在本系列的上一篇文章中，我们讲到了使用动态代理的方式完成事务处理，这种方式将service层的所有public方法都加入到事务中，这显然不是我们需要的，需要代理的只是那些需要操作数据库的方法。在本篇中，我们将讲到如何使用Java注解（Annotation）来标记需要事务处理的方法。
这是一个关于Java事务处理的系列文章，请通过以下方式下载github源代码：
Git clone https://github.com/davenkin/java_transaction_workshop.git
首先定义Transactional注解：
[java] view plain copy print ? @Target(ElementType.METHOD) @Retention(RetentionPolicy.RUNTIME) public @interface Transactional { } @Target(ElementType.METHOD) @Retention(RetentionPolicy.RUNTIME) public @interface Transactional { } 使用注解标记事务的基本原理为：依然使用上一篇中讲到的动态代理的方式，只是在InvocationHandler的invoke方法中，首先判断被代理的方法是否标记有Transactional注解，如果没有则直接调用method.invoke(proxied, objects)，否则，先准备事务，在调用method.invoke(proxied, objects)，然后根据该方法是否执行成功调用commit或rollback。定义TransactionEnabledAnnotationProxyManager如下：
[java] view plain copy print ? public class TransactionEnabledAnnotationProxyManager { private TransactionManager transactionManager; public TransactionEnabledAnnotationProxyManager(TransactionManager transactionManager) { this.transactionManager = transactionManager; } public Object proxyFor(Object object) { return Proxy.newProxyInstance(object.getClass().getClassLoader(), object.getClass().getInterfaces(), new AnnotationTransactionInvocationHandler(object, transactionManager)); } } class AnnotationTransactionInvocationHandler implements InvocationHandler { private Object proxied; private TransactionManager transactionManager; AnnotationTransactionInvocationHandler(Object object, TransactionManager transactionManager) { this.proxied = object; this.transactionManager = transactionManager; } public Object invoke(Object proxy, Method method, Object[] objects) throws Throwable { Method originalMethod = proxied.getClass().getMethod(method.getName(), method.getParameterTypes()); if (!originalMethod.isAnnotationPresent(Transactional.class)) { return method.invoke(proxied, objects); } transactionManager.start(); Object result = null; try { result = method.invoke(proxied, objects); transactionManager.commit(); } catch (Exception e) { transactionManager.rollback(); } finally { transactionManager.close(); } return result; } } public class TransactionEnabledAnnotationProxyManager { private TransactionManager transactionManager; public TransactionEnabledAnnotationProxyManager(TransactionManager transactionManager) { this.transactionManager = transactionManager; } public Object proxyFor(Object object) { return Proxy.newProxyInstance(object.getClass().getClassLoader(), object.getClass().getInterfaces(), new AnnotationTransactionInvocationHandler(object, transactionManager)); } } class AnnotationTransactionInvocationHandler implements InvocationHandler { private Object proxied; private TransactionManager transactionManager; AnnotationTransactionInvocationHandler(Object object, TransactionManager transactionManager) { this.proxied = object; this.transactionManager = transactionManager; } public Object invoke(Object proxy, Method method, Object[] objects) throws Throwable { Method originalMethod = proxied.getClass().getMethod(method.getName(), method.getParameterTypes()); if (!originalMethod.isAnnotationPresent(Transactional.class)) { return method.invoke(proxied, objects); } transactionManager.start(); Object result = null; try { result = method.invoke(proxied, objects); transactionManager.commit(); } catch (Exception e) { transactionManager.rollback(); } finally { transactionManager.close(); } return result; } }
可以看到，在AnnotationTransactionInvocationHandler的invoke方法中，我们首先获得原service的transfer方法，然后根据originalMethod.isAnnotationPresent(Transactional.class)判断该方法是否标记有Transactional注解，如果没有，则任何额外功能都不加，直接调用原来service的transfer方法；否则，将其加入到事务处理中。
在service层中，我们只需将需要加入事务处理的方法用Transactional注解标记就行了：
[java] view plain copy print ? public class AnnotationBankService implements BankService { private ConnectionHolderBankDao connectionHolderBankDao; private ConnectionHolderInsuranceDao connectionHolderInsuranceDao; public AnnotationBankService(DataSource dataSource) { connectionHolderBankDao = new ConnectionHolderBankDao(dataSource); connectionHolderInsuranceDao = new ConnectionHolderInsuranceDao(dataSource); } public void transfer(final int fromId, final int toId, final int amount) { try { connectionHolderBankDao.withdraw(fromId, amount); connectionHolderInsuranceDao.deposit(toId, amount); } catch (Exception e) { throw new RuntimeException(); } } } public class AnnotationBankService implements BankService { private ConnectionHolderBankDao connectionHolderBankDao; private ConnectionHolderInsuranceDao connectionHolderInsuranceDao; public AnnotationBankService(DataSource dataSource) { connectionHolderBankDao = new ConnectionHolderBankDao(dataSource); connectionHolderInsuranceDao = new ConnectionHolderInsuranceDao(dataSource); } public void transfer(final int fromId, final int toId, final int amount) { try { connectionHolderBankDao.withdraw(fromId, amount); connectionHolderInsuranceDao.deposit(toId, amount); } catch (Exception e) { throw new RuntimeException(); } } } 然后执行测试：
[java] view plain copy print ? @Test public void transferFailure() throws SQLException { TransactionEnabledAnnotationProxyManager transactionEnabledAnnotationProxyManager = new TransactionEnabledAnnotationProxyManager(new TransactionManager(dataSource)); BankService bankService = new AnnotationBankService(dataSource); BankService proxyBankService = (BankService) transactionEnabledAnnotationProxyManager.proxyFor(bankService); int toNonExistId = 3333; proxyBankService.transfer(1111, toNonExistId, 200); assertEquals(1000, getBankAmount(1111)); assertEquals(1000, getInsuranceAmount(2222)); } @Test public void transferFailure() throws SQLException { TransactionEnabledAnnotationProxyManager transactionEnabledAnnotationProxyManager = new TransactionEnabledAnnotationProxyManager(new TransactionManager(dataSource)); BankService bankService = new AnnotationBankService(dataSource); BankService proxyBankService = (BankService) transactionEnabledAnnotationProxyManager.proxyFor(bankService); int toNonExistId = 3333; proxyBankService.transfer(1111, toNonExistId, 200); assertEquals(1000, getBankAmount(1111)); assertEquals(1000, getInsuranceAmount(2222)); } 测试运行成功，如果将AnnotationBankService中transfer方法的Transactional注解删除，那么以上测试将抛出RuntimeException异常，该异常为transfer方法中我们人为抛出的，也即由于此时没有事务来捕捉异常，程序便直接抛出该异常而终止运行。在下一篇（本系列最后一篇）文章中，我们将讲到分布式事务的一个入门例子。
