详解MySQL与Spring的自动提交(autocommit)

1 MySQL的autocommit设置

MySQL默认是开启自动提交的，即每一条DML（增删改）语句都会被作为一个单独的事务进行隐式提交。如果修改为关闭状态，则执行DML语句之后要手动提交才能生效。
查询当前会话的自动提交是否开启：

mysql> show variables like 'autocommit';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| autocommit  | ON  |
+---------------+-------+

查询全局的自动提交是否开启：

mysql> show global variables like 'autocommit';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| autocommit  | ON  |
+---------------+-------+

通过修改autocommit变量可以关闭和开启操作

关闭当前会话的自动提交模式
mysql> set autocommit=0;

 
mysql> show variables like 'autocommit';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| autocommit  | OFF  |
+---------------+-------+

 全局的autocommit还是开启状态
mysql> show global variables like 'autocommit';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| autocommit  | ON  |
+---------------+-------+

 
 关闭全局的autocommit
mysql> set global autocommit=0;

 
mysql> show global variables like 'autocommit';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| autocommit  | OFF  |
+---------------+-------+

如果想要MySQL服务重启之后仍能生效，需要设置系统环境变量。MySQL5.7 在cnf配置文件中[mysqld]下面设置autocommit的值。

[mysqld]
...
autocommit=0

Spring中对自动提交的控制

MySQL的JDBC驱动包 mysql-connector-java 会给会话的connection默认开启自动提交，譬如 mysql-connector-java-8.0.22版本的代码：

//com.mysql.cj.protocol.a.NativeServerSession.java
  private boolean autoCommit = true;

常用的数据库连接池如HikariCP,druid等，默认也是开启自动提交，会将connection的自动提交设置都改为true。
druid在初始化DataSource的时候设置connection的autocommit为true。代码如下：

com.alibaba.druid.pool.DruidAbstractDataSource.java
  protected volatile boolean             defaultAutoCommit             = true;
  ...
  public void initPhysicalConnection(Connection conn, Map variables, Map globalVariables) throws SQLException {
    if (conn.getAutoCommit() != defaultAutoCommit) {
      //将connection的autocommit设置为true
      conn.setAutoCommit(defaultAutoCommit);
    }
    ...
 
  }

HikariCP 初始化DataSource的默认配置中autocommit也是true：

com.zaxxer.hikari.HikariConfig.java
  public HikariConfig()
  {
   ...
   isAutoCommit = true;
  }

对于事务管理器PlatformTransactionManager管理的显式事务（譬如@Transactional注解声明）在开启事务时会关闭自动提交模式。代码如下：

	@Override
	protected void doBegin(Object transaction, TransactionDefinition definition) {
		DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
		Connection con = null;

		try {
      		........

			// Switch to manual commit if necessary. This is very expensive in some JDBC drivers,
			// so we don't want to do it unnecessarily (for example if we've explicitly
			// configured the connection pool to set it already).
			if (con.getAutoCommit()) {
				txObject.setMustRestoreAutoCommit(true);
				if (logger.isDebugEnabled()) {
					logger.debug("Switching JDBC Connection [" + con + "] to manual commit");
				}
                //关闭自动提交模
                con.setAutoCommit(false);
			}

      		.......
		}

		catch (Throwable ex) {
     		.......
		}
	}