【Mybatis源码分析】04-SqlSession的创建过程

SqlSession是我们操作数据库的主要客户端API，是由SqlSessionFactory的openSession方法所创建。openSession有多个重载方法，我们看一下:

    public interface SqlSessionFactory {
      SqlSession openSession();
      SqlSession openSession(boolean autoCommit);
      SqlSession openSession(Connection connection);
      SqlSession openSession(TransactionIsolationLevel level);
      SqlSession openSession(ExecutorType execType);
      SqlSession openSession(ExecutorType execType, boolean autoCommit);
      SqlSession openSession(ExecutorType execType, TransactionIsolationLevel level);
      SqlSession openSession(ExecutorType execType, Connection connection);
      Configuration getConfiguration();
    }

看DefaultSqlSessionFactory的实现上面所有的openSession方法都是最终调用下面这个重载方法:

    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();
      }
    }

1.首先从configuration获取Environment对象，里面主要包含了DataSource和TransactionFactory对象

2.创建TransactionFactory创建Transaction

    public Transaction newTransaction(DataSource ds, TransactionIsolationLevel level, boolean autoCommit) {
      return new JdbcTransaction(ds, level, autoCommit);
    }

    protected Connection connection;
    protected DataSource dataSource;
    protected TransactionIsolationLevel level;
    // MEMO: We are aware of the typo. See #941
    protected boolean autoCommmit;
    public JdbcTransaction(DataSource ds, TransactionIsolationLevel desiredLevel, boolean desiredAutoCommit) {
      dataSource = ds;
      level = desiredLevel;
      autoCommmit = desiredAutoCommit;
    }

JdbcTransaction主要维护了一个默认autoCommit为false的Connection对象，对事物的提交，回滚，关闭等都是接见通过Connection完成的。

    protected void openConnection() throws SQLException {
      if (log.isDebugEnabled()) {
        log.debug("Opening JDBC Connection");
      }
      connection = dataSource.getConnection();
      if (level != null) {
        connection.setTransactionIsolation(level.getLevel());
      }
      setDesiredAutoCommit(autoCommmit);//默认为false
    }

    public void rollback() throws SQLException {
      if (connection != null && !connection.getAutoCommit()) {
        if (log.isDebugEnabled()) {
          log.debug("Rolling back JDBC Connection [" + connection + "]");
        }
        connection.rollback();
      }
    }
    public void close() throws SQLException {
      if (connection != null) {
        resetAutoCommit();
        if (log.isDebugEnabled()) {
          log.debug("Closing JDBC Connection [" + connection + "]");
        }
        connection.close();
      }
    }

    public void commit() throws SQLException {
      if (connection != null && !connection.getAutoCommit()) {
        if (log.isDebugEnabled()) {
          log.debug("Committing JDBC Connection [" + connection + "]");
        }
        connection.commit();
      }
    }

3.从configuration获取Executor。executor包含了刚刚创建的Transaction，所以Transaction关联了Connection和Executor，如果为指定executorType则使用默认的SimpleExecutor，如果开启了二级缓存(默认开启)，则CachingExecutor会包装SimpleExecutor，然后依次调用拦截器的plugin方法返回一个被代理过的Executor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;
    }

4.构造DefaultSqlSession对象

    new DefaultSqlSession(configuration, executor, autoCommit)

5.如果上述一步出现异常，则会调用Transaction的close方法，间接调用DataSource(PooledDataSource)的方法回收或释放连接。

    private void closeTransaction(Transaction tx) {
      if (tx != null) {
        try {
          tx.close();
        } catch (SQLException ignore) {
          // Intentionally ignore. Prefer previous error.
        }
      }
    }

以上创建DefaultSqlSession的过程主要就是为了得到Configuration和一个Executor。下篇文章分析一下DefaultSqlSession的具体实现。