1、前言

mybatis 是有事務(wù)模塊的乙各，mybatis 與 spring 結(jié)合的時候杖狼，spring 實(shí)現(xiàn)了 mybatis 的事務(wù)接口 Transaction披蕉，實(shí)現(xiàn)類為 SpringManagedTransaction。

SpringManagedTransaction 類的注釋

此類的注釋如下：

• 如果它主要掌管 jdbc 連接的整個生命周期伞访，包括獲取釋放膝藕。
• 如果 spring 的事務(wù)管理器被激活的話，那么它不會做事務(wù)的操作咐扭，他的 commit/rollback/close 不會做任何操作。
• 如果 spring 事務(wù)不激活滑废，那么它會掌管 jdbc 的事務(wù)蝗肪，事實(shí)上，我們一般都用 spring 自己的事務(wù)蠕趁，所以它其實(shí)就拿個連接而已薛闪。

我們可以點(diǎn)進(jìn)去看他的方法，事實(shí)上也是這樣：

/**
 *    Copyright 2010-2016 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.mybatis.spring.transaction;

import static org.springframework.util.Assert.notNull;

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.transaction.Transaction;
import org.springframework.jdbc.datasource.ConnectionHolder;
import org.springframework.jdbc.datasource.DataSourceUtils;
import org.springframework.transaction.support.TransactionSynchronizationManager;

/**
 * {@code SpringManagedTransaction} handles the lifecycle of a JDBC connection.
 * It retrieves a connection from Spring's transaction manager and returns it back to it
 * when it is no longer needed.
 * <p>
 * If Spring's transaction handling is active it will no-op all commit/rollback/close calls
 * assuming that the Spring transaction manager will do the job.
 * <p>
 * If it is not it will behave like {@code JdbcTransaction}.
 *
 * @author Hunter Presnall
 * @author Eduardo Macarron
 * 
 * @version $Id$
 */
public class SpringManagedTransaction implements Transaction {

  private static final Log LOGGER = LogFactory.getLog(SpringManagedTransaction.class);

  private final DataSource dataSource;

  private Connection connection;

  private boolean isConnectionTransactional;

  private boolean autoCommit;

  public SpringManagedTransaction(DataSource dataSource) {
    notNull(dataSource, "No DataSource specified");
    this.dataSource = dataSource;
  }

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

  /**
   * Gets a connection from Spring transaction manager and discovers if this
   * {@code Transaction} should manage connection or let it to Spring.
   * <p>
   * It also reads autocommit setting because when using Spring Transaction MyBatis
   * thinks that autocommit is always false and will always call commit/rollback
   * so we need to no-op that calls.
   */
  private void openConnection() throws SQLException {
    this.connection = DataSourceUtils.getConnection(this.dataSource);
    this.autoCommit = this.connection.getAutoCommit();
    this.isConnectionTransactional = DataSourceUtils.isConnectionTransactional(this.connection, this.dataSource);

    if (LOGGER.isDebugEnabled()) {
      LOGGER.debug(
          "JDBC Connection ["
              + this.connection
              + "] will"
              + (this.isConnectionTransactional ? " " : " not ")
              + "be managed by Spring");
    }
  }

  /**
   * {@inheritDoc}
   */
  @Override
  public void commit() throws SQLException {
    if (this.connection != null && !this.isConnectionTransactional && !this.autoCommit) {
      if (LOGGER.isDebugEnabled()) {
        LOGGER.debug("Committing JDBC Connection [" + this.connection + "]");
      }
      this.connection.commit();
    }
  }

  /**
   * {@inheritDoc}
   */
  @Override
  public void rollback() throws SQLException {
    if (this.connection != null && !this.isConnectionTransactional && !this.autoCommit) {
      if (LOGGER.isDebugEnabled()) {
        LOGGER.debug("Rolling back JDBC Connection [" + this.connection + "]");
      }
      this.connection.rollback();
    }
  }

  /**
   * {@inheritDoc}
   */
  @Override
  public void close() throws SQLException {
    DataSourceUtils.releaseConnection(this.connection, this.dataSource);
  }
    
  /**
   * {@inheritDoc}
   */
  @Override
  public Integer getTimeout() throws SQLException {
    ConnectionHolder holder = (ConnectionHolder) TransactionSynchronizationManager.getResource(dataSource);
    if (holder != null && holder.hasTimeout()) {
      return holder.getTimeToLiveInSeconds();
    } 
    return null;
  }

