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 ();看到这段代码你会不会有一个问题,sqlMapClient里面到底是怎么做的? 我能猜到有两种做法:
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); }这里的 getLocalSqlMapSession()返回了一个SqlMapSession对象,这个对象非常重要,它管理了一次事务的状态,为什么重要后面会讲到。
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); }注意用横线标出的那两句,首先创建了一个Transcaction,然后把它放入了session,为什么放入session,很重要,继续往下看。
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 Transaction connection = dataSource.getConnection(); if (connection == null) { throw new TransactionException("JdbcTransaction could not start transaction. Cause: The DataSource returned a null connection."); } // Isolation Level isolationLevel.applyIsolationLevel(connection); // AutoCommit if (connection.getAutoCommit()) { connection.setAutoCommit(false); } // Debug if (connectionLog.isDebugEnabled()) { connection = ConnectionLogProxy.newInstance(connection); } } public void commit() throws SQLException, TransactionException { if (connection != null) { connection.commit(); } } public void rollback() throws SQLException, TransactionException { if (connection != null) { connection.rollback(); } } public void close() throws SQLException, TransactionException { if (connection != null) { try { isolationLevel.restoreIsolationLevel(connection); } finally { connection.close(); connection = null; } } } public Connection getConnection() throws SQLException, TransactionException { if (connection == null) { init(); } return connection; } }它在Init方法中调用了 connection.setAutoCommit(false); 来开始一个事务。 但是Init方法并没有在构造方法中调用啊。那它是什么时候被调用的呢? 仔细看一下,是在 getConnection() 方法中调用。那 getConnection() 又是什么时候被调用呢。回头想一下,前面我们把这个创建出来的transcation对象放入了session,在事务中第一次操作数据库的时候,比如说add操作时,就会从session中把这个transcation对象拿出来,然后调用 getConnection()方法 。这时就会通知数据库开始一个事务。 到现在为止,我们的问题应该清楚了。我刚开始提出来的两个猜想都不完全正确。不过第一个猜想更接近一点。 正确的答案是: 调用sqlMapClient.startTransaction时并没有通知数据库开始事务。但也不是最后commit时一次提交。 而是在 调用sqlMapClient.startTransaction后,第一次执行数据库操作时通知数据库开始事务,在我们的例子中,就是 sqlMapClient.add (xxxxx)的这个时机。