Mybaits 源码解析 （十二）—– Mybatis的事务如何被Spring管理？Mybatis和Spring事务中用的Connection是同一个吗？
- 2019 年 11 月 13 日
- 筆記
不知道一些同学有没有这种疑问，为什么Mybtis中要配置dataSource，Spring的事务中也要配置dataSource？那么Mybatis和Spring事务中用的Connection是同一个吗？我们常用配置如下
<!--会话工厂 --> <bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean"> <property name="dataSource" ref="dataSource" /> </bean> <!--spring事务管理 --> <bean id="transactionManager" class="org.springframework.jdbc.datasource.DataSourceTransactionManager"> <property name="dataSource" ref="dataSource" /> </bean> <!--使用注释事务 --> <tx:annotation-driven transaction-manager="transactionManager" />
看到没，sqlSessionFactory中配置了dataSource，transactionManager也配置了dataSource，我们来回忆一下SqlSessionFactoryBean这个类
1 protected SqlSessionFactory buildSqlSessionFactory() throws IOException { 2 3 // 配置类 4 Configuration configuration; 5 // 解析mybatis-Config.xml文件， 6 // 将相关配置信息保存到configuration 7 XMLConfigBuilder xmlConfigBuilder = null; 8 if (this.configuration != null) { 9 configuration = this.configuration; 10 if (configuration.getVariables() == null) { 11 configuration.setVariables(this.configurationProperties); 12 } else if (this.configurationProperties != null) { 13 configuration.getVariables().putAll(this.configurationProperties); 14 } 15 //资源文件不为空 16 } else if (this.configLocation != null) { 17 //根据configLocation创建xmlConfigBuilder，XMLConfigBuilder构造器中会创建Configuration对象 18 xmlConfigBuilder = new XMLConfigBuilder(this.configLocation.getInputStream(), null, this.configurationProperties); 19 //将XMLConfigBuilder构造器中创建的Configuration对象直接赋值给configuration属性 20 configuration = xmlConfigBuilder.getConfiguration(); 21 } 22 23 //略.... 24 25 if (xmlConfigBuilder != null) { 26 try { 27 //解析mybatis-Config.xml文件，并将相关配置信息保存到configuration 28 xmlConfigBuilder.parse(); 29 if (LOGGER.isDebugEnabled()) { 30 LOGGER.debug("Parsed configuration file: '" + this.configLocation + "'"); 31 } 32 } catch (Exception ex) { 33 throw new NestedIOException("Failed to parse config resource: " + this.configLocation, ex); 34 } 35 } 36 37 if (this.transactionFactory == null) { 38 //事务默认采用SpringManagedTransaction，这一块非常重要 39 this.transactionFactory = new SpringManagedTransactionFactory(); 40 } 41 // 为sqlSessionFactory绑定事务管理器和数据源 42 // 这样sqlSessionFactory在创建sqlSession的时候可以通过该事务管理器获取jdbc连接，从而执行SQL 43 configuration.setEnvironment(new Environment(this.environment, this.transactionFactory, this.dataSource)); 44 // 解析mapper.xml 45 if (!isEmpty(this.mapperLocations)) { 46 for (Resource mapperLocation : this.mapperLocations) { 47 if (mapperLocation == null) { 48 continue; 49 } 50 try { 51 // 解析mapper.xml文件，并注册到configuration对象的mapperRegistry 52 XMLMapperBuilder xmlMapperBuilder = new XMLMapperBuilder(mapperLocation.getInputStream(), 53 configuration, mapperLocation.toString(), configuration.getSqlFragments()); 54 xmlMapperBuilder.parse(); 55 } catch (Exception e) { 56 throw new NestedIOException("Failed to parse mapping resource: '" + mapperLocation + "'", e); 57 } finally { 58 ErrorContext.instance().reset(); 59 } 60 61 if (LOGGER.isDebugEnabled()) { 62 LOGGER.debug("Parsed mapper file: '" + mapperLocation + "'"); 63 } 64 } 65 } else { 66 if (LOGGER.isDebugEnabled()) { 67 LOGGER.debug("Property 'mapperLocations' was not specified or no matching resources found"); 68 } 69 } 70 71 // 将Configuration对象实例作为参数， 72 // 调用sqlSessionFactoryBuilder创建sqlSessionFactory对象实例 73 return this.sqlSessionFactoryBuilder.build(configuration); 74 }
我们看第39行，Mybatis集成Spring后，默认使用的transactionFactory是SpringManagedTransactionFactory，那我们就来看看其获取Transaction的方法
private SqlSession openSessionFromConnection(ExecutorType execType, Connection connection) { try { boolean autoCommit; try { autoCommit = connection.getAutoCommit(); } catch (SQLException e) { // Failover to true, as most poor drivers // or databases won't support transactions autoCommit = true; } //从configuration中取出environment对象 final Environment environment = configuration.getEnvironment(); //从environment中取出TransactionFactory final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment); //创建Transaction final Transaction tx = transactionFactory.newTransaction(connection); //创建包含事务操作的执行器 final Executor executor = configuration.newExecutor(tx, execType); //构建包含执行器的SqlSession