：
对于同一个id=1来说，请求A与请求B都进到了update方法中，此时按id查询得到的信息是相同的，那么请求A更新了id=1的这条记录之后，此时，请求B又对id=1的记录进行更新，此时注意请求B更新id=1的记录的时候，请求B按id查询到的数据已经是旧数据了，请求B执行完之后，请求A的更新被请求B覆盖了。

如果更新的是状态，那倒无所谓，如果库存量呢？
如果更新的是库存，是在这条记录的原始值上进行+或者-操作，是不是就出问题了？

2. 分析产生的原因

事后写了个小demo来复现问题，代码大致如下：

需求： 某个分类每被访问一次，排序就+1；
开启两个客户端同时访问

@Transactional(rollbackFor = Exception.class)
public void updateStatusTest(Integer id) {
    System.out.println("start:"+Thread.currentThread().getName());
    CateGory category = categoryMapper.selectById(id);
    try {
        Thread.sleep(8000);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
    System.out.println(Thread.currentThread().getName()+":"+category);
    category.setSort(category.getSort()+1);
    categoryMapper.updateById(category);

    category = categoryMapper.selectById(id);
    System.out.println(Thread.currentThread().getName()+":"+category);
    System.out.println("end:"+Thread.currentThread().getName());
}

DAO层使用的mybatis，生成日志大致如下：
需要更新的sort值数据库初始为：0

start:Http-NIO-8062-exec-1
Creating a new SqlSession
ReGIStering transaction synchronization for SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@6df7e3e9]
JDBC Connection [com.alibaba.druid.proxy.jdbc.ConnectionProxyImpl@2d5c951c] will be managed by spring
==>  Preparing: SELECT cid,category_name,parent_id,image_url,icon_url,sort,status FROM t_admin_category WHERE cid=? 
==> Parameters: 6(Integer)
<==    Columns: cid, category_name, parent_id, image_url, icon_url, sort, status
<==        Row: 6, 1, 0, 11111, 1, 0, 1111
<==      Total: 1
Releasing transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@6df7e3e9]
 Time：27 ms - ID：com.sxl.simple.shop.admin.category.mapper.CategoryMapper.selectById
Execute SQL：
    SELECT
        cid,
        category_name,
        parent_id,
        image_url,
        icon_url,
        sort,
        status 
    FROM
        t_admin_category 
    WHERE
        cid=6

start:http-nio-8062-exec-2
Creating a new SqlSession
Registering transaction synchronization for SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@7af43046]
JDBC Connection [com.alibaba.druid.proxy.jdbc.ConnectionProxyImpl@700c5b36] will be managed by Spring
==>  Preparing: SELECT cid,category_name,parent_id,image_url,icon_url,sort,status FROM t_admin_category WHERE cid=? 
 Time：2 ms - ID：com.sxl.simple.shop.admin.category.mapper.CategoryMapper.selectById
Execute SQL：
    SELECT
        cid,
        category_name,
        parent_id,
        image_url,
        icon_url,
        sort,
        status 
    FROM
        t_admin_category 
    WHERE
        cid=6

==> Parameters: 6(Integer)
<==    Columns: cid, category_name, parent_id, image_url, icon_url, sort, status
<==        Row: 6, 1, 0, 11111, 1, 0, 1111
<==      Total: 1
Releasing transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@7af43046]
http-nio-8062-exec-1:Category{cid=6, categoryName='1', parentId=0, imageUrl='11111', iconUrl='1', sort=0, status='1111'}
Fetched SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@6df7e3e9] from current transaction
==>  Preparing: UPDATE t_admin_category SET category_name=?, parent_id=?, image_url=?, icon_url=?, sort=?, status=? WHERE cid=? 
==> Parameters: 1(String), 0(Integer), 11111(String), 1(String), 1(Integer), 1111(String), 6(Integer)
 Time：16 ms - ID：com.sxl.simple.shop.admin.category.mapper.CategoryMapper.updateById
Execute SQL：
    UPDATE
        t_admin_category 
    SET
        category_name='1',
        parent_id=0,
        image_url='11111',
        icon_url='1',
        sort=1,
        status='1111' 
    WHERE
<==    Updates: 1
        cid=6

