mybatis操作的时候跟数据库的每一次连接,都需要创建一个会话,我们用openSession()方法来创建。这个会话里面需要包含一个Executor用来执行 SQL，Executor又要指定事务类型和执行器的类型。

1.创建Transaction(两种方式)

如果配置的是 JDBC,则会使用Connection 对象的 commit()、rollback()、close()管理事务。 如果配置成MANAGED,会把事务交给容器来管理,比如 JBOSS,Weblogic。

SqlSession sqlSession = sqlSessionFactory.openSession();

public SqlSession openSession() {

    protected ExecutorType defaultExecutorType = ExecutorType.SIMPLE

    return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);

}

<environments default="development">

<environment id="development">

<transactionManager type="JDBC"/>

<dataSource type="POOLED">

<property name="driver" value="${driver}"/>

<property name="url" value="${url}"/>

<property name="username" value="${username}"/>

<property name="password" value="${password}"/>

</dataSource>

</environment>

</environments>

2.创建Executor

//ExecutorType是SIMPLE,一共有三种SIMPLE(SimpleExecutor)、REUSE(ReuseExecutor)、BATCH(BatchExecutor)
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
	Transaction tx = null;
	try {
		//xml中的development节点
		final Environment environment = configuration.getEnvironment();
		//type配置的是Jbdc所以生成的是JbdcTransactionFactory工厂类
		final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
		//Jdbc生成JbdcTransactionFactory生成JbdcTransaction
		tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
		//创建CachingExecutor执行器
		final Executor executor = configuration.newExecutor(tx, execType);
		//创建DefaultSqlSession属性包括 Configuration、Executor对象
		return new DefaultSqlSession(configuration, executor, autoCommit);
	} catch (Exception e) {
		closeTransaction(tx); // may have fetched a connection so lets call
		close();
		throw ExceptionFactory.wrapException("Error opening session. Cause: " + e, e);
	} finally {
		ErrorContext.instance().reset();
	}
}