mybatis源码分析(四) mybatis与spring事务管理分析

mybatis源码分析(四) mybatis与spring事务管理分析

 

一丶从jdbc的角度理解什么是事务

　　从mysql获取一个连接之后, 默认是自动提交, 即执行完sql之后, 就会提交事务. 这种事务的范围是一条sql语句. 　　

　　将该连接设置非自动提交, 可以执行多条sql语句, 然后由程序决定是提交事务, 还是回滚事务. 这也是我们常说的事务.

Connection connection = dataSource.getConnection();

// connection.setTransactionIsolation(level.getLevel()); //设置事务隔离级别

// 设置是否自动提交, 如果不是自动提交, 则是"开启"事务
connection.setAutoCommit(desiredAutoCommit);

// connection预编译statement, 并执行sql
Statement stmt=connection.preparedStatement();
stmt.execute(sql);

// 提交事务, 或者回滚
connection.commit();
//connection.rollback();

 

　　从jdbc使用事务的角度来看, 事务主要是围绕connection展开的, 所以谁可获得connection, 即可控制事务.

 

 

二丶mybatis是如何使用事务的

 　　mybatis将jdbc中的事务操作抽象封装成Transaction,用于管理connection的生命周期--创建, 准备, 提交/回滚 和关闭.

/**
 * Wraps a database connection.
 * Handles the connection lifecycle that comprises: its creation, preparation, commit/rollback and close.
 *
 * @author Clinton Begin
 */
public interface Transaction {

  /**
   * Retrieve inner database connection.
   * @return DataBase connection
   * @throws SQLException
   */
  Connection getConnection() throws SQLException;

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

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

  /**
   * Close inner database connection.
   * @throws SQLException
   */
  void close() throws SQLException;

  /**
   * Get transaction timeout if set.
   * @throws SQLException
   */
  Integer getTimeout() throws SQLException;

}

 

 　　mybatis提供了两种事务实现,一种是完全由jdbc实现的事务JdbcTransaction,包括实现提交和回滚.一种是供容器管理整个生命周期的事务ManagedTransaction,其中将忽略提交和回滚事务的请求, 将提交和回滚事务由容器实现, 但其实这种事务很少用.

　　JdbcTransaction:

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 autoCommit;

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

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

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

  @Override
  public void commit() throws SQLException {
    if (connection != null && !connection.getAutoCommit()) {
      if (log.isDebugEnabled()) {
        log.debug("Committing JDBC Connection [" + connection + "]");
      }
      connection.commit();
    }
  }

  @Override
  public void rollback() throws SQLException {
    if (connection != null && !connection.getAutoCommit()) {
      if (log.isDebugEnabled()) {
        log.debug("Rolling back JDBC Connection [" + connection + "]");
      }
      connection.rollback();
    }
  }

  @Override
  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) {
        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);
    }
  }

  protected void resetAutoCommit() {
    try {
      if (!connection.getAutoCommit()) {
        // MyBatis does not call commit/rollback on a connection if just selects were performed.
        // Some databases start transactions with select statements
        // and they mandate a commit/rollback before closing the connection.
        // A workaround is setting the autocommit to true before closing the connection.
        // Sybase throws an exception here.
        if (log.isDebugEnabled()) {
          log.debug("Resetting autocommit to true on JDBC Connection [" + connection + "]");
        }
        connection.setAutoCommit(true);
      }
    } catch (SQLException e) {
      if (log.isDebugEnabled()) {
        log.debug("Error resetting autocommit to true "
            + "before closing the connection.  Cause: " + e);
      }
    }
  }

  protected void openConnection() throws SQLException {
    if (log.isDebugEnabled()) {
      log.debug("Opening JDBC Connection");
    }
    connection = dataSource.getConnection();
    if (level != null) {
      connection.setTransactionIsolation(level.getLevel());
    }
    setDesiredAutoCommit(autoCommit);
  }

  @Override
  public Integer getTimeout() throws SQLException {
    return null;
  }

}

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