Releasing transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@6df7e3e9]
 Time：0 ms - ID：com.sxl.simple.shop.admin.category.mapper.CategoryMapper.selectById
Execute SQL：
Fetched SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@6df7e3e9] from current transaction
    SELECT
        cid,
        category_name,
==>  Preparing: SELECT cid,category_name,parent_id,image_url,icon_url,sort,status FROM t_admin_category WHERE cid=? 
        parent_id,
==> Parameters: 6(Integer)
        image_url,
        icon_url,
<==    Columns: cid, category_name, parent_id, image_url, icon_url, sort, status
        sort,
        status 
<==        Row: 6, 1, 0, 11111, 1, 1, 1111
    FROM
<==      Total: 1
        t_admin_category 
    WHERE
Releasing transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@6df7e3e9]
        cid=6

http-nio-8062-exec-1:Category{cid=6, categoryName='1', parentId=0, imageUrl='11111', iconUrl='1', sort=1, status='1111'}
end:http-nio-8062-exec-1
Transaction synchronization committing SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@6df7e3e9]
Transaction synchronization deregistering SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@6df7e3e9]
Transaction synchronization closing SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@6df7e3e9]
http-nio-8062-exec-2:Category{cid=6, categoryName='1', parentId=0, imageUrl='11111', iconUrl='1', sort=0, status='1111'}
Fetched SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@7af43046] from current transaction
==>  Preparing: UPDATE t_admin_category SET category_name=?, parent_id=?, image_url=?, icon_url=?, sort=?, status=? WHERE cid=? 
 Time：0 ms - ID：com.sxl.simple.shop.admin.category.mapper.CategoryMapper.updateById
Execute SQL：
    UPDATE
        t_admin_category 
    SET
        category_name='1',
        parent_id=0,
        image_url='11111',
        icon_url='1',
        sort=1,
        status='1111' 
    WHERE
        cid=6

==> Parameters: 1(String), 0(Integer), 11111(String), 1(String), 1(Integer), 1111(String), 6(Integer)
<==    Updates: 1
Releasing transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@7af43046]
Fetched SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@7af43046] from current transaction
==>  Preparing: SELECT cid,category_name,parent_id,image_url,icon_url,sort,status FROM t_admin_category WHERE cid=? 
==> Parameters: 6(Integer)
<==    Columns: cid, category_name, parent_id, image_url, icon_url, sort, status
<==        Row: 6, 1, 0, 11111, 1, 0, 1111
 Time：16 ms - ID：com.sxl.simple.shop.admin.category.mapper.CategoryMapper.selectById
Execute SQL：
    SELECT
        cid,
        category_name,
<==      Total: 1
        parent_id,
        image_url,
Releasing transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@7af43046]
        icon_url,
        sort,
        status 
    FROM
http-nio-8062-exec-2:Category{cid=6, categoryName='1', parentId=0, imageUrl='11111', iconUrl='1', sort=0, status='1111'}
        t_admin_category 
end:http-nio-8062-exec-2
    WHERE
        cid=6

大致意思：
请求A与请求B，同时操作id=6的这条记录时，查询出来id=6的这条记录，然后请求A对其进行了修改 SET　sort=1操作，完事之后，请求A的事务提交了。而此时请求B，通过id=6条件查询到的category的信息是请求A提交之前的，此时请求B执行了SET　sort=1，覆盖了请求B的修改操作。

3. 解决思路

解决方法其它很简单，我们上面可以看到，update语句是直接set sort=1的，我们只需要改为set sort = sort +1 就可以了。你不信？那我改一下试试：

@Transactional(rollbackFor = Exception.class)
public void updateStatusTest(Integer id) {
    System.out.println("start:"+Thread.currentThread().getName());
    Category category = categoryMapper.selectById(id);
    try {
        Thread.sleep(8000);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
    System.out.println(Thread.currentThread().getName()+":"+category);
//        category.setSort(category.getSort()+1);
    categoryMapper.updateStatusById(category);

    category = categoryMapper.selectById(id);
    System.out.println(Thread.currentThread().getName()+":"+category);
    System.out.println("end:"+Thread.currentThread().getName());
}

Mapper文件：

<update id="updateStatusById" parameterType="com.sxl.simple.shop.admin.category.entity.Category">
      UPDATE
        t_admin_category
      SET
        sort =sort + 1
    WHERE
        cid= #{cid,jdbcType=INTEGER}
</update>

我们执行试一下，看下日志：

start:http-nio-8062-exec-1
Creating a new SqlSession
Registering transaction synchronization for SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@5e925f3f]
JDBC Connection [com.alibaba.druid.proxy.jdbc.ConnectionProxyImpl@6c17839b] will be managed by Spring
==>  Preparing: SELECT cid,category_name,parent_id,image_url,icon_url,sort,status FROM t_admin_category WHERE cid=? 
==> Parameters: 6(Integer)
<==    Columns: cid, category_name, parent_id, image_url, icon_url, sort, status
<==        Row: 6, 1, 0, 11111, 1, 0, 1111
<==      Total: 1
Releasing transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@5e925f3f]
 Time：43 ms - ID：com.sxl.simple.shop.admin.category.mapper.CategoryMapper.selectById
Execute SQL：
    SELECT
        cid,
        category_name,
        parent_id,
        image_url,
        icon_url,
        sort,
        status 
    FROM
        t_admin_category 
    WHERE
        cid=6

