Mybatis工作机制源码分析—SqlSessionUtils.getSqlSession工作机制

       在调用SqlSessionTemplate进行dao层操作时，其会将工作委托给sqlSessionProxy属性进行，而sqlSessionProxy在进行相关method调用时，用到了JDK动态代理机制，首先SqlSessionUtils.getSqlSession获取sqlSession，本文主要以源码的形式阐述其工作机制。

SqlSessionTemplate.SqlSessionInterceptor源码

private class SqlSessionInterceptor implements InvocationHandler {public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {// 首先获取sqlSessionfinal SqlSession sqlSession = SqlSessionUtils.getSqlSession(SqlSessionTemplate.this.sqlSessionFactory,SqlSessionTemplate.this.executorType,SqlSessionTemplate.this.exceptionTranslator);try {// JDK动态代理反射机制Object result = method.invoke(sqlSession, args);if (!SqlSessionUtils.isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) {sqlSession.commit();}return result;} catch (Throwable t) {Throwable unwrapped = ExceptionUtil.unwrapThrowable(t);if (SqlSessionTemplate.this.exceptionTranslator != null && unwrapped instanceof PersistenceException) {unwrapped = SqlSessionTemplate.this.exceptionTranslator.translateExceptionIfPossible((PersistenceException) unwrapped);}throw unwrapped;} finally {SqlSessionUtils.closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);}}}

SqlSessionUtils.getSqlSession源码

      dataSource不为TransactionAwareDataSourceProxy时，采用DataSourceUtils.getConnection(dataSource)获取实际的Connection：

public static SqlSession getSqlSession(SqlSessionFactory sessionFactory, ExecutorType executorType, PersistenceExceptionTranslator exceptionTranslator) {Assert.notNull(sessionFactory, "No SqlSessionFactory specified");Assert.notNull(executorType, "No ExecutorType specified");// 事务同步资源管理SqlSessionHolder holder = (SqlSessionHolder) TransactionSynchronizationManager.getResource(sessionFactory);if (holder != null && holder.isSynchronizedWithTransaction()) {if (holder.getExecutorType() != executorType) {throw new TransientDataAccessResourceException("Cannot change the ExecutorType when there is an existing transaction");}holder.requested();if (logger.isDebugEnabled()) {logger.debug("Fetched SqlSession [" + holder.getSqlSession() + "] from current transaction");}return holder.getSqlSession();}// 获取dataSourceDataSource dataSource = sessionFactory.getConfiguration().getEnvironment().getDataSource();// SqlSessionFactoryBean unwraps TransactionAwareDataSourceProxies but // we keep this check for the case that SqlSessionUtils is called from custom codeboolean transactionAware = (dataSource instanceof TransactionAwareDataSourceProxy);Connection conn;try {// 实际Connection的获取conn = transactionAware ? dataSource.getConnection() : DataSourceUtils.getConnection(dataSource);} catch (SQLException e) {throw new CannotGetJdbcConnectionException("Could not get JDBC Connection for SqlSession", e);}if (logger.isDebugEnabled()) {logger.debug("Creating SqlSession with JDBC Connection [" + conn + "]");}// Assume either DataSourceTransactionManager or the underlying// connection pool already dealt with enabling auto commit.// This may not be a good assumption, but the overhead of checking// connection.getAutoCommit() again may be expensive (?) in some drivers// (see DataSourceTransactionManager.doBegin()). One option would be to// only check for auto commit if this function is being called outside// of DSTxMgr, but to do that we would need to be able to call// ConnectionHolder.isTransactionActive(), which is protected and not// visible to this class.// 基于Connection，创建SqlSessionSqlSession session = sessionFactory.openSession(executorType, conn);// Register session holder and bind it to enable synchronization.//// Note: The DataSource should be synchronized with the transaction// either through DataSourceTxMgr or another tx synchronization.// Further assume that if an exception is thrown, whatever started the transaction will// handle closing / rolling back the Connection associated with the SqlSession.if (TransactionSynchronizationManager.isSynchronizationActive()) {// 事务同步下的对资源相关操作if (!(sessionFactory.getConfiguration().getEnvironment().getTransactionFactory() instanceof SpringManagedTransactionFactory)&& DataSourceUtils.isConnectionTransactional(conn, dataSource)) {throw new TransientDataAccessResourceException("SqlSessionFactory must be using a SpringManagedTransactionFactory in order to use Spring transaction synchronization");}if (logger.isDebugEnabled()) {logger.debug("Registering transaction synchronization for SqlSession [" + session + "]");}holder = new SqlSessionHolder(session, executorType, exceptionTranslator);TransactionSynchronizationManager.bindResource(sessionFactory, holder);TransactionSynchronizationManager.registerSynchronization(new SqlSessionSynchronization(holder, sessionFactory));holder.setSynchronizedWithTransaction(true);holder.requested();} else {if (logger.isDebugEnabled()) {logger.debug("SqlSession [" + session + "] was not registered for synchronization because synchronization is not active");}            }return session;}

DataSourceUtils.getConnection源码

DataSourceUtils.java// 获取Connectionpublic 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");// 事务同步管理下的对应ConnectionHolder资源获取ConnectionHolder conHolder = (ConnectionHolder) TransactionSynchronizationManager.getResource(dataSource);if (conHolder != null && (conHolder.hasConnection() || conHolder.isSynchronizedWithTransaction())) {conHolder.requested();if (!conHolder.hasConnection()) {logger.debug("Fetching resumed JDBC Connection from DataSource");conHolder.setConnection(dataSource.getConnection());}return conHolder.getConnection();}// Else we either got no holder or an empty thread-bound holder here.logger.debug("Fetching JDBC Connection from DataSource");// 直接从dataSource中获取JDBC连接Connection con = dataSource.getConnection();if (TransactionSynchronizationManager.isSynchronizationActive()) {// 事务同步管理下，对ConnectionHolder资源的操作logger.debug("Registering transaction synchronization for JDBC Connection");// Use same Connection for further JDBC actions within the transaction.// Thread-bound object will get removed by synchronization at transaction completion.ConnectionHolder holderToUse = conHolder;if (holderToUse == null) {holderToUse = new ConnectionHolder(con);}else {holderToUse.setConnection(con);}holderToUse.requested();TransactionSynchronizationManager.registerSynchronization(new ConnectionSynchronization(holderToUse, dataSource));holderToUse.setSynchronizedWithTransaction(true);if (holderToUse != conHolder) {TransactionSynchronizationManager.bindResource(dataSource, holderToUse);}}return con;}

     采用源码的形式了解Mybatis如何基于Spring获取Connection。