return new DefaultSqlSession(configuration, executor, autoCommit); } catch (Exception e) { throw ExceptionFactory.wrapException("Error opening session. Cause: " + e, e); } finally { ErrorContext.instance().reset(); } } private TransactionFactory getTransactionFactoryFromEnvironment(Environment environment) { if (environment == null || environment.getTransactionFactory() == null) { return new ManagedTransactionFactory(); } //这里返回SpringManagedTransactionFactory return environment.getTransactionFactory(); } @Override public Transaction newTransaction(DataSource dataSource, TransactionIsolationLevel level, boolean autoCommit) { //创建SpringManagedTransaction return new SpringManagedTransaction(dataSource); }
SpringManagedTransaction
也就是说mybatis的执行事务的事务管理器就切换成了SpringManagedTransaction，下面我们再去看看SpringManagedTransactionFactory类的源码：
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) { Assert.notNull(dataSource, "No DataSource specified"); this.dataSource = dataSource; } public Connection getConnection() throws SQLException { if (this.connection == null) { this.openConnection(); } return this.connection; } private void openConnection() throws SQLException { //通过DataSourceUtils获取connection，这里和JdbcTransaction不一样 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"); } } public void commit() throws SQLException { if (this.connection != null && !this.isConnectionTransactional && !this.autoCommit) { if (LOGGER.isDebugEnabled()) { LOGGER.debug("Committing JDBC Connection [" + this.connection + "]"); } //通过connection提交，这里和JdbcTransaction一样 this.connection.commit(); } } public void rollback() throws SQLException { if (this.connection != null && !this.isConnectionTransactional && !this.autoCommit) { if (LOGGER.isDebugEnabled()) { LOGGER.debug("Rolling back JDBC Connection [" + this.connection + "]"); } //通过connection回滚，这里和JdbcTransaction一样 this.connection.rollback(); } } public void close() throws SQLException { DataSourceUtils.releaseConnection(this.connection, this.dataSource); } public Integer getTimeout() throws SQLException { ConnectionHolder holder = (ConnectionHolder)TransactionSynchronizationManager.getResource(this.dataSource); return holder != null && holder.hasTimeout() ? holder.getTimeToLiveInSeconds() : null; } }
org.springframework.jdbc.datasource.DataSourceUtils#getConnection
public static Connection getConnection(DataSource dataSource) throws CannotGetJdbcConnectionException { try { return doGetConnection(dataSource); } catch (SQLException ex) { throw new CannotGetJdbcConnectionException("Could not get JDBC Connection", ex); } } public static Connection doGetConnection(DataSource dataSource) throws SQLException { Assert.notNull(dataSource, "No DataSource specified"); //TransactionSynchronizationManager重点！！！有没有很熟悉的感觉？？ //还记得我们前面Spring事务源码的分析吗？@Transaction会创建Connection，并放入ThreadLocal中 //这里从ThreadLocal中获取ConnectionHolder ConnectionHolder conHolder = (ConnectionHolder)TransactionSynchronizationManager.getResource(dataSource); if (conHolder == null || !conHolder.hasConnection() && !conHolder.isSynchronizedWithTransaction()) { logger.debug("Fetching JDBC Connection from DataSource"); //如果没有使用@Transaction，那调用Mapper接口方法时，也是通过Spring的方法获取Connection Connection con = fetchConnection(dataSource); if (TransactionSynchronizationManager.isSynchronizationActive()) { logger.debug("Registering transaction synchronization for JDBC Connection"); ConnectionHolder holderToUse = conHolder; if (conHolder == null) { holderToUse = new ConnectionHolder(con); } else { conHolder.setConnection(con); } holderToUse.requested(); TransactionSynchronizationManager.registerSynchronization(new DataSourceUtils.ConnectionSynchronization(holderToUse, dataSource)); holderToUse.setSynchronizedWithTransaction(true); if (holderToUse != conHolder) { //将获取到的ConnectionHolder放入ThreadLocal中，那么当前线程调用下一个接口，下一个接口使用了Spring事务，那Spring事务也可以直接取到Mybatis创建的Connection //通过ThreadLocal保证了同一线程中Spring事务使用的Connection和Mapper代理类使用的Connection是同一个 TransactionSynchronizationManager.bindResource(dataSource, holderToUse); } } return con; } else { conHolder.requested(); if (!conHolder.hasConnection()) { logger.debug("Fetching resumed JDBC Connection from DataSource"); conHolder.setConnection(fetchConnection(dataSource)); } //所以如果我们业务代码使用了@Transaction注解，在Spring中就已经通过dataSource创建了一个Connection并放入ThreadLocal中 //那么当Mapper代理对象调用方法时，通过SqlSession的SpringManagedTransaction获取连接时，就直接获取到了当前线程中Spring事务创建的Connection并返回 return conHolder.getConnection(); } }
