在Ibatis中我们要执行一个事务，代码是这样的:

?

sqlMapClient.startTransaction ();   

sqlMapClient.add (xxxxx);   
sqlMapClient.update (xxxxx);   

sqlMapClient.commitTransaction ();  

?

1. 调用sqlMapClient.startTransaction时会通知数据库事务开始了，然后依次执行add和update，最后告诉数据库提交事务。

2.调用sqlMapClient.startTransaction时并没有通知数据库，只不过在sqlMapClient这个对象中记录了一个标志，然后下面的add操作与update操作也没有实际上执行语句，只是产生了sql语句并存储在sqlMapClient中。最终调用sqlMapClient.commitTransaction时，会把产生的这些sql语句作为一个事务一次提交执行。

?

google了一下，没发现有人提出这样的问题。只好自己看源码。下面是源码的调用关系。

SqlMapClientImpl.startTransaction() --->

?

  public void startTransaction(int transactionIsolation) throws SQLException {
    getLocalSqlMapSession().startTransaction(transactionIsolation);
  }

?

继续跟踪调用

------>SqlMapSessionImpl.startTransaction(transactionIsolation)

------>SqlMapExecutorDelegate.startTransaction(session, transactionIsolation)

------>TransactionManager.begin(session, transactionIsolation)这个方法是重点，看代码：

?

 public void begin(SessionScope session, int transactionIsolation) throws SQLException, TransactionException {
    Transaction trans = session.getTransaction();
    TransactionState state = session.getTransactionState();
    if (state == TransactionState.STATE_STARTED) {
      throw new TransactionException("TransactionManager could not start a new transaction.  " +
          "A transaction is already started.");
    } else if (state == TransactionState.STATE_USER_PROVIDED) {
      throw new TransactionException("TransactionManager could not start a new transaction.  " +
          "A user provided connection is currently being used by this session.  " +
          "The calling .setUserConnection (null) will clear the user provided transaction.");
    }

    txThrottle.increment();

    try {
      trans = transactionConfig.newTransaction(transactionIsolation);
-------------------------------
      session.setCommitRequired(false);
    } catch (SQLException e) {
      txThrottle.decrement();
      throw e;
    } catch (TransactionException e) {
      txThrottle.decrement();
      throw e;
    }

    session.setTransaction(trans);
----------------------------------------------
    session.setTransactionState(TransactionState.STATE_STARTED);
  }

?

?

transactionConfig.newTransaction(transactionIsolation);中创建了一个JdbcTransaction

看看JdbcTransaction的代码：

?

public class JdbcTransaction implements Transaction {

  private static final Log connectionLog = LogFactory.getLog(Connection.class);

  private DataSource dataSource;
  private Connection connection;
  private IsolationLevel isolationLevel = new IsolationLevel();

  public JdbcTransaction(DataSource ds, int isolationLevel) throws TransactionException {
    // Check Parameters
    dataSource = ds;
    if (dataSource == null) {
      throw new TransactionException("JdbcTransaction initialization failed.  DataSource was null.");
    }
    this.isolationLevel.setIsolationLevel(isolationLevel);
  }

  private void init() throws SQLException, TransactionException {
    // Open JDBC