Mybatis学习系列（五）：SqlSession

前面我们讲完了SqlSessionFactory的初始化，这里我们主要看一下如何通过SqlSessionFactory获取SqlSession对象的，在获取SqlSession的同时，又做了哪些操作

在前面我们了解到最终获取的SqlSessionFactory其实是DefaultSqlSessionFactory对象，我们看一下这个对象的方法：

这里可以看到，重载了获取session的方法，一共有8个方法：

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

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

  @Override
  public SqlSession openSession(ExecutorType execType) {
    return openSessionFromDataSource(execType, null, false);
  }

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

  @Override
  public SqlSession openSession(ExecutorType execType, TransactionIsolationLevel level) {
    return openSessionFromDataSource(execType, level, false);
  }

  @Override
  public SqlSession openSession(ExecutorType execType, boolean autoCommit) {
    return openSessionFromDataSource(execType, null, autoCommit);
  }

  @Override
  public SqlSession openSession(Connection connection) {
    return openSessionFromConnection(configuration.getDefaultExecutorType(), connection);
  }

  @Override
  public SqlSession openSession(ExecutorType execType, Connection connection) {
    return openSessionFromConnection(execType, connection);
  }

我们可以看到最终都是调用了openSessionFromDataSource方法：

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;
      }      
      //获取到我们的环境配置
      final Environment environment = configuration.getEnvironment();
      //通过环境获取到配置的事务工厂
      final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
      //通过事务工厂来生产一个事务
      final Transaction tx = transactionFactory.newTransaction(connection);
      //初始化Executor 执行器
      final Executor executor = configuration.newExecutor(tx, execType);
      //生成DefaultSqlSession对象
      return new DefaultSqlSession(configuration, executor, autoCommit);
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error opening session.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

1.通过环境获取到配置的事务

private TransactionFactory getTransactionFactoryFromEnvironment(Environment environment) {
    //如果没有配置环境，或者是环境中没有配置事务工厂
    //默认生成ManagedTransactionFactory
    if (environment == null || environment.getTransactionFactory() == null) {
      return new ManagedTransactionFactory();
    }
    //返回配置的事务工厂
    return environment.getTransactionFactory();
  }

我们看一下事务工厂TransactionFactory和Transaction的实现关系

2.初始化Executor 执行器

public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
    //获取执行器类型
    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);
    }
    //在执行器上添加插件链路
    executor = (Executor) interceptorChain.pluginAll(executor);
    return executor;
  }

看下Executor的实现

3.生成DefaultSqlSession对象

public DefaultSqlSession(Configuration configuration, Executor executor, boolean autoCommit) {
    this.configuration = configuration;
    this.executor = executor;
    this.dirty = false;
    this.autoCommit = autoCommit;
  }

到这里就生成了SqlSession即DefaultSqlSession对象，并且把configuration和executor设置完成。