<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN" "http://mybatis.org/dtd/mybatis-3-config.dtd"><configuration><environments default="development"><environment id="development"><!--配置事务的管理方式--><transactionManager type="JDBC" /><!-- 配置数据库连接信息 --><dataSource type="POOLED"><property name="driver" value="com.mysql.jdbc.Driver" /><property name="url" value="jdbc:mysql://localhost:3306/mybatis" /><property name="username" value="root" /><property name="password" value="XDP" /></dataSource></environment></environments>    </configuration>

Mybatis提供了一个事务接口Transaction，以及两个实现类jdbcTransaction和ManagedTransaction，当spring与Mybatis一起使用时，spring提供了一个实现类SpringManagedTransaction
Transaction接口：提供的抽象方法有获取数据库连接getConnection，提交事务commit，回滚事务rollback和关闭连接close，源码如下：

 //事务接口
public interface Transaction {/*** Retrieve inner database connection* @return DataBase connection* @throws SQLException*///获得数据库连接Connection getConnection() throws SQLException;/*** 提交* Commit inner database connection.* @throws SQLException*/void commit() throws SQLException;/*** 回滚* Rollback inner database connection.* @throws SQLException*/void rollback() throws SQLException;/*** 关闭连接* Close inner database connection.* @throws SQLException*/void close() throws SQLException;}

JdbcTransaction实现类：Transaction的实现类，通过使用jdbc提供的方式来管理事务，通过Connection提供的事务管理方法来进行事务管理，源码如下：

public class JdbcTransaction implements Transaction {private static final Log log = LogFactory.getLog(JdbcTransaction.class);/* 连接**/protected Connection connection;/* 数据源**/protected DataSource dataSource;/* 事务等级**/protected TransactionIsolationLevel level;/* 事务提交**/protected boolean autoCommmit;public JdbcTransaction(DataSource ds, TransactionIsolationLevel desiredLevel, boolean desiredAutoCommit) {dataSource = ds;level = desiredLevel;autoCommmit = desiredAutoCommit;}public JdbcTransaction(Connection connection) {this.connection = connection;}@Overridepublic Connection getConnection() throws SQLException {if (connection == null) {openConnection();}//返回连接return connection;}@Overridepublic void commit() throws SQLException {if (connection != null && !connection.getAutoCommit()) {if (log.isDebugEnabled()) {log.debug("Committing JDBC Connection [" + connection + "]");}//连接提交connection.commit();}}@Overridepublic void rollback() throws SQLException {if (connection != null && !connection.getAutoCommit()) {if (log.isDebugEnabled()) {log.debug("Rolling back JDBC Connection [" + connection + "]");}//连接回滚connection.rollback();}}@Overridepublic void close() throws SQLException {if (connection != null) {resetAutoCommit();if (log.isDebugEnabled()) {log.debug("Closing JDBC Connection [" + connection + "]");}//关闭连接connection.close();}}protected void setDesiredAutoCommit(boolean desiredAutoCommit) {try {//事务提交状态不一致时修改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. select操作没有commit和rollback事务// Some databases start transactions with select statements 一些数据库在select操作是会开启事务// 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);}}}//打开连接protected void openConnection() throws SQLException {if (log.isDebugEnabled()) {log.debug("Opening JDBC Connection");}//从数据源中获得连接connection = dataSource.getConnection();if (level != null) {connection.setTransactionIsolation(level.getLevel());}setDesiredAutoCommit(autoCommmit);}}

ManagedTransaction实现类：通过容器来进行事务管理，所有它对事务提交和回滚并不会做任何操作，源码如下：

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;}//数据源，事务等级及是否关闭事务public ManagedTransaction(DataSource ds, TransactionIsolationLevel level, boolean closeConnection) {this.dataSource = ds;this.level = level;this.closeConnection = closeConnection;}@Overridepublic Connection getConnection() throws SQLException {if (this.connection == null) {openConnection();}return this.connection;}//提交操作无效@Overridepublic void commit() throws SQLException {// Does nothing}//回滚操作无效@Overridepublic void rollback() throws SQLException {// Does nothing}@Overridepublic void close() throws SQLException {if (this.closeConnection && this.connection != null) {if (log.isDebugEnabled()) {log.debug("Closing JDBC Connection [" + this.connection + "]");}//关闭连接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());}}}

SpringManagedTransaction实现类：它其实也是通过使用JDBC来进行事务管理的，当spring的事务管理有效时，不需要操作commit/rollback/close，spring事务管理会自动帮我们完成，源码如下：

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;//获得数据源public SpringManagedTransaction(DataSource dataSource) {notNull(dataSource, "No DataSource specified");this.dataSource = dataSource;}/*** {@inheritDoc}* 返回数据库连接*/@Overridepublic 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.*从spring的事务管理中获得一个连接 */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}*/@Overridepublic 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}*/@Overridepublic 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}*/@Overridepublic void close() throws SQLException {DataSourceUtils.releaseConnection(this.connection, this.dataSource);}}

Mybatis的事务管理机制还是比较简单的，其并没有做过多的操作，只是封装一下方便别人调用而已。