MyBatis框架的使用及源码分析(五) DefaultSqlSessionFactory和DefaultSqlSession

我们回顾<MyBatis框架中Mapper映射配置的使用及原理解析(一) 配置与使用> 一文的示例

private static SqlSessionFactory getSessionFactory() {
    SqlSessionFactory sessionFactory = null;
    String resource = "mybatisConfig.xml";
    try {
        sessionFactory = new SqlSessionFactoryBuilder().build(Resources
                .getResourceAsReader(resource));
    } catch (IOException e) {
        e.printStackTrace();
    }
    return sessionFactory;
}

@Test
public void findUserById() {
    SqlSessionFactory sqlSessionFactory = getSessionFactory();
    SqlSession sqlSession = sqlSessionFactory.openSession();
    UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
    User user = userMapper.selectByPrimaryKey(1l);
    System.out.println(user.getId() + " /  " + user.getName());
}

SqlSessionFactoryBuilder 创建出SqlSessionFactory，然后从SqlSessionFactory中得到SqlSession，最后通过SqlSession得到Mapper接口对象进行数据库操作。

我们跟踪SqlSessionFactoryBuilder的源代码：

package org.apache.ibatis.session;

public class SqlSessionFactoryBuilder {

public SqlSessionFactory build(Reader reader, String environment, Properties properties) {
    try {
      XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
      return build(parser.parse());
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error building SqlSession.", e);
    } finally {
      ErrorContext.instance().reset();
      try {
        reader.close();
      } catch (IOException e) {
        // Intentionally ignore. Prefer previous error.
      }
    }
  }

public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
    try {
      XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
      return build(parser.parse());
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error building SqlSession.", e);
    } finally {
      ErrorContext.instance().reset();
      try {
        inputStream.close();
      } catch (IOException e) {
        // Intentionally ignore. Prefer previous error.
      }
    }
  }
    
  public SqlSessionFactory build(Configuration config) {
    return new DefaultSqlSessionFactory(config);
  }

}

我们可以看到这个类用很多的构造方法，但主要分为三大类：
1、通过读取字符流（Reader）的方式构件SqlSessionFactory。
2、通过字节流（InputStream）的方式构件SqlSessionFacotry。
3、通过Configuration对象构建SqlSessionFactory。
第1、2种方式是通过配置文件方式，第3种是通过Java代码方式。
build方法返回SqlSessionFactory接口的实现对象DefaultSqlSessionFactory。

我们继续跟踪DefaultSqlSessionFactory的openSession()方法：

package org.apache.ibatis.session.defaults;

/**
 * @author Clinton Begin
 */
public class DefaultSqlSessionFactory implements SqlSessionFactory {

  private final Configuration configuration;

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

  public SqlSession openSession() {
    return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
  }

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

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

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

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

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

  public SqlSession openSession(Connection connection) {
    return openSessionFromConnection(configuration.getDefaultExecutorType(), connection);
  }

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

  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);
      tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
      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();
    }
  }

  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) {
    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.
      }
    }
  }

}

这么多的openSession重载方法，都是通过传入不同的参数构造SqlSession实例，有通过设置事务是否自动提交"autoCommit"，有设置执行器类型"ExecutorType"来构造的，还有事务的隔离级别等等。
最后一个方法就告诉我们可以通过SqlSessionFactory来获取Configuration对象。

mybatis创建sqlsession经过了以下几个主要步骤： 

1.       从核心配置文件mybatis-config.xml中获取Environment（这里面是数据源）；
2.       从Environment中取得DataSource；
3.       从Environment中取得TransactionFactory；
4.       从DataSource里获取数据库连接对象Connection；
5.       在取得的数据库连接上创建事务对象Transaction；
6.       创建Executor对象（该对象非常重要，事实上sqlsession的所有操作都是通过它完成的）；
7.       创建sqlsession对象。

从源码中可以知道DefaultSqlSession是SqlSession的实例。

new DefaultSqlSession(configuration, executor, autoCommit);

那么通过此文，我们就清楚的知道了SqlSessionFactory和SqlSession具体的创建过程，知道了他们的实现类是DefaultSqlSessionFactory和DefaultSqlSession。