《深入理解mybatis原理(八)》 MyBatis事务管理机制

   MyBatis作为Java语言的数据库框架，对数据库的事务管理是其非常重要的一个方面。本文将讲述MyBatis的事务管理的实现机制。首先介绍MyBatis的事务Transaction的接口设计以及其不同实现JdbcTransaction和 ManagedTransaction；接着，从MyBatis的XML配置文件入手，讲解MyBatis事务工厂的创建和维护，进而阐述了MyBatis事务的创建和使用；最后分析JdbcTransaction和ManagedTransaction的实现和二者的不同特点。

以下是本文的组织结构：

一、概述

    对数据库的事务而言，应该具有以下几点：创建（create）、提交（commit）、回滚（rollback）、关闭（close）。对应地，MyBatis将事务抽象成了Transaction接口：其接口定义如下：

MyBatis的事务管理分为两种形式：

一、使用JDBC的事务管理机制：即利用java.sql.Connection对象完成对事务的提交（commit()）、回滚（rollback()）、关闭（close()）等

二、使用MANAGED的事务管理机制：这种机制MyBatis自身不会去实现事务管理，而是让程序的容器如（JBOSS，Weblogic）来实现对事务的管理

这两者的类图如下所示：

二、事务的配置、创建和使用

1. 事务的配置

我们在使用MyBatis时，一般会在MyBatisXML配置文件中定义类似如下的信息：

<environment>节点定义了连接某个数据库的信息，其子节点<transactionManager> 的type 会决定我们用什么类型的事务管理机制。

2.事务工厂的创建

    MyBatis事务的创建是交给TransactionFactory 事务工厂来创建的，如果我们将<transactionManager>的type 配置为"JDBC",那么，在MyBatis初始化解析<environment>节点时，会根据type="JDBC"创建一个JdbcTransactionFactory工厂，其源码如下：

  /**
   * 解析<transactionManager>节点，创建对应的TransactionFactory
   * @param context
   * @return
   * @throws Exception
   */
private TransactionFactory transactionManagerElement(XNode context) throws Exception {
  if (context != null) {
    String type = context.getStringAttribute("type");
    Properties props = context.getChildrenAsProperties();
    /*
          在Configuration初始化的时候，会通过以下语句，给JDBC和MANAGED对应的工厂类
          typeAliasRegistry.registerAlias("JDBC", JdbcTransactionFactory.class);
          typeAliasRegistry.registerAlias("MANAGED", ManagedTransactionFactory.class);
          下述的resolveClass(type).newInstance()会创建对应的工厂实例
     */
    TransactionFactory factory = (TransactionFactory) resolveClass(type).newInstance();
    factory.setProperties(props);
    return factory;
  }
  throw new BuilderException("Environment declaration requires a TransactionFactory.");
}

    /**
     * 解析<transactionManager>节点，创建对应的TransactionFactory
     * @param context
     * @return
     * @throws Exception
     */
  private TransactionFactory transactionManagerElement(XNode context) throws Exception {
    if (context != null) {
      String type = context.getStringAttribute("type");
      Properties props = context.getChildrenAsProperties();
      /*
            在Configuration初始化的时候，会通过以下语句，给JDBC和MANAGED对应的工厂类
            typeAliasRegistry.registerAlias("JDBC", JdbcTransactionFactory.class);
            typeAliasRegistry.registerAlias("MANAGED", ManagedTransactionFactory.class);
            下述的resolveClass(type).newInstance()会创建对应的工厂实例
       */
      TransactionFactory factory = (TransactionFactory) resolveClass(type).newInstance();
      factory.setProperties(props);
      return factory;
    }
    throw new BuilderException("Environment declaration requires a TransactionFactory.");
  }

     如上述代码所示，如果type = "JDBC",则MyBatis会创建一个JdbcTransactionFactory.class 实例；如果type="MANAGED"，则MyBatis会创建一个MangedTransactionFactory.class实例。

 MyBatis对<transactionManager>节点的解析会生成 TransactionFactory实例；而对<dataSource>解析会生成datasouce实例(关于dataSource的解析和原理，读者可以参照我的另一篇博文：《深入理解mybatis原理》 Mybatis数据源与连接池   
)，作为<environment>节点，会根据TransactionFactory和DataSource实例创建一个Environment对象，代码如下所示：