start:http-nio-8062-exec-2
Creating a new SqlSession
Registering transaction synchronization for SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@6c88f022]
JDBC Connection [com.alibaba.druid.proxy.jdbc.ConnectionProxyImpl@195f8b5a] will be managed by Spring
==>  Preparing: SELECT cid,category_name,parent_id,image_url,icon_url,sort,status FROM t_admin_category WHERE cid=? 
==> Parameters: 6(Integer)
<==    Columns: cid, category_name, parent_id, image_url, icon_url, sort, status
<==        Row: 6, 1, 0, 11111, 1, 0, 1111
<==      Total: 1
Releasing transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@6c88f022]
 Time：2 ms - ID：com.sxl.simple.shop.admin.category.mapper.CategoryMapper.selectById
Execute SQL：
    SELECT
        cid,
        category_name,
        parent_id,
        image_url,
        icon_url,
        sort,
        status 
    FROM
        t_admin_category 
    WHERE
        cid=6

http-nio-8062-exec-1:Category{cid=6, categoryName='1', parentId=0, imageUrl='11111', iconUrl='1', sort=0, status='1111'}
Fetched SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@5e925f3f] from current transaction
==>  Preparing: UPDATE t_admin_category SET sort =sort + 1 WHERE cid= ? 
==> Parameters: 6(Integer)
<==    Updates: 1
Releasing transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@5e925f3f]
Fetched SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@5e925f3f] from current transaction
==>  Preparing: SELECT cid,category_name,parent_id,image_url,icon_url,sort,status FROM t_admin_category WHERE cid=? 
 Time：0 ms - ID：com.sxl.simple.shop.admin.category.mapper.CategoryMapper.updateStatusById
Execute SQL：
    UPDATE
        t_admin_category 
    SET
        sort =sort + 1 
    WHERE
        cid= 6

==> Parameters: 6(Integer)
 Time：0 ms - ID：com.sxl.simple.shop.admin.category.mapper.CategoryMapper.selectById
<==    Columns: cid, category_name, parent_id, image_url, icon_url, sort, status
Execute SQL：
<==        Row: 6, 1, 0, 11111, 1, 1, 1111
    SELECT
<==      Total: 1
        cid,
Releasing transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@5e925f3f]
        category_name,
        parent_id,
http-nio-8062-exec-1:Category{cid=6, categoryName='1', parentId=0, imageUrl='11111', iconUrl='1', sort=1, status='1111'}
        image_url,
end:http-nio-8062-exec-1
        icon_url,
        sort,
Transaction synchronization committing SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@5e925f3f]
        status 
    FROM
Transaction synchronization deregistering SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@5e925f3f]
        t_admin_category 
    WHERE
Transaction synchronization closing SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@5e925f3f]
        cid=6

 Time：0 ms - ID：com.sxl.simple.shop.admin.category.mapper.CategoryMapper.updateStatusById
Execute SQL：
    UPDATE
        t_admin_category 
    SET
        sort =sort + 1 
    WHERE
        cid= 6

 Time：0 ms - ID：com.sxl.simple.shop.admin.category.mapper.CategoryMapper.selectById
Execute SQL：
    SELECT
        cid,
        category_name,
        parent_id,
        image_url,
        icon_url,
        sort,
        status 
    FROM
        t_admin_category 
    WHERE
        cid=6

http-nio-8062-exec-2:Category{cid=6, categoryName='1', parentId=0, imageUrl='11111', iconUrl='1', sort=0, status='1111'}
Fetched SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@6c88f022] from current transaction
==>  Preparing: UPDATE t_admin_category SET sort =sort + 1 WHERE cid= ? 
==> Parameters: 6(Integer)
<==    Updates: 1
Releasing transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@6c88f022]
Fetched SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@6c88f022] from current transaction
==>  Preparing: SELECT cid,category_name,parent_id,image_url,icon_url,sort,status FROM t_admin_category WHERE cid=? 
==> Parameters: 6(Integer)
<==    Columns: cid, category_name, parent_id, image_url, icon_url, sort, status
<==        Row: 6, 1, 0, 11111, 1, 2, 1111
<==      Total: 1
Releasing transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@6c88f022]
http-nio-8062-exec-2:Category{cid=6, categoryName='1', parentId=0, imageUrl='11111', iconUrl='1', sort=2, status='1111'}
end:http-nio-8062-exec-2

看到了吧，保证数据一致了吧

我之前的文章有对Mysql事务这块做过详解，想知道真正原理的小伙伴请阅读： 深入理解mysql事务
注意，我这里使用的数据库是mysql8以上，应用服务器是单机版的。