在Mybatis的配置文件中可以配置事务管理方式如下：

说明：
（1）type为"JDBC"时，使用JdbcTransaction管理事务。
（2）type为"managed"时，使用ManagedTransaction管理事务（也就是交由外部容器管理）

mybatis的事务是由TransactionFactory创建的，利用典型的简单工厂模式来创建Transaction，如下图：

说明：
Transaction：封装事务管理方法的接口
TransactionFactory：抽象事务工厂生产方法
JdbcTransactionFactory：实现TransactionFactory接口，用于生产JdbcTransaction的工厂类
ManagedTransactionFactory：实现TransactionFactory接口，用于生产ManagedTransaction的工厂类
JdbcTransaction：实现Transaction接口，只是对事务进行了一层包装、实际调用数据库连接Connection的事务管理方法
ManagedTransaction：实现Transaction没有对数据库连接做任何事务处理、交由外部容器管理

第一步：解析配置文件的transactionManager节点。

private void environmentsElement(XNode context) throws Exception {
          //只关注事务部分...
          TransactionFactory txFactory = transactionManagerElement(child.evalNode("transactionManager"));
          ...
  }
  
private TransactionFactory transactionManagerElement(XNode context) throws Exception {
    if (context != null) {
      String type = context.getStringAttribute("type");
      Properties props = context.getChildrenAsProperties();
      TransactionFactory factory = (TransactionFactory) resolveClass(type).newInstance();
      factory.setProperties(props);
      return factory;
    }
    throw new BuilderException("Environment declaration requires a TransactionFactory.");
  }

注意：两种事务工厂已经在mybatis初始化的时候完成了注册：
typeAliasRegistry.registerAlias("JDBC", JdbcTransactionFactory.class);
typeAliasRegistry.registerAlias("MANAGED", ManagedTransactionFactory.class);

第二步：利用反射机制生成具体的的factory，此处以JdbcTransactionFactory为例，查看一下JdbcTransactionFactory的源码：

public class JdbcTransactionFactory implements TransactionFactory {

  public void setProperties(Properties props) {
  }

  public Transaction newTransaction(Connection conn) {
    return new JdbcTransaction(conn);
  }

  public Transaction newTransaction(DataSource ds, TransactionIsolationLevel level, boolean autoCommit) {
    return new JdbcTransaction(ds, level, autoCommit);
  }
}

说明：
在反射过程中，JdbcTransactionFactory默认无参构造方法被调用

第三步：获取具体类型的事务，以JdbcTransaction为例。

public Transaction newTransaction(Connection conn) {
    return new JdbcTransaction(conn);
  }

说明：
JdbcTransaction是通过Connection来创建具体的实例

第四步：事务的底层实现。以JdbcTransaction为例来说明。

public class JdbcTransaction implements Transaction {

  private static final Log log = LogFactory.getLog(JdbcTransaction.class);

  protected Connection connection;
  protected DataSource dataSource;
  protected TransactionIsolationLevel level;
  protected boolean autoCommmit;

  public JdbcTransaction(DataSource ds, TransactionIsolationLevel desiredLevel, boolean desiredAutoCommit) {
    dataSource = ds;
    level = desiredLevel;
    autoCommmit = desiredAutoCommit;
  }

  public JdbcTransaction(Connection connection) {
    this.connection = connection;
  }

  public Connection getConnection() throws SQLException {
    if (connection == null) {
      openConnection();
    }
    return connection;
  }

  public void commit() throws SQLException {
    if (connection != null && !connection.getAutoCommit()) {
      connection.commit();
    }
  }

  public void rollback() throws SQLException {
    if (connection != null && !connection.getAutoCommit()) {
      connection.rollback();
    }
  }

  public void close() throws SQLException {
    if (connection != null) {
      resetAutoCommit();
      if (log.isDebugEnabled()) {
        log.debug("Closing JDBC Connection [" + connection + "]");
      }
      connection.close();
    }
  }

  protected void setDesiredAutoCommit(boolean desiredAutoCommit) {
    try {
      if (connection.getAutoCommit() != desiredAutoCommit) {
       
        connection.setAutoCommit(desiredAutoCommit);
      }
    } catch (SQLException e) {
      throw new TransactionException("Error configuring AutoCommit.  "
          + "Your driver may not support getAutoCommit() or setAutoCommit(). "
          + "Requested setting: " + desiredAutoCommit + ".  Cause: " + e, e);
    }
  }

  protected void resetAutoCommit() {
    try {
      if (!connection.getAutoCommit()) {
        connection.setAutoCommit(true);
      }
    } catch (SQLException e) {
      log.debug("Error resetting autocommit to true "
          + "before closing the connection.  Cause: " + e);
    }
  }

  protected void openConnection() throws SQLException {
    connection = dataSource.getConnection();
    if (level != null) {
      connection.setTransactionIsolation(level.getLevel());
    }
    setDesiredAutoCommit(autoCommmit);
  }
}

从源码中可知，JdbcTransaction如何管理事务的，是通过封装调用DataSource获取connection来实现的。