Java事务处理全解析(三)——丑陋的案例

在本系列的上一篇文章中,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。

请通过以下方式下载本系列文章的github源代码:

git clone https://github.com/davenkin/java_transaction_workshop.git

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。

在下一篇博文中,我们将讲到如何在不传递Connection对象的情况下完成和本文相同的事务处理功能。

你可能感兴趣的:(Java事务处理全解析(三)——丑陋的案例)