解析Mybatis之Sqlsession、Connection和Transaction原理与三者间的关系

你的名字 2024-03-02 10:59 71阅读 0赞

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[解析Mybatis之Sqlsession、Connection和Transaction原理与三者间的关系\_sqlsession和事务的关系-CSDN博客][Mybatis_Sqlsession_Connection_Transaction_sqlsession_-CSDN]

Mybatis之Sqlsession、Connection和Transaction解析关系与原理  
对于我们开发来讲，不管跟任何关系型数据库打交道都无法规避这三巨头，数据库的会话-Sqlsession、链接-Connection和事务-Transaction，今天让我们一起来梳理下这三者之间的工作原理和关系  
1、首先来解析下会话

Sqlsession  
会话是Mybatis持久化层跟关系型数据库交互的基础，所有的查询、数据更新（包含保存、更新、删除）操作都在与数据库建立会话的基础上进行的；MyBatis中的会话是SqlSession，默认实现是DefaultSqlSession。可以通过SqlSessionFactory的openSession来获取的。  
通过SqlSessionFactory获取SqlSession的代码如下：

String resource = "mybatis-config.xml";  
InputStream inputStream = Resources.getResourceAsStream(resource);  
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);  
SqlSession sqlSession = sqlSessionFactory.openSession();  
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打开一个会话的时序图大致流程如下：

第一步：通过new SqlSessionFactoryBuilder().build(inputStream)来构造SqlSessionFactory，参数是配置文件的输入流。\*  
主要实现的代码块如下：\*

public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) \{  
try \{  
XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);  
return build(parser.parse());  
\} catch (Exception e) \{  
throw ExceptionFactory.wrapException(“Error building SqlSession.”, e);  
\} finally \{  
ErrorContext.instance().reset();  
try \{  
inputStream.close();  
\} catch (IOException e) \{  
// Intentionally ignore. Prefer previous error.  
\}  
\}  
\}

public SqlSessionFactory build(Configuration config) \{  
return new DefaultSqlSessionFactory(config);  
\}

第二步：XMLConfigBuilder的parse方法会解析配置文件，解析的结果就是得出一个Configuration对象。其中一步就是根据配置文件中的datasource节点解析出数据源  
主要实现的代码块如下：

<dataSource type="POOLED">  
  
<property name="driver" value="com.mysql.cj.jdbc.Driver"/>  
<property name="url" value="jdbc:mysql://127.0.0.1:3306/test?useUnicode=true"/>  
<property name="username" value="root"/>  
<property name="password" value="12345678"/>  
<property name="poolMaximumActiveConnections" value="2"/>  
<property name="poolMaximumIdleConnections" value="2"/>  
</dataSource>  
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第三步：SqlSessionFactory的openSession会获取SqlSession。具体实现代码如下：

Transaction tx = null;  
try \{  
final Environment environment = configuration.getEnvironment();  
final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);  
tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);  
final Executor executor = configuration.newExecutor(tx, execType);  
return new DefaultSqlSession(configuration, executor, autoCommit);  
\} catch (Exception e) \{  
closeTransaction(tx); // may have fetched a connection so lets call close()  
throw ExceptionFactory.wrapException("Error opening session. Cause: " + e, e);  
\} finally \{  
ErrorContext.instance().reset();  
\}  
\}

\`\`\`yaml  
在这里插入代码片

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2、接下来再分析下Mybatis的链接

Connection  
MyBatis在第一次执行SQL操作时，在获取Statement时，会去获取数据库链接。

我们配置的数据源为POOLED，这里会使用PooledDataSource来获取connection。

private PooledConnection popConnection(String username, String password) throws SQLException \{  
boolean countedWait = false;  
PooledConnection conn = null;  
long t = System.currentTimeMillis();  
int localBadConnectionCount = 0;

