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：
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(800, 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。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。
因此，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。
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