Mybatis深入之事务管理

最后更新于:2022-04-01 16:30:09

# Mybatis之事务管理 ### 简介 Mybatis的事务管理分为两种JdbcTransaction,ManagedTransaction。其中JdbcTransaction仅仅是对数据库连接Connection的一个包装、内部管理数据库事务还是调用Connection的提交、回滚等事务操作方法。ManagedTransaction更直接、什么也没有做。直接将事务交给外部容器管理。 ### Mybatis事务管理相关类结构图 类概览: ![类概览](https://docs.gechiui.com/gc-content/uploads/sites/kancloud/2016-08-08_57a85803ca136.jpg "") 类UML图(典型的简单工厂模式来创建Transaction): ![类UML图](https://docs.gechiui.com/gc-content/uploads/sites/kancloud/2016-08-08_57a85803dc606.jpg "") - Transaction 封装事务管理方法的接口 - TransactionFactory 抽象事务工厂生产方法 - JdbcTransactionFactory实现TransactionFactory、用于生产JdbcTransaction的工厂类 - ManagedTransactionFactory实现TransactionFactory、用于生产ManagedTransaction的工厂类 - JdbcTransaction实现Transaction、只是对事务进行了一层包装、实际调用数据库连接Connection的事务管理方法 - ManagedTransaction 实现Transaction没有对数据库连接做任何事务处理、交由外部容器管理 ### 源码事务 ### 事务配置 Mybatis中关于事务的配置是通过`<transaction type="xx"/>`来指定的。配置如下: ~~~ <environments default="development"> <environment id="development"> <transactionManager type="JDBC"/> <dataSource type="POOLED"> <property name="driver" value="${jdbc.driverClassName}"/> <property name="url" value="${jdbc.url}"/> <property name="username" value="${jdbc.username}"/> <property name="password" value="${jdbc.password}"/> </dataSource> </environment> </environments> ~~~ - type为”JDBC”时、使用JdbcTransaction管理事务。 - type为”managed”时、使用ManagedTransaction管理事务(也就是交由外部容器管理) [Mybatis深入之初始化过程 ](http://blog.csdn.net/crave_shy/article/details/46013493 "Mybatis深入之初始化过程")中知道配置文件如何解析的、其中关于事务方面的解析: ~~~ private void environmentsElement(XNode context) throws Exception { //只关注事务部分... TransactionFactory txFactory = transactionManagerElement(child.evalNode("transactionManager")); ... } ~~~ ~~~ private TransactionFactory transactionManagerElement(XNode context) throws Exception { if (context != null) { String type = context.getStringAttribute("type"); Properties props = context.getChildrenAsProperties(); TransactionFactory factory = (TransactionFactory) resolveClass(type).newInstance(); factory.setProperties(props); return factory; } throw new BuilderException("Environment declaration requires a TransactionFactory."); } ~~~ ~~~ typeAliasRegistry.registerAlias("JDBC", JdbcTransactionFactory.class); typeAliasRegistry.registerAlias("MANAGED", ManagedTransactionFactory.class); ~~~ - 重点在于根据type类型判断实例化何种TransactionFactory - 前面已经知道Mybatis两种事务配置的方式、这里使用的jdbc类型的事务 - 上一篇分析DataSource实例化过程中有一段是关于根据DataSource的type来获取何种Factory的、这里原理同样 - 通过TypeAliasRegistry根据type=’JDBC’来获取TransactionFactory实现类JdbcTransactionFactory 关键在于JdbcTransactionFactory通过newInstance()使用无参构造函数时做了什么工作 ~~~ public class JdbcTransactionFactory implements TransactionFactory { public void setProperties(Properties props) { } public Transaction newTransaction(Connection conn) { return new JdbcTransaction(conn); } public Transaction newTransaction(DataSource ds, TransactionIsolationLevel level, boolean autoCommit) { return new JdbcTransaction(ds, level, autoCommit); } } ~~~ - JdbcTransactionFactory默认无参构造方法被调用 - setProperties没有做任何实质性处理 - 对比ManagedTransactionFactory不再贴代码 下面就是获取具有事务特性的数据库连接了 JdbcTransaction: ~~~ public Transaction newTransaction(Connection conn) { return new JdbcTransaction(conn); } ~~~ ManagedTransaction: ~~~ public Transaction newTransaction(Connection conn) { return new ManagedTransaction(conn, closeConnection); } ~~~ - 两者都是通过Connection来创建具体的实例 JdbcTransaction: ~~~ 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; } public Connection getConnection() throws SQLException { if (connection == null) { openConnection(); } return connection; } public void commit() throws SQLException { if (connection != null && !connection.getAutoCommit()) { if (log.isDebugEnabled()) { log.debug("Committing JDBC Connection [" + connection + "]"); } connection.commit(); } } public void rollback() throws SQLException { if (connection != null && !connection.getAutoCommit()) { if (log.isDebugEnabled()) { log.debug("Rolling back JDBC Connection [" + connection + "]"); } connection.rollback(); } } 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); } } ~~~ - 从源码中可知、JdbcTransaction如何管理事务的、如前面所说调用DataSource事务操作方法。 - 并且对select不进行事务控制 - 当使用DataSource创建数据库连接时、数据库的事务隔离级别使用DataSource默认的事务隔离级别 - 如需指定事务的隔离级别、必须手动创建JdbcTransaction(调用另一个构造函数) - 关于事务隔离级别会在补充中有 ManagedTransaction: ~~~ 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()); } } } ~~~ - 重点看一下`commit()``rollback()`方法,没有方法体。验证前面其关于事务的管理方式 到这里事务暂时告一段落、一般在使用时会与spring结合、将数据库连接、事务管理都交由spring管理。 ### 补充 数据库隔离级别: 先对不同隔离级别涉及到的名词解释: • 脏读: 对于两个事物 T1, T2, T1 读取了已经被 T2 更新但还没有被提交的字段. 之后, 若 T2 回滚, T1读取的内容就是临时且无效的. • 不可重复读: 对于两个事物 T1, T2, T1 读取了一个字段, 然后 T2 更新了该字段. 之后, T1再次读取同一个字段, 值就不同了. • 幻读: 对于两个事物 T1, T2, T1 从一个表中读取了一个字段, 然后 T2 在该表中插入了一些新的行. 之后, 如果 T1 再次读取同一个表, 就会多出几 具体的隔离级别定义: READ UNCOMMITTED(读未提交数据) 允许事务读取未被其他事务提交的变更,脏读、不可重复读和幻读的问题都会出现 READ COMMITED(读已提交数据) 只允许事务读取已经被其他事务提交的变更,可以避免脏读,但不可重复读和幻读问题仍然会出现 REPEATABLE READ(可重复读) 确保事务可以多次从一个字段中读取相同的值,在这个事务持续期间,禁止其他事务对这个字段进行更新,可以避免脏读和不可重复读,但幻读的问题依然存在 SERIALIZABLE(串行化) 确保事务可以从一个表中读取相同的行,在这个事务持续期间,禁止其他事务对该表执行插入、更新和删除操作,所有并发问题都可以避免,但性能十分低 Oracle 支持的 2 种事务隔离级别:READ COMMITED, SERIALIZABLE. Oracle 默认的事务隔离级别为: READ COMMITED Mysql 支持 4 中事务隔离级别. Mysql 默认的事务隔离级别为: REPEATABLE READ 更多内容:[Mybatis 目录](http://blog.csdn.net/crave_shy/article/details/45825599)
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