java 事务脏读 java怎么处理事务

在上一篇文章中，我们看到了一个典型的事务处理失败的案例，其主要原因在于，service层和各个DAO所使用的Connection是不一样的，而JDBC中事务处理的作用对象正是Connection对象，所以不同DAO中的操作不在同一个事务里面，从而导致事务失败。从中我们得出了教训：要避免这种失败，我们可以使所有操作共享一个Connection对象，这样应该就没有问题了。  

在本篇文章中，我们将看到一个成功的，但是丑陋的事务处理方案，它的基本思路是：在service层创建Connection对象，再将该Connection传给各个DAO类，这样就完成了Connection共享的目的。 

修改两个DAO类，使他们都接受一个Connection对象，定义UglyBankDao类如下：

public class UglyBankDao
{
    public void withdraw(int bankId, int amount, Connection connection) throws SQLException
    {
        PreparedStatement selectStatement = connection.prepareStatement("SELECT BANK_AMOUNT FROM BANK_ACCOUNT WHERE BANK_ID = ?");
        selectStatement.setInt(1, bankId);
        ResultSet resultSet = selectStatement.executeQuery();
        resultSet.next();
        int previousAmount = resultSet.getInt(1);
        resultSet.close();
        selectStatement.close();
         
        int newAmount = previousAmount - amount;
        PreparedStatement updateStatement = connection.prepareStatement("UPDATE BANK_ACCOUNT SET BANK_AMOUNT = ? WHERE BANK_ID = ?");
        updateStatement.setInt(1, newAmount);
        updateStatement.setInt(2, bankId);
        updateStatement.execute();
         
        updateStatement.close();
    }
}

使用同样的方法，定义UglyInsuranceDao类：

public class UglyInsuranceDao
{
    public void deposit(int insuranceId, int amount, Connection connection) throws SQLException
    {
        PreparedStatement selectStatement = connection.prepareStatement("SELECT INSURANCE_AMOUNT FROM INSURANCE_ACCOUNT WHERE INSURANCE_ID = ?");
        selectStatement.setInt(1, insuranceId);
        ResultSet resultSet = selectStatement.executeQuery();
        resultSet.next();
        int previousAmount = resultSet.getInt(1);
        resultSet.close();
        selectStatement.close();
        
        
        int newAmount = previousAmount + amount;
        PreparedStatement updateStatement = connection.prepareStatement("UPDATE INSURANCE_ACCOUNT SET INSURANCE_AMOUNT = ? WHERE INSURANCE_ID = ?");
        updateStatement.setInt(1, newAmount);
        updateStatement.setInt(2, insuranceId);
        updateStatement.execute();
        
        updateStatement.close();
    }
}

然后修改Service类，在UglyBankService类的transfer方法中，首先创建一个Connection对象，然后在将该对象依次传给UglyBankDao的withdraw方法和UglyInsuranceDao类的deposit方法，这样service层和DAO层使用相同的Connection对象。定义UglyBankService类如下：

public class UglyBankService implements BankService
{
    private DataSource dataSource;
    private UglyBankDao uglyBankDao;
    private UglyInsuranceDao uglyInsuranceDao;
      
    public UglyBankService(DataSource dataSource)
    {
        this.dataSource = dataSource;
    }
      
    public void transfer(int fromId, int toId, int amount)
    {
        Connection connection = null;
        try
        {
            connection = dataSource.getConnection();
            connection.setAutoCommit(false);
      
            uglyBankDao.withdraw(fromId, amount, connection);
            uglyInsuranceDao.deposit(toId, amount, connection);
      
            connection.commit();
        } catch (Exception e)
        {
            try
            {
                assert connection != null;
                connection.rollback();
            } catch (SQLException e1)
            {
                e1.printStackTrace();
            }
        } finally
        {
            try
            {
                assert connection != null;
                connection.close();
            } catch (SQLException e)
            {
                e.printStackTrace();
            }
        }
    }
      
    public void setUglyBankDao(UglyBankDao uglyBankDao)
    {
        this.uglyBankDao = uglyBankDao;
    }
      
    public void setUglyInsuranceDao(UglyInsuranceDao uglyInsuranceDao)
    {
        this.uglyInsuranceDao = uglyInsuranceDao;
    }
}

通过上面共享Connection对象的方法虽然可以完成事务处理的目的，但是这样做法是丑陋的，原因在于：为了完成事务处理的目的，我们需要将一个底层的Connection类在service层和DAO层之间进行传递，而DAO层的方法也要接受这个Connection对象，这种做法显然是不好的，这就是典型的API污染。