Mybatis 源码之获取映射器类

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获取 SqlSession 对象

定位: org.apache.ibatis.session.SqlSessionFactory#openSession
作用: 创建 SqlSession 实例

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private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
  Transaction tx = null;
  try {
    // 获取 mybatis-config.xml 配置文件中配置的 Environment 对象
    final Environment environment = configuration.getEnvironment();
    // 获取 TransactionFactory 对象
    final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
    // 创建 Transaction 对象
    tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
    // 根据配置创建 Executor 对象
    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 {
      // 获取当前连接的事务是否为自动提交方法
      autoCommit = connection.getAutoCommit();
    } catch (SQLException e) {
      // Failover to true, as most poor drivers
      // or databases won't support transactions
      // 当前数据库驱动提供的连接不支持事务，则可能会抛出异常
      autoCommit = true;
    }
    // 获取 mybatis-config.xml 配置文件中配置的 Environment 对象
    final Environment environment = configuration.getEnvironment();
    // 获取 TransactionFactory 对象
    final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
    // 创建 Transaction 对象
    final Transaction tx = transactionFactory.newTransaction(connection);
    // 根据配置创建 Executor 对象
    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();
  }
}

创建执行器

定位: org.apache.ibatis.session.Configuration#newExecutor

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// 产生执行器
public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
  executorType = executorType == null ? defaultExecutorType : executorType;
  executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
  Executor executor;
  // 根据参数，选择合适的 Executor 实现
  if (ExecutorType.BATCH == executorType) {
    executor = new BatchExecutor(this, transaction);
  } else if (ExecutorType.REUSE == executorType) {
    executor = new ReuseExecutor(this, transaction);
  } else {
    executor = new SimpleExecutor(this, transaction);
  }
  // 根据配置决定是否开启二级缓存的功能
  if (cacheEnabled) {
    // 装饰器模式
    executor = new CachingExecutor(executor);
  }
  // 此处引入插件, 通过插件可以改变 Executor 行为
  executor = (Executor) interceptorChain.pluginAll(executor);
  return executor;
}

DefaultSqlSession 对象

DefaultSqlSession 实现了 SqlSession 接口的所有方法，包括对数据库的增删改查、事务管理等功能。