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。