}

綜上而言俺陋，mybatis 就像個工具人豁延，spring 只需要它解析 mapper 接口、mapper.xml 文件之類的功能（mybatis 主體功能）腊状，對于事務(wù)這種東西诱咏，spring 選擇了自己實(shí)現(xiàn)。那么缴挖，spring 事務(wù)是怎么實(shí)現(xiàn)的呢袋狞？

2、關(guān)于事務(wù)實(shí)現(xiàn)

首先映屋，不管是 spring苟鸯、mybatis 抑或是其他的任何框架類，對于事務(wù)的實(shí)現(xiàn)本質(zhì)上還是利用數(shù)據(jù)源的事務(wù)實(shí)現(xiàn)棚点。就比如 mybatis早处，在最后提交事務(wù)的時候，還是使用 connection.commit()瘫析、connection.rollback()砌梆。可以試著點(diǎn)進(jìn)去 commit 等方法颁股，發(fā)現(xiàn)調(diào)用的是各個數(shù)據(jù)源的實(shí)現(xiàn)（而數(shù)據(jù)源的實(shí)現(xiàn)么库，最后都是數(shù)據(jù)庫 sql 語句的實(shí)現(xiàn)，比如一個很簡單的事務(wù)操作語句：
set autocommit = 0甘有；
update xxx set xx = yy ...诉儒；
commit；
如果有人能講清楚這個問題亏掀，而不是說一堆廢話忱反，我想可能更好理解把）泛释。代碼如下：

/**
 *    Copyright 2009-2017 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.jdbc;

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;
import org.apache.ibatis.transaction.TransactionException;

/**
 * {@link Transaction} that makes use of the JDBC commit and rollback facilities directly.
 * It relies on the connection retrieved from the dataSource to manage the scope of the transaction.
 * Delays connection retrieval until getConnection() is called.
 * Ignores commit or rollback requests when autocommit is on.
 *
 * @author Clinton Begin
 *
 * @see JdbcTransactionFactory
 */
public class JdbcTransaction implements Transaction {

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

  protected Connection connection;
  protected DataSource dataSource;
  protected TransactionIsolationLevel level;
  // MEMO: We are aware of the typo. See #941
  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;
  }

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

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

  @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 {
      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;
  }
  
}

3、spring 事務(wù)實(shí)現(xiàn)

實(shí)際上温算，spring 的源碼異常龐大復(fù)雜怜校，所以基本上是不可能看完的，光 transaction 都有一個模塊注竿，這邊頂多講講他的實(shí)現(xiàn)思路茄茁。

spring中的事務(wù)實(shí)現(xiàn)從原理上說比較簡單，通過aop在方法執(zhí)行前后增加數(shù)據(jù)庫事務(wù)的操作巩割。

• 1.在方法開始時判斷是否開啟新事務(wù)裙顽，需要開啟事務(wù)則設(shè)置事務(wù)手動提交 set autocommit=0;
• 2.在方法執(zhí)行完成后手動提交事務(wù) commit;
• 3.在方法拋出指定異常后調(diào)用rollback回滾事務(wù)

具體邏輯看 TransactionIntercepto 的 invoke 方法，會發(fā)現(xiàn)有創(chuàng)建事務(wù)宣谈，提交事務(wù)愈犹，出錯會滾的代碼，我也就看了一下闻丑，沒具體細(xì)看漩怎。不過我們使用 @Transactional 開啟事務(wù)的時候，
我們先使用：sudo tcpdump -i lo0 -s 0 -l -w - port 3306 | strings
抓包 mysql 的 sql 打印代碼嗦嗡，會發(fā)現(xiàn)有如下的代碼打友浮：

mysql 的 tcp 通信

當(dāng)然，我在實(shí)際操作的過程中發(fā)現(xiàn)侥祭，我不好抓這個包怪得，可能是不熟悉這個軟件把催什。所以我直接去 mysql 的日志查看爆哑，事實(shí)也確實(shí)是類似的（記得開啟 mysql 日志）：

mysql 日志

4殊者、感想

我覺得大部分人都不會好好講話鲸沮，至少都不會完整的把一個事情講明白往扔。spring 的事務(wù)編程模型確實(shí)很復(fù)雜逾礁，但是我有時候其實(shí)想知道他為啥有效移宅，而不是整個模型是怎樣做的鲫售，因?yàn)槿绻浪麨樯队行Ш竺婢吐芯烤托辛讼跖　Ｒ苍S

5径筏、參考資料

https://xie.infoq.cn/article/52f38883e28821c9cf0a608ea