Spring AOP实现声明式事务代码分析

时间:2023-02-08 22:59:33

众所周知,Spring的声明式事务是利用AOP手段实现的,所谓“深入一点,你会更快乐”,本文试图给出相关代码分析。

AOP联盟为增强定义了org.aopalliance.aop.Advice接口,Spring由Advice接口扩展了5中类型的增强(接口),AOP联盟自身提供了IntroductionInterceptor->MethodInterceptor->Interceptor->Advice,而MethodInterceptor就代表环绕增强,表示在目标方法执行前后实施增强。要进行事务操作,正是要在目标方法前后加入相应的代码,因此,Spring为我们提供了TransactionInterceptor类。

TransactionInterceptor的invoke方法调用了父类TransactionAspectSupport的invokeWithinTransactionf方法,

if (txAttr == null || !(tm instanceof CallbackPreferringPlatformTransactionManager)) {
// Standard transaction demarcation with getTransaction and commit/rollback calls.
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.
retVal = invocation.proceedWithInvocation();
}
catch (Throwable ex) {
// target invocation exception
completeTransactionAfterThrowing(txInfo, ex);
throw ex;
}
finally {
cleanupTransactionInfo(txInfo);
}
commitTransactionAfterReturning(txInfo);
return retVal;
}

瞬间,我们看到了我们期望看到的代码,其中completeTransactionAfterThrowing里面做的是rollback的相关操作。

Spring 提供了多种不同的方案实现对 bean 的 aop proxy, 包括 ProxyFactoryBean, 便利的 TransactionProxyFactoryBean 以及 AutoProxyCreator 等,

这里重点说一下最常用的 ProxyFactoryBean, TransactionProxyFactoryBean, BeanNameAutoProxyCreator, DefaultAdvisorAutoProxyCreator 的联系和区别

1. ProxyFactoryBean : 使用率最高的 proxy 方式, 它通过配置 interceptorNames 属性决定加入哪些 advisor (method interceptor 将会被自动包装成 advisor),

注意是 "interceptorNames" 而不是 "interceptors",

原因是 ProxyFactoryBean 可能返回非 singleton 的 proxy 实例, 而 advisior 可能也是非 singleton 的,

因此不能通过 interceptor reference 来注入

2. TransactionProxyFactoryBean : 特定用于 transaction proxy, 注意其 super class 是 AbstractSingletonProxyFactoryBean, 也就是说,

TransactionProxyFactoryBean 永远无法返回非 singleton 的 proxy 实例 !

如果你需要非 singleton 的 proxy 实例, 请考虑使用 ProxyFactoryBean.

3. BeanNameAutoProxyCreator : 故名思义, 根据 bean name 进行 auto proxy, bean name 的 match 规则参见 org.springframework.util.PatternMatchUtils

4. DefaultAdvisorAutoProxyCreator : 更强大的 auto proxy creator, 强大之处在于它会 cahce 容器中所有注册的 advisor, 然后搜索容器中所有的 bean ,

如果某个 bean 满足 advisor 中的 Pointcut, 那么将会被自动代理, 与 BeanNameAutoProxyCreator 相比, 省去了配置 beanNames 的工作,

5. AnnotationAwareAspectJAutoProxyCreator -> @Aspect  <aop:aspectj-autoproxy/>

-> @Transactinal <tx:annotation-driven transaction-manager="txManager"/>

AbstractAutoProxyCreator实现了BeanPostProcessor,Spring默认会自动创建代理。

// AbstractAutowireCapableBeanFactory
public Object applyBeanPostProcessorsBeforeInitialization(Object existingBean, String beanName)
throws BeansException { Object result = existingBean;
for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {
result = beanProcessor.postProcessBeforeInitialization(result, beanName);
if (result == null) {
return result;
}
}
return result;
} public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
throws BeansException { Object result = existingBean;
for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {
result = beanProcessor.postProcessAfterInitialization(result, beanName);
if (result == null) {
return result;
}
}
return result;
}

 
Spring AOP实现声明式事务代码分析
 

我们来看下AbstractAutoProxyCreator里的重点代码

// AbstractAutoProxyCreator
public Object postProcessBeforeInitialization(Object bean, String beanName) {
return bean;
} /**
* Create a proxy with the configured interceptors if the bean is
* identified as one to proxy by the subclass.
* @see #getAdvicesAndAdvisorsForBean
*/
public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
if (bean != null) {
Object cacheKey = getCacheKey(bean.getClass(), beanName);
if (!this.earlyProxyReferences.contains(cacheKey)) {
return wrapIfNecessary(bean, beanName, cacheKey);
}
}
return bean;
}
/**
* Wrap the given bean if necessary, i.e. if it is eligible for being proxied.
* @param bean the raw bean instance
* @param beanName the name of the bean
* @param cacheKey the cache key for metadata access
* @return a proxy wrapping the bean, or the raw bean instance as-is
*/
protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
if (this.targetSourcedBeans.contains(beanName)) {
return bean;
}
if (this.nonAdvisedBeans.contains(cacheKey)) {
return bean;
}
if (isInfrastructureClass(bean.getClass()) || shouldSkip(bean.getClass(), beanName)) {
this.nonAdvisedBeans.add(cacheKey);
return bean;
} // Create proxy if we have advice.
Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
if (specificInterceptors != DO_NOT_PROXY) {
// 有AnnotationAwareAspectJAutoProxyCreator 这个processor时
this.advisedBeans.add(cacheKey);
Object proxy = createProxy(bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));
this.proxyTypes.put(cacheKey, proxy.getClass());
return proxy;
} this.nonAdvisedBeans.add(cacheKey);
return bean;
}

Spring AOP实现声明式事务代码分析

protected Object createProxy(
Class<?> beanClass, String beanName, Object[] specificInterceptors, TargetSource targetSource) { ProxyFactory proxyFactory = new ProxyFactory();
// Copy our properties (proxyTargetClass etc) inherited from ProxyConfig.
proxyFactory.copyFrom(this); if (!shouldProxyTargetClass(beanClass, beanName)) {
// Must allow for introductions; can't just set interfaces to
// the target's interfaces only.
Class<?>[] targetInterfaces = ClassUtils.getAllInterfacesForClass(beanClass, this.proxyClassLoader);
for (Class<?> targetInterface : targetInterfaces) {
proxyFactory.addInterface(targetInterface);
}
} Advisor[] advisors = buildAdvisors(beanName, specificInterceptors);
for (Advisor advisor : advisors) {
proxyFactory.addAdvisor(advisor);
} proxyFactory.setTargetSource(targetSource);
customizeProxyFactory(proxyFactory); proxyFactory.setFrozen(this.freezeProxy);
if (advisorsPreFiltered()) {
proxyFactory.setPreFiltered(true);
} return proxyFactory.getProxy(this.proxyClassLoader);
}

至于事务切面和其他切面形成切面chain时的调用关系,请参考http://wely.iteye.com/blog/2313924的解释。

  本文并未介绍事务属性、事务状态、事务管理器以及事务自身更底层的一些内容,这些内容留待我们研究了mysql的事务后再详细介绍。