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：
[java] view plain copy print ? 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(); } } 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：
[java] view plain copy print ? 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(); } } 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：
[java] view plain copy print ? 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; } } 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。
转载地址：http://www.davenkin.me/post/2013-02-22/40049367747
