java事务学习笔记（二）--失败案例

来源：互联网发布：印刷体汉字识别数据集编辑：程序博客网时间：2024/05/18 00:53

在本系列的上一篇文章中，我们讲到了Java事务处理的基本问题，并且讲到了Service层和DAO层，在本篇文章中，我们将以BankService为例学习一个事务处理失败的案例。

请通过以下方式下载github源代码：

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

BankService的功能为：某个用户有两个账户，分别为银行账户和保险账户，并且有各自的账户号，BankService的transfer方法从该用户的银行账户向保险账户转帐，两个DAO分别用于对两个账户表的存取操作。

定义一个BankService接口如下：

package davenkin;                                             public interface BankService {    public void transfer(int fromId, int toId, int amount);}

在两个DAO对象中，我们通过传入的同一个DataSource获得Connection，然后通过JDBC提供的API直接对数据库进行操作。

定义操作银行账户表的DAO类如下：

public class FailureBankDao {    private DataSource dataSource;                                               public FailureBankDao(DataSource dataSource) {        this.dataSource = dataSource;    }                                                                                          public void withdraw(int bankId, int amount) throws SQLException {        Connection connection = dataSource.getConnection();        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();        connection.close();                                               }}

FailureBankDao的withdraw方法从银行账户表（BANK_ACCOUNT）中帐号为bankId的用户账户中取出数量为amount的金额。

采用同样的方法，定义保险账户的DAO类如下：

public class FailureInsuranceDao {    private DataSource dataSource;                                           public FailureInsuranceDao(DataSource dataSource){        this.dataSource = dataSource;    }                                           public void deposit(int insuranceId, int amount) throws SQLException {        Connection connection = dataSource.getConnection();        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();        connection.close();    }}

FailureInsuranceDao类的deposit方法向保险账户表（INSURANCE_ACCOUNT）存入amount数量的金额，这样在BankService中，我们可以先调用FailureBankDao的withdraw方法取出一定金额的存款，再调用FailureInsuranceDao的deposit方法将该笔存款存入保险账户表中，一切看似OK，实现BankService接口如下：

public class FailureBankService implements BankService{    private FailureBankDao failureBankDao;    private FailureInsuranceDao failureInsuranceDao;    private DataSource dataSource;                                        public FailureBankService(DataSource dataSource) {        this.dataSource = dataSource;    }                                        public void transfer(int fromId, int toId, int amount) {        Connection connection = null;        try {            connection = dataSource.getConnection();            connection.setAutoCommit(false);                                                failureBankDao.withdraw(fromId, amount);            failureInsuranceDao.deposit(toId, amount);                                                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 setFailureBankDao(FailureBankDao failureBankDao) {        this.failureBankDao = failureBankDao;    }                                        public void setFailureInsuranceDao(FailureInsuranceDao failureInsuranceDao) {        this.failureInsuranceDao = failureInsuranceDao;    }}

在FailureBankService的transfer方法中，我们首先通过DataSource获得Connection，然后调用connection.setAutoCommit(false)已开启手动提交模式，如果一切顺利，则commit，如果出现异常，则rollback。接下来，开始测试我们的BankService吧。

为了准备测试数据，我们定义个BankFixture类，该类负责在每次测试之前准备测试数据，分别向银行账户（1111）和保险账户（2222）中均存入1000元。BankFixture还提供了两个helper方法（getBankAmount和getInsuranceAmount）帮助我们从数据库中取出数据以做数据验证。我们使用HSQL数据库的in-memory模式，这样不用启动数据库server，方便测试。BankFixture类定义如下：

