Mybatis 源码阅读----事务模块（Transaction）

该章将介绍Mybatis对事务的封装，事务模块所在的包位置如下：

各个类之间的关系图如下：

1、Transaction接口如下

public interface Transaction {

  /**
   * Retrieve inner database connection.
   * 获取数据库连接
   * @return DataBase connection
   * @throws SQLException
   */
  Connection getConnection() throws SQLException;

  /**
   * 提交事务
   * Commit inner database connection.
   * @throws SQLException
   */
  void commit() throws SQLException;

  /**
   * 回滚
   * Rollback inner database connection.
   * @throws SQLException
   */
  void rollback() throws SQLException;

  /**
   * 关闭数据库连接
   * Close inner database connection.
   * @throws SQLException
   */
  void close() throws SQLException;

  /**
   * 事务超时时间
   * Get transaction timeout if set.
   * @throws SQLException
   */
  Integer getTimeout() throws SQLException;

}

2、其实现类JdbcTransaction实现Transaction接口其中定义字段如下

 /**
   *  connection 对象
   */
  protected Connection connection;

  /**
   *  数据源
   */
  protected DataSource dataSource;

  /**
   * 数据库隔离级别
   */
  protected TransactionIsolationLevel level;

  /**
   *  是否自动提交
   */
  protected boolean autoCommit;

在JdbcTransaction的构造函数中首先会初始化connection之外的三个字段，而connection是通过getConnection() 方法延时初始化化。
构造器如下：

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

getConnection(）方法如下

public Connection getConnection() throws SQLException {
    // 如果connection为空，则创建
    if (connection == null) {
      openConnection();
    }
    return connection;
  }

openConnection()方法主要是从数据源中获取连接并设置相关属性，如下

protected void openConnection() throws SQLException {
    if (log.isDebugEnabled()) {
      log.debug("Opening JDBC Connection");
    }
    // 从数据源中获取连接
    connection = dataSource.getConnection();
    if (level != null) {
      // 设置数据路隔离级别
      connection.setTransactionIsolation(level.getLevel());
    }
    // 设置autoCommit 属性
    setDesiredAutoCommit(autoCommit);
  }

setDesiredAutoCommit() 方法如下

protected void setDesiredAutoCommit(boolean desiredAutoCommit) {
    try {
      // 将事务提交方式设置为 desiredAutoCommit
      if (connection.getAutoCommit() != desiredAutoCommit) {
        if (log.isDebugEnabled()) {
          log.debug("Setting autocommit to " + desiredAutoCommit + " on JDBC Connection [" + connection + "]");
        }
        connection.setAutoCommit(desiredAutoCommit);
      }
    } catch (SQLException e) {
      // Only a very poorly implemented driver would fail here,
      // and there's not much we can do about that.
      throw new TransactionException("Error configuring AutoCommit.  "
          + "Your driver may not support getAutoCommit() or setAutoCommit(). "
          + "Requested setting: " + desiredAutoCommit + ".  Cause: " + e, e);
    }
  }

事务提交方法如下：

public void commit() throws SQLException {
    // connection 不为 null  并且 connection不是自动提交
    if (connection != null && !connection.getAutoCommit()) {
      if (log.isDebugEnabled()) {
        log.debug("Committing JDBC Connection [" + connection + "]");
      }
      // 手动提交事务
      connection.commit();
    }
  }

事务回滚的方法如下：

public void rollback() throws SQLException {
    // 非自动提交，则进行回滚
    if (connection != null && !connection.getAutoCommit()) {
      if (log.isDebugEnabled()) {
        log.debug("Rolling back JDBC Connection [" + connection + "]");
      }
      connection.rollback();
    }
  }

关闭connection链接方法如下：

 public void close() throws SQLException {
    if (connection != null) {
      // 将connection重置为自动提交
      resetAutoCommit();
      if (log.isDebugEnabled()) {
        log.debug("Closing JDBC Connection [" + connection + "]");
      }
      // 关闭connection
      connection.close();
    }
  }

resetAutoCommit()方法如下：

protected void resetAutoCommit() {
    try {
      if (!connection.getAutoCommit()) {
       // MyBatis does not call commit/rollback on a connection if just selects were performed.
       // 当执行查询时，mybatis不需要提交或者回滚
        // Some databases start transactions with select statements
        // 一些数库在执行查询的时候开启了事务
        // and they mandate a commit/rollback before closing the connection.
        // 并且在关闭链接之前需要进行提交或者回滚
        // A workaround is setting the autocommit to true before closing the connection.
        // 需要在关闭链接之前将事务设置为自动提交
        // Sybase throws an exception here.
        if (log.isDebugEnabled()) {
          log.debug("Resetting autocommit to true on JDBC Connection [" + connection + "]");
        }
        // 将链接设置为自动提交
        connection.setAutoCommit(true);
      }
    } catch (SQLException e) {
      if (log.isDebugEnabled()) {
        log.debug("Error resetting autocommit to true "
            + "before closing the connection.  Cause: " + e);
      }
    }
  }

3.ManagedTransaction 的实现更加简单，它同样依赖其中的 dataSource 宇段获取连接， 但其commit（）、 rollback（）方法都是空实现，事务的提交和回滚都是依靠容器管理的。Managed Transaction 中通过 closeConnection 字段的值控制数据库连接 关闭行为。比较简单可自行看一下源码

4.TranscationFactory定义如下：

public interface TransactionFactory {

  /**
   * 设置工厂属性（运用了java8中的新特性，在接口中允许default方法）
   * Sets transaction factory custom properties.
   * @param props
   */
  default void setProperties(Properties props) {
    // NOP
  }

  /**
   *
   * 创建transaction
   * Creates a {@link Transaction} out of an existing connection.
   * @param conn Existing database connection
   * @return Transaction
   * @since 3.1.0
   */
  Transaction newTransaction(Connection conn);

  /**
   * Creates a {@link Transaction} out of a datasource.
   * @param dataSource DataSource to take the connection from
   * @param level Desired isolation level
   * @param autoCommit Desired autocommit
   * @return Transaction
   * @since 3.1.0
   */
  Transaction newTransaction(DataSource dataSource, TransactionIsolationLevel level, boolean autoCommit);

}

而TransactionFactory的实现类为JdbcTransactionFactory和ManagedTransactionFactory,主要是为了创建JdbcTranscation 和 ManagedTransaction,比较简单，就不贴出来代码了。至此Transaction模块分析完毕。

参考文献 《Mybatis技术内幕》