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");
      //是和默认的环境相同时，解析之
      if (isSpecifiedEnvironment(id)) {
        //1.解析<transactionManager>节点，决定创建什么类型的TransactionFactory
        TransactionFactory txFactory = transactionManagerElement(child.evalNode("transactionManager"));
        //2. 创建dataSource
        DataSourceFactory dsFactory = dataSourceElement(child.evalNode("dataSource"));
        DataSource dataSource = dsFactory.getDataSource();
        //3. 使用了Environment内置的构造器Builder，传递id 事务工厂TransactionFactory和数据源DataSource
        Environment.Builder environmentBuilder = new Environment.Builder(id)
            .transactionFactory(txFactory)
            .dataSource(dataSource);
        configuration.setEnvironment(environmentBuilder.build());
      }
    }
  }
}

  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");
        //是和默认的环境相同时，解析之
        if (isSpecifiedEnvironment(id)) {
          //1.解析<transactionManager>节点，决定创建什么类型的TransactionFactory
          TransactionFactory txFactory = transactionManagerElement(child.evalNode("transactionManager"));
          //2. 创建dataSource
          DataSourceFactory dsFactory = dataSourceElement(child.evalNode("dataSource"));
          DataSource dataSource = dsFactory.getDataSource();
          //3. 使用了Environment内置的构造器Builder，传递id 事务工厂TransactionFactory和数据源DataSource
          Environment.Builder environmentBuilder = new Environment.Builder(id)
              .transactionFactory(txFactory)
              .dataSource(dataSource);
          configuration.setEnvironment(environmentBuilder.build());
        }
      }
    }
  }

     Environment表示着一个数据库的连接，生成后的Environment对象会被设置到Configuration实例中，以供后续的使用。

    上述一直在讲事务工厂TransactionFactory来创建的Transaction，现在让我们看一下MyBatis中的TransactionFactory的定义吧。

3. 事务工厂TransactionFactory

    事务工厂Transaction定义了创建Transaction的两个方法：一个是通过指定的Connection对象创建Transaction，另外是通过数据源DataSource来创建Transaction。与JDBC 和MANAGED两种Transaction相对应，TransactionFactory有两个对应的实现的子类：如下所示：

4. 事务Transaction的创建

          通过事务工厂TransactionFactory很容易获取到Transaction对象实例。我们以JdbcTransaction为例，看一下JdbcTransactionFactory是怎样生成JdbcTransaction的，代码如下：

public class JdbcTransactionFactory implements TransactionFactory {

  public void setProperties(Properties props) {
  }

    /**
     * 根据给定的数据库连接Connection创建Transaction
     * @param conn Existing database connection
     * @return
     */
  public Transaction newTransaction(Connection conn) {
    return new JdbcTransaction(conn);
  }

    /**
     * 根据DataSource、隔离级别和是否自动提交创建Transacion
     *
     * @param ds
     * @param level Desired isolation level
     * @param autoCommit Desired autocommit
     * @return
     */
  public Transaction newTransaction(DataSource ds, TransactionIsolationLevel level, boolean autoCommit) {
    return new JdbcTransaction(ds, level, autoCommit);
  }
}

public class JdbcTransactionFactory implements TransactionFactory {

  public void setProperties(Properties props) {
  }

    /**
     * 根据给定的数据库连接Connection创建Transaction
     * @param conn Existing database connection
     * @return
     */
  public Transaction newTransaction(Connection conn) {
    return new JdbcTransaction(conn);
  }

    /**
     * 根据DataSource、隔离级别和是否自动提交创建Transacion
     *
     * @param ds
     * @param level Desired isolation level
     * @param autoCommit Desired autocommit
     * @return
     */
  public Transaction newTransaction(DataSource ds, TransactionIsolationLevel level, boolean autoCommit) {
    return new JdbcTransaction(ds, level, autoCommit);
  }
}

    如上说是，JdbcTransactionFactory会创建JDBC类型的Transaction，即JdbcTransaction。类似地，ManagedTransactionFactory也会创建ManagedTransaction。下面我们会分别深入JdbcTranaction 和ManagedTransaction，看它们到底是怎样实现事务管理的。

