给自己的每日一句

不从恶人的计谋，不站罪人的道路，不坐亵慢人的座位，惟喜爱耶和华的律法，昼夜思想，这人便为有福！他要像一棵树栽在溪水旁，按时候结果子，叶子也不枯干。凡他所做的尽都顺利

前言

        InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();

作为一个开发界的菜鸟，当我研究Mybatis的源码的时候，总是好奇，上边这三行代码单当中走过的时候，发生了什么样的事情呢？我们这篇文章只研究openSession这个方法。

sqlSessionFactory.openSession()

public interface SqlSessionFactory {

  SqlSession openSession();

  SqlSession openSession(boolean autoCommit);
  SqlSession openSession(Connection connection);
  SqlSession openSession(TransactionIsolationLevel level);

  SqlSession openSession(ExecutorType execType);
  SqlSession openSession(ExecutorType execType, boolean autoCommit);
  SqlSession openSession(ExecutorType execType, TransactionIsolationLevel level);
  SqlSession openSession(ExecutorType execType, Connection connection);

  Configuration getConfiguration();

}

SqlSessionFactory是SqlSession的工厂，各种各样的重载的openSession方法。

实现类有两个，默认走的都是第一个DefaultSqlSessionFactory

  @Override
  public SqlSession openSession() {
    return openSessionFromDataSource(
    				//默认的是SimpleExcutor,
    				configuration.getDefaultExecutorType(), null, false);
  }

  private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
    Transaction tx = null;
    try {
      //final可以修饰局部变量
      //获取Mybatis-config.xml中的Environment标签对应的对象
      final Environment environment = configuration.getEnvironment();
      //通过environment获取事务工厂
      final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
      //通过事务工厂和数据源获取事务对象
      tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
      //configuration.newExecutor(事务对象，Executor类型对象)
      final Executor executor = configuration.newExecutor(tx, execType);
      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();
    }
  }

通过Environment 获取TransactionFactory 对象
通过TransactionFactory和Environment中的DateSource获取Transaction
通过Transaction 和 ExcutorType获取 Executor 对象
通过Executor configuration autoCommit获取SqlSession对象、

我们可以得出以下结论
1：openSession()默认创建的是DefaultSqlSession对象，而且三个入参是：ExcutorType、TransatinIsolationLevel、AutoCommit
2：默认的Exuecutor是SimpleExecutor
3：Excutor对象是由Configuration创建的，默认void：SimpleExcutor，param：Transaction、ExecutorType