Mybatis中的事务管理器详述

喜欢ヅ旅行 2022-06-16 11:29 580阅读 0赞

在上篇文章<[Mybatis中的数据源与连接池详解 ][Mybatis_]>中，我们结合源码对mybatis中的数据源和连接池进行了比较详细的说明。在这篇文章中，我们讲讲相关的另外一个主题——事务管理器。

在前面的文章中，我们知道mybatis支持两种事务类型，分别为JdbcTransaction和ManagedTransaction。接下来，我们从mybatis的xml配置文件入手，讲解事务管理器工厂的创建，然后讲述事务的创建和使用，最后分析这两种事务的实现和两者的区别。

我们先看看配置文件中相关的配置：

![Center][]

Mybatis定义了一个事务类型接口Transaction，JdbcTransaction和ManagedTransaction两种事务类型都实现了Transaction接口。我们看看Transaction这个接口的定义：

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; }

在事务接口中，定义了若干方法，如下结构所示：

![Center 1][]

事务的继承关系如下：

![Center 2][]

JdbcTransaction和ManagedTransaction的区别如下：

JdbcTransaction：利用java.sql.Connection对象完成对事务的提交（commit()）、回滚（rollback()）、关闭（close()）等；

ManagedTransaction：MyBatis自身不会去实现事务管理，而是让程序的容器来实现对事务的管理；

那么在mybatis中又是怎么使用事务管理器的呢？首先需要根据xml中的配置确定需要创建什么样的事务管理器，然后从事务管理器中获取相应的事务。

在mybatis初始化的时候，在解析<transactionManager>节点的时候，根据设置的type类型去初始化相应的事务管理器，解析源码如下所示：

/** * 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."); }

从代码可以看出来，如果type配置成JDBC，则创建一个JdbcTransactionFactory实例，如果type配置成MANAGED，则会创建一个ManagedTransactionFactory实例。这两个事务管理器类型都实现了mybatis定义的TransactionFactory接口。

![Center 3][]

事务管理器工厂接口的定义如下所示：

public interface TransactionFactory { /** * Sets transaction factory custom properties. * @param props */ void setProperties(Properties props); /** * Creates a {@link Transaction} out of an existing connection. * @param conn Existing database connection * @return Transaction * @since 3.1.0 */ Transaction newTransaction(Connection conn); /** * Creates a {@link Transaction} out of a datasource. * @param dataSource DataSource to take the connection from * @param level Desired isolation level * @param autoCommit Desired autocommit * @return Transaction * @since 3.1.0 */ Transaction newTransaction(DataSource dataSource, TransactionIsolationLevel level, boolean autoCommit); }

从接口定义看，不管是JdbcTransactionFactory，还是ManagedTransactionFactory，都需要自行实现事务Transaction的创建工作。我们从源码上看看这两个类都是怎么定义实现的。

JdbcTransactionFactory定义如下：

public class JdbcTransactionFactory implements TransactionFactory { @Override public void setProperties(Properties props) { } @Override public Transaction newTransaction(Connection conn) { return new JdbcTransaction(conn); } @Override public Transaction newTransaction(DataSource ds, TransactionIsolationLevel level, boolean autoCommit) { return new JdbcTransaction(ds, level, autoCommit); } }

ManagedTransactionFactory定义如下：

public class ManagedTransactionFactory implements TransactionFactory { private boolean closeConnection = true; @Override public void setProperties(Properties props) { if (props != null) { String closeConnectionProperty = props.getProperty("closeConnection"); if (closeConnectionProperty != null) { closeConnection = Boolean.valueOf(closeConnectionProperty); } } } @Override public Transaction newTransaction(Connection conn) { return new ManagedTransaction(conn, closeConnection); } @Override public Transaction newTransaction(DataSource ds, TransactionIsolationLevel level, boolean autoCommit) { // Silently ignores autocommit and isolation level, as managed transactions are entirely // controlled by an external manager. It's silently ignored so that // code remains portable between managed and unmanaged configurations. return new ManagedTransaction(ds, level, closeConnection); } }

从源码看，JdbcTransactionFactory会创建JDBC类型的事务JdbcTransaction，而ManagedTransactionFactory则会创建ManagedTransaction。

接下来，我们再来看看这两种类型的事务类型。

先从JdbcTransaction看起吧，这种类型的事务就是使用Java自带的Connection来实现事务的管理。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：

public class JdbcTransaction implements Transaction { private static final Log log = LogFactory.getLog(JdbcTransaction.class); protected Connection connection; protected DataSource dataSource; protected TransactionIsolationLevel level; 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; } }

最后我们再来看看ManagedTransaction对象，这个对象因为是将事务的管理交给容器去控制，所以，这里的ManagedTransaction是没有做任何控制的。我们先来看看源码：

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; } }

从源码我们可以看到，ManagedTransaction的commit和rollback方法是没有做任何事情的，它将事务交由了更上层的容易来进行控制和实现。至此，关于事务管理器我们描述的已经差不多了，如果需要深究可以自己再去研究研究。

如果想了解我最新的博文，请关注我的博客，谢谢。如果想看到更多相关技术文章并愿意支持我继续写下去，欢迎给我打赏，对您的打赏我表示感谢。![吐舌头][tongue.gif]

![20170430151631454][]![20170430151912379][]

[Mybatis_]: http://blog.csdn.net/majinggogogo/article/details/71715846
[Center]: /images/20220616/1aae8ea0c55b4e24a9ea0041456070eb.png
[Center 1]: /images/20220616/bc3e52281b3d41d381c2d8150e190a8f.png
[Center 2]: /images/20220616/fe229e23aea041109731bb58798cae8f.png
[Center 3]: /images/20220616/8cb80a76b7ee41648bce9911e66aee97.png
[tongue.gif]: /images/20220616/f45a199f870f4a1493685bd44f3432d1.png
[20170430151631454]: https://img-blog.csdn.net/20170430151631454
[20170430151912379]: /images/20220616/da49913f548542c88259acb1288ce12f.png