5. JdbcTransaction

    JdbcTransaction直接使用JDBC的提交和回滚事务管理机制 。它依赖与从dataSource中取得的连接connection 来管理transaction 的作用域，connection对象的获取被延迟到调用getConnection()方法。如果autocommit设置为on，开启状态的话，它会忽略commit和rollback。

    直观地讲，就是JdbcTransaction是使用的java.sql.Connection 上的commit和rollback功能，JdbcTransaction只是相当于对java.sql.Connection事务处理进行了一次包装（wrapper），Transaction的事务管理都是通过java.sql.Connection实现的。JdbcTransaction的代码实现如下：

/**
 * @see JdbcTransactionFactory
 */
/**
 * @author Clinton Begin
 */
public class JdbcTransaction implements Transaction {

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

  //数据库连接
  protected Connection connection;
  //数据源
  protected DataSource dataSource;
  //隔离级别
  protected TransactionIsolationLevel level;
  //是否为自动提交
  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;
  }

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

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

    /**
     * close()功能 使用connection的close()
     * @throws SQLException
     */
  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");
    }
    connection = dataSource.getConnection();
    if (level != null) {
      connection.setTransactionIsolation(level.getLevel());
    }
    setDesiredAutoCommit(autoCommmit);
  }

}

/**
 * @see JdbcTransactionFactory
 */
/**
 * @author Clinton Begin
 */
public class JdbcTransaction implements Transaction {

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

  //数据库连接
  protected Connection connection;
  //数据源
  protected DataSource dataSource;
  //隔离级别
  protected TransactionIsolationLevel level;
  //是否为自动提交
  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;
  }

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

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

    /**
     * close()功能 使用connection的close()
     * @throws SQLException
     */
  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");
    }
    connection = dataSource.getConnection();
    if (level != null) {
      connection.setTransactionIsolation(level.getLevel());
    }
    setDesiredAutoCommit(autoCommmit);
  }

}

6. ManagedTransaction

    ManagedTransaction让容器来管理事务Transaction的整个生命周期，意思就是说，使用ManagedTransaction的commit和rollback功能不会对事务有任何的影响，它什么都不会做，它将事务管理的权利移交给了容器来实现。看如下Managed的实现代码大家就会一目了然：

/**
 *
 * 让容器管理事务transaction的整个生命周期
 * connection的获取延迟到getConnection()方法的调用
 * 忽略所有的commit和rollback操作
 * 默认情况下，可以关闭一个连接connection，也可以配置它不可以关闭一个连接
 * 让容器来管理transaction的整个生命周期
 * @see ManagedTransactionFactory
 */
/**
 * @author Clinton Begin
 */
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("Opening JDBC Connection");
    }
    this.connection = this.dataSource.getConnection();
    if (this.level != null) {
      this.connection.setTransactionIsolation(this.level.getLevel());
    }
  }

}

/**
 * 
 * 让容器管理事务transaction的整个生命周期
 * connection的获取延迟到getConnection()方法的调用
 * 忽略所有的commit和rollback操作
 * 默认情况下，可以关闭一个连接connection，也可以配置它不可以关闭一个连接
 * 让容器来管理transaction的整个生命周期
 * @see ManagedTransactionFactory
 */
/**
 * @author Clinton Begin
 */
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("Opening JDBC Connection");
    }
    this.connection = this.dataSource.getConnection();
    if (this.level != null) {
      this.connection.setTransactionIsolation(this.level.getLevel());
    }
  }

}

注意：如果我们使用MyBatis构建本地程序，即不是WEB程序，若将type设置成"MANAGED"，那么，我们执行的任何update操作，即使我们最后执行了commit操作，数据也不会保留，不会对数据库造成任何影响。因为我们将MyBatis配置成了“MANAGED”，即MyBatis自己不管理事务，而我们又是运行的本地程序，没有事务管理功能，所以对数据库的update操作都是无效的。

以上就是 《深入理解mybatis原理》 MyBatis事务管理机制 的全部内容，如有错误或者不准确的地方，请读者指正，共同进步！

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本文源自  http://blog.csdn.net/luanlouis/，如需转载，请注明出处，谢谢！

本文转载自：http://blog.csdn.net/luanlouis/article/details/37992171