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Java事务处理全解析（二）——失败的案例
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。
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：
package davenkin.step1_failure;import javax.sql.DataSource;import java.sql.Connection;import java.sql.PreparedStatement;import java.sql.ResultSet;import java.sql.SQLException;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：
package davenkin.step1_failure;import javax.sql.DataSource;import java.sql.Connection;import java.sql.PreparedStatement;import java.sql.ResultSet;import java.sql.SQLException;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：
package davenkin.step1_failure;import davenkin.BankService;import javax.sql.DataSource;import java.sql.Connection;import java.sql.SQLException;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：
package davenkin;import org.junit.Before;import javax.sql.DataSource;import java.sql.*;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：
package davenkin.step1_failure;import davenkin.BankFixture;import org.junit.Test;import java.sql.SQLException;import static junit.framework.Assert.assertEquals;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。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。
Java事务处理全解析（二）——失败的案例