mybatis-build

SqlSessionFactoryBuilder
build方法
构造 SqlSessionFactory 类型的默认实现 DefaultSqlSessionFactory(Configuration configuration)

// mybatis-config.xml 配置信息
Configuration
protected Environment environment;

Environment
// 事务工厂 (JdbcTransactionFactory)
private final TransactionFactory transactionFactory;
// 数据源
private final DataSource dataSource;

DefaultSqlSessionFactory 实现接口 SqlSessionFactory

// DefaultSqlSessionFactory 中只有一个成员变量 configuration
private final Configuration configuration;    

// 创建 SqlSession
SqlSession sqlSession = sqlSessionFactory.openSession()
// execType 执行器类型，默认是ExecutorType.SIMPLE
// level 事务隔离级别 null
// autoCommit 自动提交，默认false
openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit)

// 从配置中获取环境信息
final Environment environment = configuration.getEnvironment();

// 从环境信息中获取事务工厂，这里是 JdbcTransactionFactory
final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);

// 根据数据源创建事务，这里是 JdbcTransaction
Transaction tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);

// 使用 configuration 创建执行器 这里是 CachingExecutor 托委给 SimpleExecutor
final Executor executor = configuration.newExecutor(tx, execType);

return new DefaultSqlSession(configuration, executor, autoCommit); 

// Jdbc事务，用于获取连接Connection，管理连接的commit 或 rollback
JdbcTransaction
protected Connection connection;
// 数据源
protected DataSource dataSource;
// 事务隔离级别
protected TransactionIsolationLevel level;

// 获取连接
@Override
public Connection getConnection() throws SQLException {
    if (connection == null) {
      openConnection();
    }
    return connection;
}
// 打开连接
protected void openConnection() throws SQLException {
    if (log.isDebugEnabled()) {
      log.debug("Opening JDBC Connection");
    }
    connection = dataSource.getConnection();
    if (level != null) {
      connection.setTransactionIsolation(level.getLevel());
    }
    setDesiredAutoCommit(autoCommmit);
}

// 简单的执行器
SimpleExecutor
protected Configuration configuration;
protected Transaction transaction;

// 默认的 SqlSession
DefaultSqlSession
private final Configuration configuration;
private final Executor executor;
private boolean dirty; // 构造方法中设置默认值为false

https://stackoverflow.com/questions/15198319/why-do-we-use-a-datasource-instead-of-a-drivermanager