while (conn == null) \{  
synchronized (state) \{  
if (!state.idleConnections.isEmpty()) \{  
// Pool has available connection  
conn = state.idleConnections.remove(0);  
if (log.isDebugEnabled()) \{  
log.debug("Checked out connection " + conn.getRealHashCode() + " from pool.");  
\}  
\} else \{  
// Pool does not have available connection  
if (state.activeConnections.size() < poolMaximumActiveConnections) \{  
// Can create new connection  
conn = new PooledConnection(dataSource.getConnection(), this);  
if (log.isDebugEnabled()) \{  
log.debug("Created connection " + conn.getRealHashCode() + ".");  
\}  
\} else \{  
// Cannot create new connection  
PooledConnection oldestActiveConnection = state.activeConnections.get(0);  
long longestCheckoutTime = oldestActiveConnection.getCheckoutTime();  
if (longestCheckoutTime > poolMaximumCheckoutTime) \{  
// Can claim overdue connection  
state.claimedOverdueConnectionCount++;  
state.accumulatedCheckoutTimeOfOverdueConnections += longestCheckoutTime;  
state.accumulatedCheckoutTime += longestCheckoutTime;  
state.activeConnections.remove(oldestActiveConnection);  
if (!oldestActiveConnection.getRealConnection().getAutoCommit()) \{  
try \{  
oldestActiveConnection.getRealConnection().rollback();  
\} catch (SQLException e) \{  
/\*  
Just log a message for debug and continue to execute the following  
statement like nothing happend.  
Wrap the bad connection with a new PooledConnection, this will help  
to not intterupt current executing thread and give current thread a  
chance to join the next competion for another valid/good database  
connection. At the end of this loop, bad \{@link @conn\} will be set as null.  
\*/  
log.debug("Bad connection. Could not roll back");  
\}  
\}  
conn = new PooledConnection(oldestActiveConnection.getRealConnection(), this);  
conn.setCreatedTimestamp(oldestActiveConnection.getCreatedTimestamp());  
conn.setLastUsedTimestamp(oldestActiveConnection.getLastUsedTimestamp());  
oldestActiveConnection.invalidate();  
if (log.isDebugEnabled()) \{  
log.debug("Claimed overdue connection " + conn.getRealHashCode() + ".");  
\}  
\} else \{  
// Must wait  
try \{  
if (!countedWait) \{  
state.hadToWaitCount++;  
countedWait = true;  
\}  
if (log.isDebugEnabled()) \{  
log.debug("Waiting as long as " + poolTimeToWait + " milliseconds for connection.");  
\}  
long wt = System.currentTimeMillis();  
state.wait(poolTimeToWait);  
state.accumulatedWaitTime += System.currentTimeMillis() - wt;  
\} catch (InterruptedException e) \{  
break;  
\}  
\}  
\}  
\}  
if (conn != null) \{  
// ping to server and check the connection is valid or not  
if (conn.isValid()) \{  
if (!conn.getRealConnection().getAutoCommit()) \{  
conn.getRealConnection().rollback();  
\}  
conn.setConnectionTypeCode(assembleConnectionTypeCode(dataSource.getUrl(), username, password));  
conn.setCheckoutTimestamp(System.currentTimeMillis());  
conn.setLastUsedTimestamp(System.currentTimeMillis());  
state.activeConnections.add(conn);  
state.requestCount++;  
state.accumulatedRequestTime += System.currentTimeMillis() - t;  
\} else \{  
if (log.isDebugEnabled()) \{  
log.debug("A bad connection (" + conn.getRealHashCode() + ") was returned from the pool, getting another connection.");  
\}  
state.badConnectionCount++;  
localBadConnectionCount++;  
conn = null;  
if (localBadConnectionCount > (poolMaximumIdleConnections + poolMaximumLocalBadConnectionTolerance)) \{  
if (log.isDebugEnabled()) \{  
log.debug("PooledDataSource: Could not get a good connection to the database.");  
\}  
throw new SQLException("PooledDataSource: Could not get a good connection to the database.");  
\}  
\}  
\}  
\}

if (conn == null) \{  
if (log.isDebugEnabled()) \{  
log.debug("PooledDataSource: Unknown severe error condition. The connection pool returned a null connection.");  
\}  
throw new SQLException("PooledDataSource: Unknown severe error condition. The connection pool returned a null connection.");  
\}

return conn;

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\}  
这里进行了数据库的链接进行了池化管理：如果idle的connection，就直接取出一个返回。数据库链接的获取底层代码如下：

获取链接后的第一件事，就是设置connection的autoCommit属性。这里可以看出MyBatis通过自身的数据源PooledDataSource来进行数据库链接的管理。

3、然后再来说下事务

Transaction  
在执行sqlSession.commit时，会去提交事务。

UserMapperExt userMapperExt = sqlSession.getMapper(UserMapperExt.class);

userMapperExt.insert(new UserDTO("houliu",23));  
userMapperExt.findUserListByName("zhangsan");  
userMapperExt.update("name" ,"wangwu",22);  
sqlSession.commit();  
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执行commit后，会调用如下代码：

一个sqlSession中可以进行多个事务提交：

SqlSession sqlSession1 = sqlSessionFactory.openSession();  
UserMapperExt userMapperExt1 = sqlSession1.getMapper(UserMapperExt.class);

userMapperExt1.insert(new UserDTO("houliu",23));  
userMapperExt1.findUserListByName("zhangsan");  
userMapperExt1.update("name" ,"wangwu",22);  
sqlSession1.commit();

//SqlSession sqlSession2 = sqlSessionFactory.openSession();  
UserMapperExt userMapperExt2 = sqlSession1.getMapper(UserMapperExt.class);

userMapperExt2.insert(new UserDTO("houliu",23));  
userMapperExt2.findUserListByName("zhangsan");  
userMapperExt2.update("name" ,"wangwu",22);  
sqlSession1.commit();  
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原生jdbc中一个connection可以执行多次commit：

Class.forName(“com.mysql.cj.jdbc.Driver”); //classLoader,加载对应驱动  
Connection connection = DriverManager.getConnection(“jdbc:mysql://127.0.0.1:3306/test?useUnicode=true”, “root”, “12345678”);  
connection.setAutoCommit(false);  
PreparedStatement preparedStatement = connection.prepareStatement(“update cnt\_user set age = 201 where name = ‘zhangsan’”);  
preparedStatement.execute();

connection.commit();  
preparedStatement = connection.prepareStatement("update cnt\_user set age = 233 where name = 'zhangsan'");  
preparedStatement.execute();

preparedStatement = connection.prepareStatement("insert into cnt\_user (age , name) values(100 ,'liusi')");  
preparedStatement.execute();

connection.commit();  
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可以看出，事务是依附在SqlSession上的。

好了，讲完了上面三者的原理后，最后我们来总结下三者的关系  
\*\*

Sqlsession、Connection和Transaction之间关系  
链接可以通过数据库链接池被复用。在MyBatis中，不同时刻的SqlSession可以复用同一个Connection，同一个SqlSession中可以提交多个事务。因此，链接—会话—事务的关系如下：

1个connection可以有多个sqlSession

[Mybatis_Sqlsession_Connection_Transaction_sqlsession_-CSDN]: https://blog.csdn.net/AiMaiShanHuHai/article/details/102984764