MyBatis源码分析（三）-SqlSession理解

SqlSession理解
一、创建SqlSession
1.创建事务
a.事务工厂根据DataSource创建一个事务对象（Connection对象，事务级别，是否自动提交）
b.事务工厂根据Connection创建一个事务对象（事务级别和是否自动提交已经被设置在Connection对象中）
c.系统默认的事务工厂是ManagedTransactionFactory，默认是关闭连接的 private boolean closeConnection = true;
2.创建Executor（statement）对象
a.根据配置文件中的ExecutorType决定创建Executor的类型（SIMPLE【SimpleExecutor】, REUSE【ReuseExecutor】, BATCH【BatchExecutor】）
b.如果配置cacheEnabled=true，则创建executor = new CachingExecutor(executor);
3.返回SqlSession，默认返回的是DefaultSqlSession对象（该对象维护Configuration对象、Executor对象、是否自动提交）
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();
    }
二.DefaultSqlSession对象，SqlSession对象的使用
（一）在使用DefaultSqlSession之前要现要求Mapper.xml的加载过程
对mapper节点的解析，XMLMapperBuilder对象进行xml文件解析和MapperBuilderAssistant对象将解析后的数据组装成对象交付给Configuration对象
1.传递mapper.xml文件路径、inputstream、Configuration对象
2.XMLMapperBuilder对象的configurationElement(parser.evalNode("/mapper"));解析mpper.xml文件
3.XMLMapperBuilder对象以buildStatementFromContext(context.evalNodes("select|insert|update|delete"));为例进行分析
4.XMLMapperBuilder对象的buildStatementFromContext(List<XNode> list, String requiredDatabaseId)，主要是XMLStatementBuilder对象在干活
5.final XMLStatementBuilder statementParser = new XMLStatementBuilder(configuration, builderAssistant, context, requiredDatabaseId);
6.XMLStatementBuilder对象开始干活：statementParser.parseStatementNode();最终将所有的配置信息放到MapperBuilderAssistant对象中，然后由MapperBuilderAssistant把真实的MappedStatement放到Configuration对象中
（二）使用SqlSession的重要一步就是首先要根据ID获取MappedStatement
1.SqlSession session = sqlSessionFactory.openSession();
2.BoundBlogMapper mapper = session.getMapper(BoundBlogMapper.class);//真实是从Configuration对象中进行查找的
3.Blog b = mapper.selectSomething(1);
4.session.close();