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Mybatis与Spring集成：SqlSessionTemplate
SqlSessionTemplate构造方法
public SqlSessionTemplate(SqlSessionFactory sqlSessionFactory, ExecutorType executorType,PersistenceExceptionTranslator exceptionTranslator) {notNull(sqlSessionFactory, "Property 'sqlSessionFactory' is required");notNull(executorType, "Property 'executorType' is required");this.sqlSessionFactory = sqlSessionFactory;this.executorType = executorType;this.exceptionTranslator = exceptionTranslator;//使用JDK动态代理，创建SqlSessionFactory的代理类的实例 this.sqlSessionProxy = (SqlSession) newProxyInstance(SqlSessionFactory.class.getClassLoader(),new Class[] { SqlSession.class },new SqlSessionInterceptor());}
核心在SqlSessionInterceptor的invoke方法中
private class SqlSessionInterceptor implements InvocationHandler {@Overridepublic Object invoke(Object proxy, Method method, Object[] args) throws Throwable {//重要：获取sqlSession，具体操作见下方详解SqlSession sqlSession = getSqlSession(SqlSessionTemplate.this.sqlSessionFactory,SqlSessionTemplate.this.executorType,SqlSessionTemplate.this.exceptionTranslator);try {//调用真实SqlSession的操作方法Object result = method.invoke(sqlSession, args);//判断当前sqlSession是否被Spring托管；未被Spring托管则自动commitif (!isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) {// force commit even on non-dirty sessions because some databases require a commit/rollback before calling close()sqlSession.commit(true);}return result;} catch (Throwable t) {Throwable unwrapped = unwrapThrowable(t);if (SqlSessionTemplate.this.exceptionTranslator != null && unwrapped instanceof PersistenceException) {closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);sqlSession = null;Throwable translated = SqlSessionTemplate.this.exceptionTranslator.translateExceptionIfPossible((PersistenceException) unwrapped);if (translated != null) {unwrapped = translated;}}throw unwrapped;} finally {if (sqlSession != null) {closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);}}}}
SqlSessionUtils.getSqlSession方法
//从事务管理器中获取sqlSession或创建一个 public static SqlSession getSqlSession(SqlSessionFactory sessionFactory, ExecutorType executorType, PersistenceExceptionTranslator exceptionTranslator) {notNull(sessionFactory, NO_SQL_SESSION_FACTORY_SPECIFIED);notNull(executorType, NO_EXECUTOR_TYPE_SPECIFIED);//根据sqlSessionFactory从当前线程对应的资源map中获取SqlSessionHolder，当sqlSessionFactory创建了sqlSession，就会在事务管理器中添加一对映射：key为sqlSessionFactory，value为SqlSessionHolder，该类保存sqlSession及执行方式 SqlSessionHolder holder = (SqlSessionHolder) TransactionSynchronizationManager.getResource(sessionFactory);//返回holder中的sqlSession，见下方SqlSession session = sessionHolder(executorType, holder);if (session != null) {return session;}if (LOGGER.isDebugEnabled()) {LOGGER.debug("Creating a new SqlSession");}// 如果holder中返回的sqlSession为空，则创建sqlSession，并注册到holder中去session = sessionFactory.openSession(executorType);registerSessionHolder(sessionFactory, executorType, exceptionTranslator, session);return session;}
private static SqlSession sessionHolder(ExecutorType executorType, SqlSessionHolder holder) {SqlSession session = null;if (holder != null && holder.isSynchronizedWithTransaction()) {// 在同一个事务中，执行类型不能变化，原因就是同一个事务中同一个sqlSessionFactory创建的sqlSession会被重用 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");}session = holder.getSqlSession();}return session;}
如何将sqlSession注册到holder中去？
//开启了事务管理时（如springboot常用的的@Transactional），注册session//Register session holder if synchronization is active (i.e. a Spring TX is active).private static void registerSessionHolder(SqlSessionFactory sessionFactory, ExecutorType executorType,PersistenceExceptionTranslator exceptionTranslator, SqlSession session) {SqlSessionHolder holder;///判断同步是否激活，只要SpringTX被激活，就是true if (TransactionSynchronizationManager.isSynchronizationActive()) {//加载环境变量，判断注册的事务管理器是否是Spring管理事务，如果是，则将sqlSession加载进事务管理的本地线程缓存中；否则报错Environment environment = sessionFactory.getConfiguration().getEnvironment();if (environment.getTransactionFactory() instanceof SpringManagedTransactionFactory) {if (LOGGER.isDebugEnabled()) {LOGGER.debug("Registering transaction synchronization for SqlSession [" + session + "]");}holder = new SqlSessionHolder(session, executorType, exceptionTranslator);// 以sessionFactory为key，hodler为value，加入到TransactionSynchronizationManager管理的本地缓存ThreadLocal<Map<Object, Object>>中 TransactionSynchronizationManager.bindResource(sessionFactory, holder);TransactionSynchronizationManager.registerSynchronization(new SqlSessionSynchronization(holder, sessionFactory));//设置当前holder和当前事务同步holder.setSynchronizedWithTransaction(true);//增加引用数 holder.requested();} else {if (TransactionSynchronizationManager.getResource(environment.getDataSource()) == null) {if (LOGGER.isDebugEnabled()) {LOGGER.debug("SqlSession [" + session + "] was not registered for synchronization because DataSource is not transactional");}} else {throw new TransientDataAccessResourceException("SqlSessionFactory must be using a SpringManagedTransactionFactory in order to use Spring transaction synchronization");}}} else {if (LOGGER.isDebugEnabled()) {LOGGER.debug("SqlSession [" + session + "] was not registered for synchronization because synchronization is not active");}}
}
这里用到了TransactionSynchronizationManager类：
TransactionSynchronizationManager.bindResource：在调用一个需要事务的组件的时候，管理器首先判断当前线程有没有事务，如果没有事务则启动一个事务，并把事务与当前线程绑定。Spring使用TransactionSynchronizationManager的bindResource方法将当前线程与一个事务绑定，采用的方式是ThreadLocal。
public static void bindResource(Object key, Object value) throws IllegalStateException {Object actualKey = TransactionSynchronizationUtils.unwrapResourceIfNecessary(key);Assert.notNull(value, "Value must not be null");// 此处resources是一个全局变量： //private static final ThreadLocal<Map<Object, Object>> resources = new NamedThreadLocal("Transactional resources");Map<Object, Object> map = (Map)resources.get();if (map == null) {map = new HashMap();resources.set(map);}//判断以此sessionFactory为key的Map是否存在。如果存在则此线程已经绑定了 Object oldValue = ((Map)map).put(actualKey, value);if (oldValue instanceof ResourceHolder && ((ResourceHolder)oldValue).isVoid()) {oldValue = null;}if (oldValue != null) {throw new IllegalStateException("Already value [" + oldValue + "] for key [" + actualKey + "] bound to thread [" + Thread.currentThread().getName() + "]");} else {if (logger.isTraceEnabled()) {logger.trace("Bound value [" + value + "] for key [" + actualKey + "] to thread [" + Thread.currentThread().getName() + "]");}}}
可以看出：多线程获取SqlSession时，先从当前线程中获取SqlSessionHolder中的sqlSession；如果为空，再新建sqlSession。所以，多线程公用的是一个SqlSessionTemplate（默认bean的单例注入），但不共用一个SqlSession。
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