想看怎么获取connHolder
org.springframework.transaction.support.TransactionSynchronizationManager#getResource
//保存数据库连接的ThreadLocal private static final ThreadLocal<Map<Object, Object>> resources = new NamedThreadLocal<>("Transactional resources"); @Nullable public static Object getResource(Object key) { Object actualKey = TransactionSynchronizationUtils.unwrapResourceIfNecessary(key); //获取ConnectionHolder Object value = doGetResource(actualKey); .... return value; } @Nullable private static Object doGetResource(Object actualKey) { /** * 从threadlocal <Map<Object, Object>>中取出来当前线程绑定的map * map里面存的是<dataSource,ConnectionHolder> */ Map<Object, Object> map = resources.get(); if (map == null) { return null; } //map中取出来对应dataSource的ConnectionHolder Object value = map.get(actualKey); // Transparently remove ResourceHolder that was marked as void... if (value instanceof ResourceHolder && ((ResourceHolder) value).isVoid()) { map.remove(actualKey); // Remove entire ThreadLocal if empty... if (map.isEmpty()) { resources.remove(); } value = null; } return value; }
我们看到直接从ThreadLocal中取出来的conn,而spring自己的事务也是操作的这个ThreadLocal中的conn来进行事务的开启和回滚，由此我们知道了在同一线程中Spring事务中的Connection和Mybaits中Mapper代理对象中操作数据库的Connection是同一个，当取出来的conn为空时候,调用org.springframework.jdbc.datasource.DataSourceUtils#fetchConnection获取，然后把从数据源取出来的连接返回
private static Connection fetchConnection(DataSource dataSource) throws SQLException { //从数据源取出来conn Connection con = dataSource.getConnection(); if (con == null) { throw new IllegalStateException("DataSource returned null from getConnection(): " + dataSource); } return con; }
我们再来回顾一下上篇文章中的SqlSessionInterceptor
1 private class SqlSessionInterceptor implements InvocationHandler { 2 private SqlSessionInterceptor() { 3 } 4 5 public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { 6 SqlSession sqlSession = SqlSessionUtils.getSqlSession(SqlSessionTemplate.this.sqlSessionFactory, SqlSessionTemplate.this.executorType, SqlSessionTemplate.this.exceptionTranslator); 7 8 Object unwrapped; 9 try { 10 Object result = method.invoke(sqlSession, args); 11 // 如果当前操作没有在一个Spring事务中，则手动commit一下 12 // 如果当前业务没有使用@Transation,那么每次执行了Mapper接口的方法直接commit 13 // 还记得我们前面讲的Mybatis的一级缓存吗，这里一级缓存不能起作用了，因为每执行一个Mapper的方法，sqlSession都提交了 14 // sqlSession提交，会清空一级缓存 15 if (!SqlSessionUtils.isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) { 16 sqlSession.commit(true); 17 } 18 19 unwrapped = result; 20 } catch (Throwable var11) { 21 unwrapped = ExceptionUtil.unwrapThrowable(var11); 22 if (SqlSessionTemplate.this.exceptionTranslator != null && unwrapped instanceof PersistenceException) { 23 SqlSessionUtils.closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory); 24 sqlSession = null; 25 Throwable translated = SqlSessionTemplate.this.exceptionTranslator.translateExceptionIfPossible((PersistenceException)unwrapped); 26 if (translated != null) { 27 unwrapped = translated; 28 } 29 } 30 31 throw (Throwable)unwrapped; 32 } finally { 33 if (sqlSession != null) { 34 SqlSessionUtils.closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory); 35 } 36 37 } 38 return unwrapped; 39 } 40 }
看第15和16行，如果我们没有使用@Transation，Mapper方法执行完后，sqlSession将会提交，也就是说通过org.springframework.jdbc.datasource.DataSourceUtils#fetchConnection获取到的Connection将会commit，相当于Connection是自动提交的，也就是说如果不使用@Transation，Mybatis将没有事务可言。
如果使用了@Transation呢？那在调用Mapper代理类的方法之前就已经通过Spring的事务生成了Connection并放入ThreadLocal，并且设置事务不自动提交，当前线程多个Mapper代理对象调用数据库操作方法时，将从ThreadLocal获取Spring创建的connection,在所有的Mapper方法调用完后，Spring事务提交或者回滚，到此mybatis的事务是怎么被spring管理的就显而易见了
还有文章开头的问题，为什么Mybtis中要配置dataSource，Spring的事务中也要配置dataSource？
因为Spring事务在没调用Mapper方法之前就需要开一个Connection，并设置事务不自动提交，那么transactionManager中自然要配置dataSource。那如果我们的Service没有用到Spring事务呢，难道就不需要获取数据库连接了吗？当然不是，此时通过SpringManagedTransaction调用org.springframework.jdbc.datasource.DataSourceUtils#getConnection#fetchConnection方法获取，并将dataSource作为参数传进去，实际上获取的Connection都是通过dataSource来获取的。
