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spring源码系列9:事务代理的创建

我们总结出AOP公式

  • JDK动态代理(Proxy+InvocationHandler)+advised
  • CGLB动态代理(Enhancer+MethodInterceptor)+advised

本质都是在内存中生成了新的字节码类。

这节我们看看事务是如何利用AOP实现的。

事务代理生成过程

一、@EnableTransactionManagement配置事务环境

@EnableTransactionManagement的@Import(TransactionManagementConfigurationSelector.class)引入TransactionManagementConfigurationSelector.class。此类

@Override
	protected String[] selectImports(AdviceMode adviceMode) {
		switch (adviceMode) {
			case PROXY:
				return new String[] {AutoProxyRegistrar.class.getName(),
						ProxyTransactionManagementConfiguration.class.getName()};
			case ASPECTJ:
				return new String[] {
						TransactionManagementConfigUtils.TRANSACTION_ASPECT_CONFIGURATION_CLASS_NAME};
			default:
				return null;
		}
	}
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PROXY:情况下,会注册两个类:

>AutoProxyRegistrar:

实现了ImportBeanDefinitionRegistrar。其接口方法registerBeanDefinitions会向仓库注册Bean定义

InfrastructureAdvisorAutoProxyCreator。 InfrastructureAdvisorAutoProxyCreator 继承了AbstractAdvisorAutoProxyCreator间接继承了 AbstractAutoProxyCreator

在AOP代理生成那一节,我们讲过。 AnnotationAwareAspectJAutoProxyCreator 也是间接继承了 AbstractAutoProxyCreator

在AOP实现原理中 AbstractAutoProxyCreator 做了大部分工作。

从这一点看, 事务代理对象创建过程,与AOP代理对象过程是一样的,关键就在这个AbstractAutoProxyCreator类

>ProxyTransactionManagementConfiguration

是一个@Configuration。有三个@Bean注解方法。

  • transactionAdvisor()
  • transactionAttributeSource()
  • transactionInterceptor()
@Configuration
public class ProxyTransactionManagementConfiguration extends AbstractTransactionManagementConfiguration {

	@Bean(name = TransactionManagementConfigUtils.TRANSACTION_ADVISOR_BEAN_NAME)
	@Role(BeanDefinition.ROLE_INFRASTRUCTURE)
	public BeanFactoryTransactionAttributeSourceAdvisor transactionAdvisor() {
		BeanFactoryTransactionAttributeSourceAdvisor advisor = new BeanFactoryTransactionAttributeSourceAdvisor();
		advisor.setTransactionAttributeSource(transactionAttributeSource());
		advisor.setAdvice(transactionInterceptor());
		advisor.setOrder(this.enableTx.<Integer>getNumber("order"));
		return advisor;
	}

	@Bean
	@Role(BeanDefinition.ROLE_INFRASTRUCTURE)
	public TransactionAttributeSource transactionAttributeSource() {
		return new AnnotationTransactionAttributeSource();
	}

	@Bean
	@Role(BeanDefinition.ROLE_INFRASTRUCTURE)
	public TransactionInterceptor transactionInterceptor() {
		TransactionInterceptor interceptor = new TransactionInterceptor();
		interceptor.setTransactionAttributeSource(transactionAttributeSource());
		if (this.txManager != null) {
			interceptor.setTransactionManager(this.txManager);
		}
		return interceptor;
	}

}
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1.BeanFactoryTransactionAttributeSourceAdvisor:

首先:transactionAdvisor()方法会向仓库中注册一个BeanFactoryTransactionAttributeSourceAdvisor。

spring源码系列9:事务代理的创建

从其继承关系上看,他是一个Advisor,并且还是PointcutAdvisor.关于Advisor,上节我们分析过他是封装了(Advice+Pointcut)

既然都有了Advisor了,那Advice和Pointcut在哪里呢?

