mybatis之获取SqlSession对象

详细步骤

在openSession()处加上断点并执行—>DefaultSqlSessionFactory类的openSession()方法

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

进入openSessionFromDataSource，里面包含数据源和事务以及创建了四大对象的executor

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();
  }
}

executor 非常重要，进入configuration.newExecutor(tx, execType);看executor 创建过程，在这里会根据defaultExecutorType 类型创建不同的executor 【默认是simple，详细见mybatis官方文档】，如果开启了二级缓存，
会创建CachingExecutor。
最后执行interceptorChain.pluginAll(executor);使用每个拦截器对executor 进行包装并返回【与以后写插件息息相关】。

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;
}

最后加到DefaultSqlSessionFactory类创建new DefaultSqlSession(configuration, executor, autoCommit);并返回。