mybatis源码系列七-----sqlsession的开启

　　前面已经介绍了初始化sqlsessionFactory对象时是如何加载mybatis配置文件的各个节点的。在初始化sqlsessionFactory对象后，我们可以通过该对象创建sqlSession对象，主要是两种创建方式：

　　1）sqlSessionFactory.openSession(boolean autoCommit)

　　2)sqlSessionFactory.openSession();

　　autoCommit的作用主要是mybatis执行sql操作完成后是否自动提交，如果autoCommit为true，则自动提交事务，否则需要手动提交事务。

　　下面来看下sqlSessionFactory对象是如何创建sqlSession对象的。

  sqlSessionFactory.openSession(autoCommit)

  //configuration.getDefaultExecutorType()：如果未在mybatis配置文件settings节点中配置defaultExecutorType的值，则默认未simple
  public SqlSession openSession(boolean autoCommit) {
    return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, autoCommit);
  }
  
  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);                //创建事务
      final Executor executor = configuration.newExecutor(tx, execType);                                    //创建executor对象
      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();
    }
  }

  //mybatis中有两种事务管理器：
  //1）如果是配置的为JDBC，则直接使用jdbc的事务
  //2）如果是配置的MANAGED，则是让容器来管理事务，例如mybatis与spring整合，则让spring容器来管理事务。
  tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);

  final Executor executor = configuration.newExecutor(tx, execType);
    
  //在sqlSession对象中，执行都是通过executor对象来进行的
  //来看下如何创建executor对象的
  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);                    //因为我们这配置文件中未配置defaultExecutorType的值，默认未simple
    }
    if (cacheEnabled) {                            //如果在mybatis配置文件中settings节点中配置了开启缓存cacheEnabled，则executor的后续操作直接从缓存中读取
      executor = new CachingExecutor(executor);
    }
    executor = (Executor) interceptorChain.pluginAll(executor);            //前面配置的plugins节点，后面可以在executor执行操作时进行拦截处理
    return executor;
  }
  
  public Object pluginAll(Object target) {
    for (Interceptor interceptor : interceptors) {
      target = interceptor.plugin(target);
    }
    return target;
  }
  
  //创建defaultSqlSession对象
  public DefaultSqlSession(Configuration configuration, Executor executor, boolean autoCommit) {
    this.configuration = configuration;
    this.executor = executor;
    this.dirty = false;　　　　　　　　　　　　　　　　//注意这个属性，在执行commit()与rollback()方法时，会用到这个属性
    this.autoCommit = autoCommit;
  }