BeanFactoryTransactionAttributeSourceAdvisor有一个pointcut 属性,会new 一个TransactionAttributeSourcePointcut。

private final TransactionAttributeSourcePointcut pointcut = new TransactionAttributeSourcePointcut() {
		@Override
		protected TransactionAttributeSource getTransactionAttributeSource() {
			return transactionAttributeSource;
		}
	};
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spring源码系列9:事务代理的创建

从其继承关系,我们可以看出他其实就是一个Pointcut。

至此,就剩下Advice没有被发现。

2.TransactionAttributeSource: 其次:transactionAttributeSource()会向仓库中注册一个AnnotationTransactionAttributeSource。这个AnnotationTransactionAttributeSource干嘛用的呢?

BeanFactoryTransactionAttributeSourceAdvisor.setTransactionAttributeSource(transactionAttributeSource())

属性值
private TransactionAttributeSource transactionAttributeSource;
private final TransactionAttributeSourcePointcut pointcut = new TransactionAttributeSourcePointcut() {
		@Override
		protected TransactionAttributeSource getTransactionAttributeSource() {
			return transactionAttributeSource;
		}
	};

TransactionAttributeSourcePointcut的matche方法
@Override
	public boolean matches(Method method, Class<?> targetClass) {
		if (targetClass != null && TransactionalProxy.class.isAssignableFrom(targetClass)) {
			return false;
		}
		TransactionAttributeSource tas = getTransactionAttributeSource();
		return (tas == null || tas.getTransactionAttribute(method, targetClass) != null);
	}
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AnnotationTransactionAttributeSource对象会赋值给BeanFactoryTransactionAttributeSourceAdvisor的 transactionAttributeSource 属性。 pointcut 属性初始化时,new 一个 TransactionAttributeSourcePointcut类 并实现 getTransactionAttributeSource() 方法, getTransactionAttributeSource() 方法正好返回了 transactionAttributeSource 属性。

也就是说TransactionAttributeSourcePointcut的getTransactionAttributeSource()方法返回的是 AnnotationTransactionAttributeSource

3.TransactionInterceptor: 最后:transactionInterceptor()方法,会向仓库中注册一个TransactionInterceptor类。

spring源码系列9:事务代理的创建

TransactionInterceptor从继承关系看他是一个Advice. 也就是增强器,是对事务真正处理地方。

有了Advice+Pointcut。Advice+Pointcut = Advisor 。 Advisor+TargetSource = Advised

有了Advised ,这样spring事务不正是套用了AOP的基础吗。

二、代理的生成

在AOP源码分析那一节,我们讲过,postProcessAfterInitialization后置初始化方法中,wrapIfNecessary 满足条件,才创建代理。

@Override
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;
}
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而这个条件就是:getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null); 能够获取适用于当前bean的Advisor

1.Advisor寻找

我们回顾上节:

getAdvicesAndAdvisorsForBean 经过 AbstractAdvisorAutoProxyCreator.findEligibleAdvisors 的调用, AopUtils.findAdvisorsThatCanApply(candidateAdvisors, beanClass) 的调用,最终会调用 AopUtils.canApply 来判断某个Advisor是否适用于当前类。

我们来看看canApply方法

public static boolean canApply(Advisor advisor, Class<?> targetClass, boolean hasIntroductions) {
		if (advisor instanceof IntroductionAdvisor) {
			return ((IntroductionAdvisor) advisor).getClassFilter().matches(targetClass);
		}
		else if (advisor instanceof PointcutAdvisor) {
			PointcutAdvisor pca = (PointcutAdvisor) advisor;
			return canApply(pca.getPointcut(), targetClass, hasIntroductions);
		}
		else {
			// It doesn't have a pointcut so we assume it applies.
			return true;
		}
	}
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上文提到,

  • BeanFactoryTransactionAttributeSourceAdvisor 是一个PointcutAdvisor类型的Advisor
  • BeanFactoryTransactionAttributeSourceAdvisor中new 一个TransactionAttributeSourcePointcut。

所以此处会走: canApply(pca.getPointcut(), targetClass, hasIntroductions)分支。

pca.getPointcut() 返回的是 TransactionAttributeSourcePointcut

进一步分析重载方法canApply(Pointcut pc, Class<?> targetClass, boolean hasIntroductions)

public static boolean canApply(Pointcut pc, Class<?> targetClass, boolean hasIntroductions) {
		Assert.notNull(pc, "Pointcut must not be null");
		if (!pc.getClassFilter().matches(targetClass)) {
			return false;
		}

