ibatis DAO 事务

ibatis DAO 框架提供了事务管理模块。而这个事务管理可以应用到很多场合，包括JDBC、Hibernate、JTA、SQLMAP等。
下面以最简单的JDBC来分析一下其如何实现事务管理。
首先来看一段代码：

public class OrderService {

  private DaoManager daoManager;

  private OrderDao orderDao;

  public OrderService() {
    daoManager = DaoConfig.getDaoManager();
    orderDao = (OrderDao) daoManager.getDao(OrderDao.class);
  }

  public void method() {
    try {
      //  a separate transaction
      orderDao.method1();   //第一个事务

      daoManager.startTransaction(); //开始第二个事务

      orderDao.method1();
      orderDao.method2();

      daoManager.commitTransaction();//提交第二个事务
    } finally {
      daoManager.endTransaction();
    }
  }
  }

在method()方法里有着两个事务，如果在方法里不显式的调用daoManager.startTransaction()，则每个DAO的一次方法调用就是一个独立的事务。
  ibatis DAO事务，有两个核心接口DaoTransactionManager和DaoTransaction
  对应着不同的数据库持久层实现，两个接口分别对应着不同实现
  查看ibatis 代码，可以发现这些manager实现事务，就是调用事务源的事务操作方法

  JdbcDaoTransactionManager

public void commitTransaction(DaoTransaction trans) {
    ((JdbcDaoTransaction) trans).commit();
  }

 

JdbcDaoTransaction

public JdbcDaoTransaction(DataSource dataSource) {
    try {
      connection = dataSource.getConnection();
      if (connection == null) {
        throw new DaoException("Could not start transaction.  Cause: The DataSource returned a null connection.");
      }
      if (connection.getAutoCommit()) {
        connection.setAutoCommit(false);
      }
      if (connectionLog.isDebugEnabled()) {
        connection = ConnectionLogProxy.newInstance(connection);
      }
    } catch (SQLException e) {
      throw new DaoException("Error starting JDBC transaction.  Cause: " + e);
    }
  }
  
  public void commit() {
    try {
      try {
        connection.commit();
      } finally {
        connection.close();
      }
    } catch (SQLException e) {
      throw new DaoException("Error committing JDBC transaction.  Cause: " + e);
    }
  }

 

  那么DaoTransactionManager以什么依据处理事务呢？DaoTransactionState看看DaoTransactionState的代码，非常简单，四个常量来表示事务处于的不同的状态

  public static final DaoTransactionState ACTIVE = new DaoTransactionState();
  public static final DaoTransactionState INACTIVE = new DaoTransactionState();
  public static final DaoTransactionState COMMITTED = new DaoTransactionState();
  public static final DaoTransactionState ROLLEDBACK = new DaoTransactionState(); 

 

那么实际程序中是如何控制事务的呢
  在第一段代码中，我们是这样取得DAO
  orderDao = (OrderDao) daoManager.getDao(OrderDao.class);
  实际daoManager返回的并不是orderDao的具体实现类，它返回的DaoProxy

DaoProxy

public Object invoke(Object proxy, Method method, Object[] args)
      throws Throwable {
    Object result = null;
    if (PASSTHROUGH_METHODS.contains(method.getName())) {
      try {
        result = method.invoke(daoImpl.getDaoInstance(), args);
      } catch (Throwable t) {
        throw ClassInfo.unwrapThrowable(t);
      }
    } else {
      StandardDaoManager daoManager = daoImpl.getDaoManager();
      DaoContext context = daoImpl.getDaoContext();

      if (daoManager.isExplicitTransaction()) {
        // Just start the transaction (explicit)
        try {
          context.startTransaction();
          result = method.invoke(daoImpl.getDaoInstance(), args);
        } catch (Throwable t) {
          throw ClassInfo.unwrapThrowable(t);
        }
      } else {
        // Start, commit and end the transaction (autocommit)
        try {
          context.startTransaction();
          result = method.invoke(daoImpl.getDaoInstance(), args);
          context.commitTransaction();
        } catch (Throwable t) {
          throw ClassInfo.unwrapThrowable(t);
        } finally {
          context.endTransaction();
        }
      }

    }
    return result;
  }

 

  看到这段代码就非常清楚了，每调用DAO的一次方法时，如果不显式的调用daoManager.startTransaction()，就会成为单独的一个事务。再看看ibatis为我们提供的摸板JdbcDaoTemplate

protected Connection getConnection() {
    DaoTransaction trans = daoManager.getTransaction(this);
    if (!(trans instanceof ConnectionDaoTransaction)) {
      throw new DaoException("The DAO manager of type " + daoManager.getClass().getName() +
          " cannot supply a JDBC Connection for this template, and is therefore not" +
          "supported by JdbcDaoTemplate.");
    }
    return ((ConnectionDaoTransaction) trans).getConnection();
  }

 

  ibatis控制多个DAO的事务实际是让这些DAO共用了一个DaoTransaction（ThreadLocal），一个Connection
 
  这里是一个事务源的情况，如果多个事务源之间要完成全局事务，还是老老实实用分布式事务管理服务吧（jta）