public class BankFixture{                                      protected final DataSource dataSource = DataSourceFactory.createDataSource();                                      @Before    public void setUp() throws SQLException    {        Connection connection = dataSource.getConnection();        Statement statement = connection.createStatement();                                          statement.execute("DROP TABLE BANK_ACCOUNT IF EXISTS");        statement.execute("DROP TABLE INSURANCE_ACCOUNT IF EXISTS");        statement.execute("CREATE TABLE BANK_ACCOUNT (\n" +                "BANK_ID INT,\n" +                "BANK_AMOUNT INT,\n" +                "PRIMARY KEY(BANK_ID)\n" +                ");");                                          statement.execute("CREATE TABLE INSURANCE_ACCOUNT (\n" +                "INSURANCE_ID INT,\n" +                "INSURANCE_AMOUNT INT,\n" +                "PRIMARY KEY(INSURANCE_ID)\n" +                ");");                                          statement.execute("INSERT INTO BANK_ACCOUNT VALUES (1111, 1000);");        statement.execute("INSERT INTO INSURANCE_ACCOUNT VALUES (2222, 1000);");                                          statement.close();        connection.close();    }                                      protected int getBankAmount(int bankId) throws SQLException    {        Connection connection = dataSource.getConnection();        PreparedStatement selectStatement = connection.prepareStatement("SELECT BANK_AMOUNT FROM BANK_ACCOUNT WHERE BANK_ID = ?");        selectStatement.setInt(1, bankId);        ResultSet resultSet = selectStatement.executeQuery();        resultSet.next();        int amount = resultSet.getInt(1);        resultSet.close();        selectStatement.close();        connection.close();        return amount;    }                                      protected int getInsuranceAmount(int insuranceId) throws SQLException    {        Connection connection = dataSource.getConnection();        PreparedStatement selectStatement = connection.prepareStatement("SELECT INSURANCE_AMOUNT FROM INSURANCE_ACCOUNT WHERE INSURANCE_ID = ?");        selectStatement.setInt(1, insuranceId);        ResultSet resultSet = selectStatement.executeQuery();        resultSet.next();        int amount = resultSet.getInt(1);        resultSet.close();        selectStatement.close();        connection.close();        return amount;    }                                  }

编写的Junit测试继承自BankFixture类，测试代码如下：

public class FailureBankServiceTest extends BankFixture{    @Test    public void transferSuccess() throws SQLException    {        FailureBankDao failureBankDao = new FailureBankDao(dataSource);        FailureInsuranceDao failureInsuranceDao = new FailureInsuranceDao(dataSource);                                        FailureBankService bankService = new FailureBankService(dataSource);        bankService.setFailureBankDao(failureBankDao);        bankService.setFailureInsuranceDao(failureInsuranceDao);                                        bankService.transfer(1111, 2222, 200);                                        assertEquals(800, getBankAmount(1111));        assertEquals(1200, getInsuranceAmount(2222));                                    }                                    @Test    public void transferFailure() throws SQLException    {        FailureBankDao failureBankDao = new FailureBankDao(dataSource);        FailureInsuranceDao failureInsuranceDao = new FailureInsuranceDao(dataSource);                                        FailureBankService bankService = new FailureBankService(dataSource);        bankService.setFailureBankDao(failureBankDao);        bankService.setFailureInsuranceDao(failureInsuranceDao);                                        int toNonExistId = 3333;        bankService.transfer(1111, toNonExistId, 200);                                        assertEquals(1000, getInsuranceAmount(2222));        assertEquals(1000, getBankAmount(1111));    }}

运行测试，第一个测试（transferSuccess）成功，第二个测试（transferFailure）失败。

分析错误，原因在于：我们分别从FailureBankService，FailureBankDao和FailureInsuranceDao中调用了三次dataSource.getConnection()，亦即我们创建了三个不同的Connection对象，而Java事务是作用于Connection之上的，所以从在三个地方我们开启了三个不同的事务，而不是同一个事务。

第一个测试之所以成功，是因为在此过程中没有任何异常发生。虽然在FailureBankService中将Connection的提交模式改为了手动提交，但是由于两个DAO使用的是各自的Connection对象，所以两个DAO中的Connection依然为默认的自动提交模式。

在第二个测试中，我们给出一个不存在的保险账户id（toNonExistId），就是为了使程序产生异常，然后在assertion语句中验证两张表均没有任何变化，但是测试在第二个assertion语句处出错。原因在于，发生异常时，银行账户中的金额已经减少，而虽然程序发生了rollback，但是调用的是FailureBankService中Connection的rollback，而不是FailureInsuranceDao中Connection的，对保险账户的操作根本就没有执行，所以保险账户中依然为1000，而银行账户却变为了800。

综上，为了使两个DAO在同一个事务中，我们应该在整个事务处理过程中使用一个Connection对象，在下一篇文章中，我们将讲到通过共享Connection对象的方式达到事务处理的目的。

转载地址：http://www.davenkin.me/post/2013-02-17/40049132188

0 0