// 构造SqlSession

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public interface SqlSessionFactory {

//8个方法可以用来创建SqlSession实例
SqlSession openSession();

//自动提交
SqlSession openSession(boolean autoCommit);
//连接
SqlSession openSession(Connection connection);
//事务隔离级别
SqlSession openSession(TransactionIsolationLevel level);

//执行器的类型
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的实现类是DefaultSqlSessionFactory

public class DefaultSqlSessionFactory implements SqlSessionFactory {

  private final Configuration configuration;

  public DefaultSqlSessionFactory(Configuration configuration) {
    this.configuration = configuration;
  }

  //最终都会调用2种方法：openSessionFromDataSource,openSessionFromConnection
  //以下6个方法都会调用openSessionFromDataSource
  @Override
  public SqlSession openSession() {
    return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
  }

  @Override
  public SqlSession openSession(boolean autoCommit) {
    return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, autoCommit);
  }

  @Override
  public SqlSession openSession(ExecutorType execType) {
    return openSessionFromDataSource(execType, null, false);
  }

  @Override
  public SqlSession openSession(TransactionIsolationLevel level) {
    return openSessionFromDataSource(configuration.getDefaultExecutorType(), level, false);
  }

  @Override
  public SqlSession openSession(ExecutorType execType, TransactionIsolationLevel level) {
    return openSessionFromDataSource(execType, level, false);
  }

  @Override
  public SqlSession openSession(ExecutorType execType, boolean autoCommit) {
    return openSessionFromDataSource(execType, null, autoCommit);
  }

  //以下2个方法都会调用openSessionFromConnection
  @Override
  public SqlSession openSession(Connection connection) {
    return openSessionFromConnection(configuration.getDefaultExecutorType(), connection);
  }

  @Override
  public SqlSession openSession(ExecutorType execType, Connection connection) {
    return openSessionFromConnection(execType, connection);
  }

  @Override
  public Configuration getConfiguration() {
    return configuration;
  }

  private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
    Transaction tx = null;
    try {
      final Environment environment = configuration.getEnvironment();
      final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
      //通过事务工厂来产生一个事务
      tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
      //生成一个执行器(事务包含在执行器里)
      final Executor executor = configuration.newExecutor(tx, execType);
      //然后产生一个DefaultSqlSession
      return new DefaultSqlSession(configuration, executor, autoCommit);
    } catch (Exception e) {
      //如果打开事务出错，则关闭它
      closeTransaction(tx); // may have fetched a connection so lets call close()
      throw ExceptionFactory.wrapException("Error opening session.  Cause: " + e, e);
    } finally {
      //最后清空错误上下文
      ErrorContext.instance().reset();
    }
  }

  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;
      }      
      final Environment environment = configuration.getEnvironment();
      final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
      final Transaction tx = transactionFactory.newTransaction(connection);
      final Executor executor = configuration.newExecutor(tx, execType);
      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) {
    //如果没有配置事务工厂，则返回托管事务工厂
    if (environment == null || environment.getTransactionFactory() == null) {
      return new ManagedTransactionFactory();
    }
    return environment.getTransactionFactory();
  }

  private void closeTransaction(Transaction tx) {
    if (tx != null) {
      try {
        tx.close();
      } catch (SQLException ignore) {
        // Intentionally ignore. Prefer previous error.
      }
    }
  }

}

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