mybatis源碼解讀(四)——事務的配置

　　上一篇博客我們介紹了mybatis中關於數據源的配置原理，本篇博客介紹mybatis的事務管理。

　　對於事務，我們是在mybatis-configuration.xml 文件中配置的：

　　

　　關於解析 <environments />標簽在上一篇數據源的配置我們已經介紹了，不了解的可以參考上篇博客。

1、mybatis 支持的事務類圖

　　mybatis 支持的所有事務的所有類都在如下包中：

　　

　　下面通過類圖來理解該包中所有類的關系：

　　

2、mybatis 支持的兩種事務類型管理器

　　通過配置文件中的 type 屬性：

<transactionManager type="JDBC" />

　　我們可以配置不同的事務管理器來管理事務。在mybatis中支持兩種事務類型管理器，分別是：

　　①、type = "JDBC"：這個配置就是直接使用了 JDBC 的提交和回滾設置，它依賴於從數據源得到的連接來管理事務作用域。

　　②、type="MANAGED"：這個配置幾乎沒做什么。它從來不提交或回滾一個連接，而是讓容器來管理事務的整個生命周期（比如 JEE 應用服務器的上下文）。 默認情況下它會關閉連接，然而一些容器並不希望這樣，因此需要將 closeConnection 屬性設置為 false 來阻止它默認的關閉行為。例如:

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

　　注意：和數據源配置一樣，通常項目中我們不會單獨使用 mybatis 來管理事務。比如選擇框架 Spring +mybatis，這時候沒有必要配置事務管理器， 因為 Spring 模塊會使用自帶的管理器來覆蓋前面的配置。

　　再回頭看看在 mybatis 的 org.apache.ibatis.transaction 包下的所有類，也就是上面的類圖。mybatis的事務首先會定義一個事務接口 Transaction,

3、初始化事務管理器

　　我們說事務（Transaction），一般是指要做的或所做的事情。在數據庫中，事務具有如下四個屬性：

　　①、原子性（atomicity）：一個事務是一個不可分割的工作單位，事務中包括的諸操作要么都做，要么都不做。

　　②、一致性（consistency）：事務必須是使數據庫從一個一致性狀態變到另一個一致性狀態。一致性與原子性是密切相關的。

　　③、隔離性（isolation）：一個事務的執行不能被其他事務干擾。即一個事務內部的操作及使用的數據對並發的其他事務是隔離的，並發執行的各個事務之間不能互相干擾。

　　④、持久性（durability）：持久性也稱永久性（permanence），指一個事務一旦提交，它對數據庫中數據的改變就應該是永久性的。接下來的其他操作或故障不應該對其有任何影響。

　　這也就是常說的事務 ACID 特性。而在程序中，對於事務的操作通常是：

　　1、創建事務（create）

　　2、提交事務（commit）

　　3、回滾事務（rollback）

　　4、關閉事務（close）

　　在mybatis的 org.apache.ibatis.transaction 包下的 Transaction 接口便為我們定義了這一套操作：

　　

/**
 *    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;
  
}

　　同時，mybatis為了獲取事務采用了工廠模式，定義了一個工廠接口：TransactionFactory

　　

　　通過實現該工廠接口，mybatis提供了兩種不同的事務管理器：

　　

　　這兩種事務管理器的獲取也是采用了工廠模式。下面我們來分別看看這兩種事務管理器。

4、JdbcTransaction

　　當在配置文件中配置：type = "JDBC"時，就是用 JdbcTransaction 來管理事務。使用了 JDBC 的提交和回滾設置，它依賴於從數據源得到的連接來管理事務作用域。

　　代碼如下：

public class JdbcTransaction implements Transaction {

  private static final Log log = LogFactory.getLog(JdbcTransaction.class);
  //數據庫連接
  protected Connection connection;
  //數據源
  protected DataSource dataSource;
  //隔離級別
  protected TransactionIsolationLevel level;
  //是否自動提交
  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;
  }

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

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

  //調用connection.close()來實現
  @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 {
      //事務提交狀態不一致
      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的代碼實現幾乎都是一個空的方法，它選擇讓容器來管理事務的整個生命周期（比如 JEE 應用服務器的上下文）。

/**
 *    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;
  }

}