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資料庫的方法。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。

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原始碼：

首先定義Transactional註解：

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
public @interface Transactional
{
}
 

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，直接呼叫原來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：

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();
        }
    }
}
 

@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));
    }