MyBatis的事务

Mybatis管理事务是分为两种方式:

(1)使用JDBC的事务管理机制,就是利用java.sql.Connection对象完成对事务的提交
(2)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（JBOSS,WebLogic）来实现对事务的管理
在Mybatis的配置文件中可以配置事务管理方式如下：
在这里插入图片描述

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：

<?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><!--映入外部文件定义的属性，供此配置文件使用--><properties resource="jdbc.properties"></properties><environments default="development"><!-- 连接环境信息，取一个任意唯一的名字 --><environment id="development"><!-- mybatis使用jdbc事务管理方式 --><transactionManager type="jdbc"/><!-- mybatis使用连接池方式来获取连接 --><dataSource type="pooled"><!-- 配置与数据库交互的4个必要属性 --><property name="driver" value="${jdbc.driver}"/><property name="url" value="${jdbc.url}"/><property name="username" value="${jdbc.username}"/><property name="password" value="${jdbc.password}"/></dataSource></environment></environments><!-- 加载映射文件--><mappers><mapper resource="com/mybatis/EmployeeMapper.xml"/></mappers>   
</configuration>

<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，如果我们将的type 配置为"JDBC",那么，在MyBatis初始化解析节点时，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.");
}

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：《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TransactionFactoryTransactionFactory txFactory = transactionManagerElement(child.evalNode("transactionManager"));//2. 创建dataSourceDataSourceFactory 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DataSourceEnvironment.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。

3. 事务工厂TransactionFactory

事务工厂Transaction定义了创建Transaction的两个方法：

通过指定的Connection对象创建Transaction
通过数据源DataSource来创建Transaction

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);}
}

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);}}

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());}}}

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。