mybatis 源码分析之openSession

private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
    Connection connection = null;
    try {
      final Environment environment = configuration.getEnvironment();
      final DataSource dataSource = getDataSourceFromEnvironment(environment);
      TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
      connection = dataSource.getConnection();
      if (level != null) {
        connection.setTransactionIsolation(level.getLevel());
      }
      //设置日志代理如果需要的话
      connection = wrapConnection(connection);
      Transaction tx = transactionFactory.newTransaction(connection, autoCommit);
      Executor executor = configuration.newExecutor(tx, execType);
      return new DefaultSqlSession(configuration, executor, autoCommit);
    } catch (Exception e) {
      closeConnection(connection);
      throw ExceptionFactory.wrapException("Error opening session.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

可以看出，创建sqlsession经过了以下几个主要步骤：

1)       从配置中获取Environment；

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。

public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
    executorType = executorType == null ? defaultExecutorType : executorType;
    executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
    Executor executor;
    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);
    }
    if (cacheEnabled) {
      executor = new CachingExecutor(executor);
    }
    executor = (Executor) interceptorChain.pluginAll(executor);
    return executor;
  }

 可以看出，如果不开启cache的话，创建的Executor只是3中基础类型之一，BatchExecutor专门用于执行批量sql操作，ReuseExecutor会重用statement执行sql操作，SimpleExecutor只是简单执行sql没有什么特别的。开启cache的话（默认是开启的并且没有任何理由去关闭它），就会创建CachingExecutor，它以前面创建的Executor作为唯一参数。CachingExecutor在查询数据库前先查找缓存，若没找到的话调用delegate（就是构造时传入的Executor对象）从数据库查询，并将查询结果存入缓存中。

Executor对象是可以被插件拦截的，如果定义了针对Executor类型的插件，最终生成的Executor对象是被各个插件插入后的代理对象