Mybatis学习笔记-事务管理详解

Mybatis作为最流行的持久层框架，当然支持事务的统一管理。
不多说，先来张图热热场…

该图大致概括了mybatis的事务体系。

• mybatis事务的分类
  1）JDBC ：直接简单使用了JDBC 的提交和回滚设置。
  2）MANAGED：这个配置几乎没做什么。它从来不提交或回滚一个连接，通常情况下，在mybatis与spring集成时这样设置，因为spring提供了同意的事务管理，mybatis在事务上不需要做任何事情。

• 配置mybatis事务管理器
  在mybatis核心配置文件中

<environments default="mysql_environment">        <environment id="mysql_environment">        <!-- 配置mybatis的事务 -->            <transactionManager type="JDBC" />            <dataSource type="POOLED">        <property name="driver" value="com.mysql.jdbc.Driver" /><property name="url" value="jdbc:mysql://localhost:3306/test" />                <property name="username" value="zhangdd" />                <property name="password" value="zd1991.." />            </dataSource>        </environment></environments> 

其中<transactionManager/>节点指定了事务管理器的类型，该节点只有一个type属性。

• mybatis创建事务
  应用启动时，mybatis会加载核心配置文件，当加载到<environments/>节点时，会根据<transactionManager/>和<dataSource>来创建一个Environment对象:
  public Environment(String id, TransactionFactory transactionFactory, DataSource dataSource) {，从这个构造器签名中我们可以看到，事务（Transaction）是由事务工厂（TransactionFactory）创建的。

创建事务与创建数据源类似，TransactionFactory是事务工程的顶级接口，其下有两个实现类：JdbcTransactionFactory和ManagedTransactionFactory，mybatis根据<transactionManager/>的type属性来选择用哪种工厂类创建事务。

JDBC Transaction是创建如下：
JdbcTransactionFactory:

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);  }}

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 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()) {      if (log.isDebugEnabled()) {        log.debug("Committing JDBC Connection [" + connection + "]");      }      connection.commit();    }  }  public void rollback() throws SQLException {    if (connection != null && !connection.getAutoCommit()) {      if (log.isDebugEnabled()) {        log.debug("Rolling back JDBC Connection [" + connection + "]");      }      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) {        if (log.isDebugEnabled()) {          log.debug("Setting autocommit to " + desiredAutoCommit + " on JDBC Connection [" + connection + "]");        }        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()) {        if (log.isDebugEnabled()) {          log.debug("Resetting autocommit to true on JDBC Connection [" + connection + "]");        }        connection.setAutoCommit(true);      }    } catch (SQLException e) {      log.debug("Error resetting autocommit to true "          + "before closing the connection.  Cause: " + e);    }  }  protected void openConnection() throws SQLException {    if (log.isDebugEnabled()) {      log.debug("Openning JDBC Connection");    }    connection = dataSource.getConnection();    if (level != null) {      connection.setTransactionIsolation(level.getLevel());    }    setDesiredAutoCommit(autoCommmit);  }}

很显然，JdbcTransaction是通过底层的Connection对象来管理事务的。

ManagedTransactionFactory:

public class ManagedTransactionFactory implements TransactionFactory {  private boolean closeConnection = true;  public void setProperties(Properties props) {    if (props != null) {      String closeConnectionProperty = props.getProperty("closeConnection");      if (closeConnectionProperty != null) {        closeConnection = Boolean.valueOf(closeConnectionProperty);      }    }  }  public Transaction newTransaction(Connection conn) {    return new ManagedTransaction(conn, closeConnection);  }  public Transaction newTransaction(DataSource ds, TransactionIsolationLevel level, boolean autoCommit) {    return new ManagedTransaction(ds, level, closeConnection);  }}

ManagedTransaction:

public class ManagedTransaction implements Transaction {  private static final Log log = LogFactory.getLog(ManagedTransaction.class);  private DataSource dataSource;  private TransactionIsolationLevel level;  private Connection connection;  private boolean closeConnection;  public ManagedTransaction(Connection connection, boolean closeConnection) {    this.connection = connection;    this.closeConnection = closeConnection;  }  public ManagedTransaction(DataSource ds, TransactionIsolationLevel level, boolean closeConnection) {    this.dataSource = ds;    this.level = level;    this.closeConnection = closeConnection;  }  public Connection getConnection() throws SQLException {    if (this.connection == null) {      openConnection();    }    return this.connection;  }  public void commit() throws SQLException {    // Does nothing  }  public void rollback() throws SQLException {    // Does nothing  }  public void close() throws SQLException {    if (this.closeConnection && this.connection != null) {      if (log.isDebugEnabled()) {        log.debug("Closing JDBC Connection [" + this.connection + "]");      }      this.connection.close();    }  }  protected void openConnection() throws SQLException {    if (log.isDebugEnabled()) {      log.debug("Openning JDBC Connection");    }    this.connection = this.dataSource.getConnection();    if (this.level != null) {      this.connection.setTransactionIsolation(this.level.getLevel());    }  }}

从上面的代码中可以提炼出最重要的几行：

public void commit() throws SQLException {    // Does nothing  }  public void rollback() throws SQLException {    // Does nothing  }

是的，就想你看到的那样，commit()和rollback()什么都没有做。
当将事务类型配置成Managed时，mybatis所有的DML操作都将失效，事务会交由spring或web容器统一管理。