mybatis事务管理机制详解

1.mybatis事务的配置和使用

mybatis事务有两种使用方式：

(a)：使用JDBC的事务管理机制：即使用java.Sql.Connection对象完成对事务的提交，回滚和关闭操作。

(b)：使用MANAGED的事务管理机制：mybatis本身不会去实现事务管理的相关操作，而是交个外部容器来管理事务。当与spring整合使用后，一般使用spring来管理事务。

在配置文件中的配置：

    <environments default="development">
        <environment id="development">
            //JDBC的事务方式
            <transactionManager type="JDBC" />
           <dataSource type="POOLED">
                <property name="driver" value="${driver}" />
                <property name="url" value="${url}" />
                <property name="username" value="${username}" />
                <property name="password" value="${password}" />
            </dataSource>
        </environment>
    </environments>

在这段配置信息中，有三个节点：<transactionManager />,<dataSource>,<environment>。其中，<dataSource>节点封装了数据源的信息，<environment>封装了数据库的连接，包括数据源和事务，然后把该节点信息封装到了Configuration对象中，方便后面使用，现在看下mybatis在初始化加载配置文件时，对<environment/>节点的解析：

  private void environmentsElement(XNode context) throws Exception {
    if (context != null) {
      if (environment == null) {
        environment = context.getStringAttribute("default");
      }
      for (XNode child : context.getChildren()) {
        String id = child.getStringAttribute("id");
        if (isSpecifiedEnvironment(id)) {
          // 解析 <transactionManager>节点，获取事务工厂对象 进入该方法
          TransactionFactory txFactory = transactionManagerElement(child.evalNode("transactionManager"));
          // 解析<DataSource>节点，获取数据源对象
          DataSourceFactory dsFactory = dataSourceElement(child.evalNode("dataSource"));
          DataSource dataSource = dsFactory.getDataSource();
          Environment.Builder environmentBuilder = new Environment.Builder(id)
              .transactionFactory(txFactory)
              .dataSource(dataSource);
          configuration.setEnvironment(environmentBuilder.build());
        }
      }
    }
  }

 进入解析<transactionManager/>节点的方法：

  private TransactionFactory transactionManagerElement(XNode context) throws Exception {
    if (context != null) {
       // 获取type的属性值，JDBC或者MANAGED
      String type = context.getStringAttribute("type");
      Properties props = context.getChildrenAsProperties();
      //生成一个TransactionFactory实例 
      TransactionFactory factory = (TransactionFactory) resolveClass(type).newInstance();
      factory.setProperties(props);
      return factory;
    }
    throw new BuilderException("Environment declaration requires a TransactionFactory.");
  }

2.事务工厂TransactionFactory

TransactionFactory定义了创建Transaction的两种方法，如下：

public interface TransactionFactory {

  /**
   * Sets transaction factory custom properties.
   * @param props
   */
  void setProperties(Properties props);

  /**
   * Creates a {@link Transaction} out of an existing connection.
   * @param conn Existing database connection
   * @return Transaction
   * @since 3.1.0
   * 方法一：通过指定的Connection对象来创建事务 
   */
  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
   * 方法二：通过DataSource，隔离级别，是否自动提交 来创建事务
   */
  Transaction newTransaction(DataSource dataSource, TransactionIsolationLevel level, boolean autoCommit);

}

TransactionFactory有两个子类：JdbcTransactionFactory和ManagedTransactionFactory

2.1 JdbcTransactionFactory，会创建JDBC类型的事务，就是JdbcTransaction，看下源码：

public class JdbcTransactionFactory implements TransactionFactory {

  public void setProperties(Properties props) {
  }
  // JDBCTransaction的创建有两种方法  
  public Transaction newTransaction(Connection conn) {
    return new JdbcTransaction(conn);
  }

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

2.2 ManagedTransactionFactory，会创建MANAGED类型的事务，ManagedTransaction，同样有两种方法：

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) {
    // Silently ignores autocommit and isolation level, as managed transactions are entirely
    // controlled by an external manager.  It's silently ignored so that
    // code remains portable between managed and unmanaged configurations.
    return new ManagedTransaction(ds, level, closeConnection);
  }
}

3.JdbcTransaction和ManagedTransaction

 3.1 JdbcTransaction

从源码可知：jdbcTransaction从DataSource获取连接对象Connection，然后利用Connection对象管理事务的commit和rollback，实际在事务处理上，jdbcTransaction是对java.sql.Connection的一个包装，它是使用Connection对象来管理事务的。

public class JdbcTransaction implements Transaction {

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

  protected Connection connection;// 连接对象
  protected DataSource dataSource; //数据源
  protected TransactionIsolationLevel level; // 隔离级别
  protected boolean autoCommmit; //是否自动提交

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

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

  // 获取连接对象
  public Connection getConnection() throws SQLException {
    if (connection == null) {
      openConnection();
    }
    return connection;
  }
  // 使用Connection对象提交
  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) {
      // 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) {
      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");
    }
    // 从数据源获取连接对象 
    connection = dataSource.getConnection();
    if (level != null) {
      connection.setTransactionIsolation(level.getLevel());
    }
    setDesiredAutoCommit(autoCommmit);
  }

}

3.2 ManagedTransaction 

ManagedTransaction对事务的commit和rollback交给了容器去管理，自己本身并没有做任何处理，看源码：

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 Connection getConnection() throws SQLException {
    if (this.connection == null) {
      openConnection();
    }
    return this.connection;
  }
   
  // 没有做任何处理，因为交给了外部容器去做这件事了
  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("Opening JDBC Connection");
    }
    this.connection = this.dataSource.getConnection();
    if (this.level != null) {
      this.connection.setTransactionIsolation(this.level.getLevel());
    }
  }

}