在本系列的上一篇文章中,我們講到了使用Template模式進行事務管理,這固然是一種很好的方法,但是不那麼完美的地方在於我們依然需要在service層中編寫和事務處理相關的代碼,即我們需要在service層中聲明一個TransactionTemplate。在本篇文章中,我們將使用Java提供的動態代理(Dynamic Proxy)功能來完成事務處理,你將看到無論是在service層還是DAO層都不會有事務處理代碼,即他們根本就意識不到事務處理的存在。使用動態代理完成事務處理也是AOP的一種典型應用。
這是一個關於Java事務處理的系列文章,請通過以下方式下載github源代碼:
Git clone https://github.com/davenkin/java_transaction_workshop.git
Java動態代理的基本原理爲:被代理對象需要實現某個接口(這是前提),代理對象會攔截對被代理對象的方法調用,在其中可以全然拋棄被代理對象的方法實現而完成另外的功能,也可以在被代理對象方法調用的前後增加一些額外的功能。在本篇文章中,我們將攔截service層的transfer方法,在其調用之前加入事務準備工作,然後調用原來的transfer方法,之後根據transfer方法是否執行成功決定commit還是rollback。
首先定義一個TransactionEnabledProxyManager類:
- public class TransactionEnabledProxyManager
- {
- private TransactionManager transactionManager;
- public TransactionEnabledProxyManager(TransactionManager transactionManager)
- {
- this.transactionManager = transactionManager;
- }
- public Object proxyFor(Object object)
- {
- return Proxy.newProxyInstance(object.getClass().getClassLoader(), object.getClass().getInterfaces(), new TransactionInvocationHandler(object, transactionManager));
- }
- }
- class TransactionInvocationHandler implements InvocationHandler
- {
- private Object proxy;
- private TransactionManager transactionManager;
- TransactionInvocationHandler(Object object, TransactionManager transactionManager)
- {
- this.proxy = object;
- this.transactionManager = transactionManager;
- }
- public Object invoke(Object o, Method method, Object[] objects) throws Throwable
- {
- transactionManager.start();
- Object result = null;
- try
- {
- result = method.invoke(proxy, objects);
- transactionManager.commit();
- } catch (Exception e)
- {
- transactionManager.rollback();
- } finally
- {
- transactionManager.close();
- }
- return result;
- }
- }
public class TransactionEnabledProxyManager
{
private TransactionManager transactionManager;
public TransactionEnabledProxyManager(TransactionManager transactionManager)
{
this.transactionManager = transactionManager;
}
public Object proxyFor(Object object)
{
return Proxy.newProxyInstance(object.getClass().getClassLoader(), object.getClass().getInterfaces(), new TransactionInvocationHandler(object, transactionManager));
}
}
class TransactionInvocationHandler implements InvocationHandler
{
private Object proxy;
private TransactionManager transactionManager;
TransactionInvocationHandler(Object object, TransactionManager transactionManager)
{
this.proxy = object;
this.transactionManager = transactionManager;
}
public Object invoke(Object o, Method method, Object[] objects) throws Throwable
{
transactionManager.start();
Object result = null;
try
{
result = method.invoke(proxy, objects);
transactionManager.commit();
} catch (Exception e)
{
transactionManager.rollback();
} finally
{
transactionManager.close();
}
return result;
}
}
通過調用該類的proxyFor方法,傳入需要被代理的對象(本例中爲service對象),返回一個代理對象。此後,在調用代理對象的transfer方法時,會自動調用TransactionIvocationHandler的invoke方法,在該方法中,我們首先開始事務,然後執行:
result = method.invoke(proxy, objects);
上面一行代碼執行的是原service層的transfer方法,如果方法執行成功則commit,否則rollback事務。
由於與事務處理相關的代碼都被轉移到了代理對象中,在service層中我們只需調用DAO即可:
- public class BareBankService implements BankService
- {
- private ConnectionHolderBankDao connectionHolderBankDao;
- private ConnectionHolderInsuranceDao connectionHolderInsuranceDao;
- public BareBankService(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 BareBankService implements BankService
{
private ConnectionHolderBankDao connectionHolderBankDao;
private ConnectionHolderInsuranceDao connectionHolderInsuranceDao;
public BareBankService(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();
}
}
}
如何,上面的BareBankService中沒有任何事務處理的影子,我們只需關注核心業務邏輯即可。
然後在客戶代碼中,我們需要先創建代理對象(這在spring中通常是通過配置實現的):
- @Test
- public void transferFailure() throws SQLException
- {
- TransactionEnabledProxyManager transactionEnabledProxyManager = new TransactionEnabledProxyManager(new TransactionManager(dataSource));
- BankService bankService = new BareBankService(dataSource);
- BankService proxyBankService = (BankService) transactionEnabledProxyManager.proxyFor(bankService);
- int toNonExistId = 3333;
- proxyBankService.transfer(1111, toNonExistId, 200);
- assertEquals(1000, getBankAmount(1111));
- assertEquals(1000, getInsuranceAmount(2222));
- }
@Test
public void transferFailure() throws SQLException
{
TransactionEnabledProxyManager transactionEnabledProxyManager = new TransactionEnabledProxyManager(new TransactionManager(dataSource));
BankService bankService = new BareBankService(dataSource);
BankService proxyBankService = (BankService) transactionEnabledProxyManager.proxyFor(bankService);
int toNonExistId = 3333;
proxyBankService.transfer(1111, toNonExistId, 200);
assertEquals(1000, getBankAmount(1111));
assertEquals(1000, getInsuranceAmount(2222));
}
在上面的測試代碼中,我們首先創建一個BareBankService對象,然後調用transactionEnabledProxyManager的proxyFor方法生成對原BareBankService對象的代理對象,最後在代理對象上調用transfer方法,測試運行成功。
可以看到,通過以上動態代理實現,BareBankService中的所有public方法都被代理了,即他們都被加入到事務中。這對於service層中的所有方法都需要和數據庫打交道的情況是可以的,本例即如此(有且只有一個transfer方法),然而對於service層中不需要和數據庫打交道的public方法,這樣做雖然也不會出錯,但是卻顯得多餘。在下一篇文章中,我們將講到使用Java註解(annotation)的方式來聲明一個方法是否需要事務,就像Spring中的Transactional註解一樣。
轉載地址:http://www.davenkin.me/post/2013-02-24/40049235086