ibatis DAO 框架提供了事務(wù)管理模塊。而這個(gè)事務(wù)管理可以應(yīng)用到很多場(chǎng)合，包括JDBC、Hibernate、JTA、SQLMAP等。

下面以最簡(jiǎn)單的JDBC來(lái)分析一下ibatis DAO如何實(shí)現(xiàn)事務(wù)管理。

首先來(lái)看一段代碼：

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();   //第一個(gè)事務(wù)

      daoManager.startTransaction(); //開(kāi)始第二個(gè)事務(wù)

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

      daoManager.commitTransaction();//提交第二個(gè)事務(wù)
    } finally {
      daoManager.endTransaction();
    }
  }
  }

在method()方法里有著兩個(gè)事務(wù)，如果在方法里不顯式的調(diào)用daoManager.startTransaction()，則每個(gè)ibatis DAO的一次方法調(diào)用就是一個(gè)獨(dú)立的事務(wù)。

 ibatis DAO事務(wù)，有兩個(gè)核心接口DaoTransactionManager和DaoTransaction

對(duì)應(yīng)著不同的數(shù)據(jù)庫(kù)持久層實(shí)現(xiàn)，兩個(gè)接口分別對(duì)應(yīng)著不同實(shí)現(xiàn)；

查看ibatis 代碼，可以發(fā)現(xiàn)這些manager實(shí)現(xiàn)事務(wù)，就是調(diào)用事務(wù)源的事務(wù)管理方法。

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以什么依據(jù)進(jìn)行事務(wù)管理呢？DaoTransactionState看看DaoTransactionState的代碼，非常簡(jiǎn)單，四個(gè)常量來(lái)表示事務(wù)處于的不同的狀態(tài)。

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();

那么實(shí)際程序中是如何進(jìn)行事務(wù)管理的呢？在第一段代碼中，我們是這樣取得DAO
orderDao = (OrderDao) daoManager.getDao(OrderDao.class);
實(shí)際daoManager返回的并不是orderDao的具體實(shí)現(xiàn)類，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;
  }

看到這段代碼就非常清楚了，每調(diào)用ibatis DAO的一次方法時(shí)，如果不顯式的調(diào)用daoManager.startTransaction()，就會(huì)成為單獨(dú)的一個(gè)事務(wù)。再看看ibatis為我們提供的摸板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控制多個(gè)DAO的事務(wù)實(shí)際是讓這些DAO共用了一個(gè)DaoTransaction（ThreadLocal），一個(gè)Connection
 
  這里是一個(gè)事務(wù)源的情況，如果多個(gè)事務(wù)源之間要完成全局事務(wù)，還是老老實(shí)實(shí)用分布式事務(wù)管理服務(wù)吧。

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