在本系列的上一篇文章中,我們講到了使用動態代理的方式完成事務處理,這種方式將service層的所有public方法都加入到事務中,這顯然不是我們需要的,需要代理的只是那些需要操作數據庫的方法。在本篇中,我們將講到如何使用Java註解(Annotation)來標記需要事務處理的方法。
首先定義Transactional註解:
package davenkin.step6_annotation;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
public @interface Transactional
{
}
使用註解標記事務的基本原理爲:依然使用上一篇中講到的動態代理的方式,只是在InvocationHandler的invoke方法中,首先判斷被代理的方法是否標記有Transactional註解,如果沒有則直接調用method.invoke(proxied, objects),否則,先準備事務,在調用method.invoke(proxied, objects),然後根據該方法是否執行成功調用commit或rollback。定義TransactionEnabledAnnotationProxyManager如下:
package davenkin.step6_annotation;
import davenkin.step3_connection_holder.TransactionManager;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
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,我們首先獲得原service的transfer方法,然後根據originalMethod.isAnnotationPresent(Transactional.class)判斷該方法是否標記有Transactional註解,如果沒有,則任何額外功能都不加,直接調用原來service的transfer方法;否則,將其加入到事務處理中。
在service層中,我們只需將需要加入事務處理的方法用Transactional註解標記就行了:
package davenkin.step6_annotation;
import davenkin.BankService;
import davenkin.step3_connection_holder.ConnectionHolderBankDao;
import davenkin.step3_connection_holder.ConnectionHolderInsuranceDao;
import javax.sql.DataSource;
public class AnnotationBankService implements BankService
{
private ConnectionHolderBankDao connectionHolderBankDao;
private ConnectionHolderInsuranceDao connectionHolderInsuranceDao;
public AnnotationBankService(DataSource dataSource)
{
connectionHolderBankDao = new ConnectionHolderBankDao(dataSource);
connectionHolderInsuranceDao = new ConnectionHolderInsuranceDao(dataSource);
}
@Transactional
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();
}
}
}
然後執行測試:
@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));
}
測試運行成功,如果將AnnotationBankService中transfer方法的Transactional註解刪除,那麼以上測試將拋出RuntimeException異常,該異常爲transfer方法中我們人爲拋出的,也即由於此時沒有事務來捕捉異常,程序便直接拋出該異常而終止運行。在下一篇(本系列最後一篇)文章中,我們將講到分佈式事務的一個入門例子。