mybatis源码解读(四)——事务的配置

　　上一篇博客我们介绍了mybatis中关于数据源的配置原理，本篇博客介绍mybatis的事务管理。

　　对于事务，我们是在mybatis-configuration.xml 文件中配置的：

　　

　　关于解析 <environments />标签在上一篇数据源的配置我们已经介绍了，不了解的可以参考上篇博客。

1、mybatis 支持的事务类图

　　mybatis 支持的所有事务的所有类都在如下包中：

　　

　　下面通过类图来理解该包中所有类的关系：

　　

2、mybatis 支持的两种事务类型管理器

　　通过配置文件中的 type 属性：

<transactionManager type="JDBC" />

　　我们可以配置不同的事务管理器来管理事务。在mybatis中支持两种事务类型管理器，分别是：

　　①、type = "JDBC"：这个配置就是直接使用了 JDBC 的提交和回滚设置，它依赖于从数据源得到的连接来管理事务作用域。

　　②、type="MANAGED"：这个配置几乎没做什么。它从来不提交或回滚一个连接，而是让容器来管理事务的整个生命周期（比如 JEE 应用服务器的上下文）。 默认情况下它会关闭连接，然而一些容器并不希望这样，因此需要将 closeConnection 属性设置为 false 来阻止它默认的关闭行为。例如:

<transactionManager type="MANAGED">
  <property name="closeConnection" value="false"/>
</transactionManager>

　　注意：和数据源配置一样，通常项目中我们不会单独使用 mybatis 来管理事务。比如选择框架 Spring +mybatis，这时候没有必要配置事务管理器， 因为 Spring 模块会使用自带的管理器来覆盖前面的配置。

　　再回头看看在 mybatis 的 org.apache.ibatis.transaction 包下的所有类，也就是上面的类图。mybatis的事务首先会定义一个事务接口 Transaction,

3、初始化事务管理器

　　我们说事务（Transaction），一般是指要做的或所做的事情。在数据库中，事务具有如下四个属性：

　　①、原子性（atomicity）：一个事务是一个不可分割的工作单位，事务中包括的诸操作要么都做，要么都不做。

　　②、一致性（consistency）：事务必须是使数据库从一个一致性状态变到另一个一致性状态。一致性与原子性是密切相关的。

　　③、隔离性（isolation）：一个事务的执行不能被其他事务干扰。即一个事务内部的操作及使用的数据对并发的其他事务是隔离的，并发执行的各个事务之间不能互相干扰。

　　④、持久性（durability）：持久性也称永久性（permanence），指一个事务一旦提交，它对数据库中数据的改变就应该是永久性的。接下来的其他操作或故障不应该对其有任何影响。

　　这也就是常说的事务 ACID 特性。而在程序中，对于事务的操作通常是：

　　1、创建事务（create）

　　2、提交事务（commit）

　　3、回滚事务（rollback）

　　4、关闭事务（close）

　　在mybatis的 org.apache.ibatis.transaction 包下的 Transaction 接口便为我们定义了这一套操作：

　　

/**
 *    Copyright 2009-2016 the original author or authors.
 *
 *    Licensed under the Apache License, Version 2.0 (the "License");
 *    you may not use this file except in compliance with the License.
 *    You may obtain a copy of the License at
 *
 *       http://www.apache.org/licenses/LICENSE-2.0
 *
 *    Unless required by applicable law or agreed to in writing, software
 *    distributed under the License is distributed on an "AS IS" BASIS,
 *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *    See the License for the specific language governing permissions and
 *    limitations under the License.
 */
package org.apache.ibatis.transaction;

import java.sql.Connection;
import java.sql.SQLException;

/**
 * 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;
  
}

　　同时，mybatis为了获取事务采用了工厂模式，定义了一个工厂接口：TransactionFactory

　　

　　通过实现该工厂接口，mybatis提供了两种不同的事务管理器：

　　

　　这两种事务管理器的获取也是采用了工厂模式。下面我们来分别看看这两种事务管理器。

4、JdbcTransaction

　　当在配置文件中配置：type = "JDBC"时，就是用 JdbcTransaction 来管理事务。使用了 JDBC 的提交和回滚设置，它依赖于从数据源得到的连接来管理事务作用域。

　　代码如下：

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;
  }

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

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

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

  //调用connection.close()来实现
  @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 {
      //事务提交状态不一致
      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(autoCommmit);
  }

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

5、ManagedTransaction

　　ManagedTransaction的代码实现几乎都是一个空的方法，它选择让容器来管理事务的整个生命周期（比如 JEE 应用服务器的上下文）。

/**
 *    Copyright 2009-2016 the original author or authors.
 *
 *    Licensed under the Apache License, Version 2.0 (the "License");
 *    you may not use this file except in compliance with the License.
 *    You may obtain a copy of the License at
 *
 *       http://www.apache.org/licenses/LICENSE-2.0
 *
 *    Unless required by applicable law or agreed to in writing, software
 *    distributed under the License is distributed on an "AS IS" BASIS,
 *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *    See the License for the specific language governing permissions and
 *    limitations under the License.
 */
package org.apache.ibatis.transaction.managed;

import java.sql.Connection;
import java.sql.SQLException;
import javax.sql.DataSource;

import org.apache.ibatis.logging.Log;
import org.apache.ibatis.logging.LogFactory;
import org.apache.ibatis.session.TransactionIsolationLevel;
import org.apache.ibatis.transaction.Transaction;

/**
 * {@link Transaction} that lets the container manage the full lifecycle of the transaction.
 * Delays connection retrieval until getConnection() is called.
 * Ignores all commit or rollback requests.
 * By default, it closes the connection but can be configured not to do it.
 *
 * @author Clinton Begin
 *
 * @see ManagedTransactionFactory
 */
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;
  }

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

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

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

  @Override
  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());
    }
  }

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

}