源码角度，分析@Transactional实现原理

@Override
public TransactionAttribute parseTransactionAnnotation(AnnotatedElement ae) {
    //这里就是分析Method是否被@Transactional注解标注，有的话，不用说BeanFactoryTransactionAttributeSourceAdvisor适配当前bean，进行代理，并且注入切点
    //BeanFactoryTransactionAttributeSourceAdvisor
   AnnotationAttributes attributes = AnnotatedElementUtils.getMergedAnnotationAttributes(ae, Transactional.class);
   if (attributes != null) {
      return parseTransactionAnnotation(attributes);
   }
   else {
      return null;
   }
}

@Override
public Object intercept(Object proxy, Method method, Object[] args, MethodProxy methodProxy) throws Throwable {
   Object oldProxy = null;
   boolean setProxyContext = false;
   Class<?> targetClass = null;
   Object target = null;
   try {
      if (this.advised.exposeProxy) {
         // Make invocation available if necessary.
         oldProxy = AopContext.setCurrentProxy(proxy);
         setProxyContext = true;
      }
      // May be null. Get as late as possible to minimize the time we
      // "own" the target, in case it comes from a pool...
      target = getTarget();
      if (target != null) {
         targetClass = target.getClass();
      }
       //follow
      List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);
      Object retVal;
      // Check whether we only have one InvokerInterceptor: that is,
      // no real advice, but just reflective invocation of the target.
      if (chain.isEmpty() && Modifier.isPublic(method.getModifiers())) {
         // We can skip creating a MethodInvocation: just invoke the target directly.
         // Note that the final invoker must be an InvokerInterceptor, so we know
         // it does nothing but a reflective operation on the target, and no hot
         // swapping or fancy proxying.
         Object[] argsToUse = AopProxyUtils.adaptArgumentsIfNecessary(method, args);
         retVal = methodProxy.invoke(target, argsToUse);
      }
      else {
         // We need to create a method invocation...
         retVal = new CglibMethodInvocation(proxy, target, method, args, targetClass, chain, methodProxy).proceed();
      }
      retVal = processReturnType(proxy, target, method, retVal);
      return retVal;
   }
   finally {
      if (target != null) {
         releaseTarget(target);
      }
      if (setProxyContext) {
         // Restore old proxy.
         AopContext.setCurrentProxy(oldProxy);
      }
   }
}

@Override
public Object invoke(final MethodInvocation invocation) throws Throwable {
 // Work out the target class: may be {@code null}.
 // The TransactionAttributeSource should be passed the target class
 // as well as the method, which may be from an interface.
 Class<?> targetClass = (invocation.getThis() != null ? AopUtils.getTargetClass(invocation.getThis()) : null);
 // Adapt to TransactionAspectSupport's invokeWithinTransaction...
 return invokeWithinTransaction(invocation.getMethod(), targetClass, new InvocationCallback() {
  @Override
  public Object proceedWithInvocation() throws Throwable {
   return invocation.proceed();
  }
 });
}

protected Object invokeWithinTransaction(Method method, Class<?> targetClass, final InvocationCallback invocation)
      throws Throwable {
   // If the transaction attribute is null, the method is non-transactional.
   final TransactionAttribute txAttr = getTransactionAttributeSource().getTransactionAttribute(method, targetClass);
   final PlatformTransactionManager tm = determineTransactionManager(txAttr);
   final String joinpointIdentification = methodIdentification(method, targetClass);
   if (txAttr == null || !(tm instanceof CallbackPreferringPlatformTransactionManager)) {
      // Standard transaction demarcation with getTransaction and commit/rollback calls.
       //开启事务
      TransactionInfo txInfo = createTransactionIfNecessary(tm, txAttr, joinpointIdentification);
      Object retVal = null;
      try {
         // This is an around advice: Invoke the next interceptor in the chain.
         // This will normally result in a target object being invoked.
          //方法调用
         retVal = invocation.proceedWithInvocation();
      }
      catch (Throwable ex) {
         // target invocation exception
     //回滚事务
         completeTransactionAfterThrowing(txInfo, ex);
         throw ex;
      }
      finally {
         cleanupTransactionInfo(txInfo);
      }
       //提交事务
      commitTransactionAfterReturning(txInfo);
      return retVal;
   }
   else {
      // It's a CallbackPreferringPlatformTransactionManager: pass a TransactionCallback in.
      try {
         Object result = ((CallbackPreferringPlatformTransactionManager) tm).execute(txAttr,
               new TransactionCallback<Object>() {
                  @Override
                  public Object doInTransaction(TransactionStatus status) {
                     TransactionInfo txInfo = prepareTransactionInfo(tm, txAttr, joinpointIdentification, status);
                     try {
                        return invocation.proceedWithInvocation();
                     }
                     catch (Throwable ex) {
                        if (txAttr.rollbackOn(ex)) {
                           // A RuntimeException: will lead to a rollback.
                           if (ex instanceof RuntimeException) {
                              throw (RuntimeException) ex;
                           }
                           else {
                              throw new ThrowableHolderException(ex);
                           }
                        }
                        else {
                           // A normal return value: will lead to a commit.
                           return new ThrowableHolder(ex);
                        }
                     }
                     finally {
                        cleanupTransactionInfo(txInfo);
                     }
                  }
               });
         // Check result: It might indicate a Throwable to rethrow.
         if (result instanceof ThrowableHolder) {
            throw ((ThrowableHolder) result).getThrowable();
         }
         else {
            return result;
         }
      }
      catch (ThrowableHolderException ex) {
         throw ex.getCause();
      }
   }
}