		MethodMatcher methodMatcher = pc.getMethodMatcher();
		if (methodMatcher == MethodMatcher.TRUE) {
			// No need to iterate the methods if we're matching any method anyway...
			return true;
		}

		IntroductionAwareMethodMatcher introductionAwareMethodMatcher = null;
		if (methodMatcher instanceof IntroductionAwareMethodMatcher) {
			introductionAwareMethodMatcher = (IntroductionAwareMethodMatcher) methodMatcher;
		}

		Set<Class<?>> classes = new LinkedHashSet<Class<?>>(ClassUtils.getAllInterfacesForClassAsSet(targetClass));
		classes.add(targetClass);
		for (Class<?> clazz : classes) {
			Method[] methods = ReflectionUtils.getAllDeclaredMethods(clazz);
			for (Method method : methods) {
				if ((introductionAwareMethodMatcher != null &&
						introductionAwareMethodMatcher.matches(method, targetClass, hasIntroductions)) ||
						methodMatcher.matches(method, targetClass)) {
					return true;
				}
			}
		}

		return false;
	}
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  • 先ClassFilter.matches 校验一次。 TransactionAttributeSourcePointcut 的父类 StaticMethodMatcherPointcut.classFilter= ClassFilter.TRUE 表示类检查全部通过
  • 再MethodMatcher.matches 方法进行校验
abstract class TransactionAttributeSourcePointcut extends StaticMethodMatcherPointcut implements Serializable {
	@Override
	public boolean matches(Method method, Class<?> targetClass) {
		if (targetClass != null && TransactionalProxy.class.isAssignableFrom(targetClass)) {
			return false;
		}
		TransactionAttributeSource tas = getTransactionAttributeSource();
		return (tas == null || tas.getTransactionAttribute(method, targetClass) != null);
	}
}
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matches方法会调用 getTransactionAttributeSource() 获取一个 TransactionAttributeSource 对象,通过 TransactionAttributeSource.getTransactionAttribute(method, targetClass) ,来判断适应性。

关于 getTransactionAttributeSource() 上文讲过。会返回一个 AnnotationTransactionAttributeSource 实例对象。

也就是说: TransactionAttributeSourcePointcut 的 matches()方法是通过AnnotationTransactionAttributeSource.getTransactionAttribute(method, targetClass)来实现的

来看看getTransactionAttribute()方法

@Override
	public TransactionAttribute getTransactionAttribute(Method method, Class<?> targetClass) {
		if (method.getDeclaringClass() == Object.class) {
			return null;
		}

		// First, see if we have a cached value.
		Object cacheKey = getCacheKey(method, targetClass);
		TransactionAttribute cached = this.attributeCache.get(cacheKey);
		if (cached != null) {
			// Value will either be canonical value indicating there is no transaction attribute,
			// or an actual transaction attribute.
			if (cached == NULL_TRANSACTION_ATTRIBUTE) {
				return null;
			}
			else {
				return cached;
			}
		}
		else {
			// We need to work it out.
			TransactionAttribute txAttr = computeTransactionAttribute(method, targetClass);
			// Put it in the cache.
			if (txAttr == null) {
				this.attributeCache.put(cacheKey, NULL_TRANSACTION_ATTRIBUTE);
			}
			else {
				String methodIdentification = ClassUtils.getQualifiedMethodName(method, targetClass);
				if (txAttr instanceof DefaultTransactionAttribute) {
					((DefaultTransactionAttribute) txAttr).setDescriptor(methodIdentification);
				}
				if (logger.isDebugEnabled()) {
					logger.debug("Adding transactional method '" + methodIdentification + "' with attribute: " + txAttr);
				}
				this.attributeCache.put(cacheKey, txAttr);
			}
			return txAttr;
		}
	}
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这里用了一个缓存,但重点在这个 TransactionAttribute txAttr = computeTransactionAttribute(method, targetClass);

protected TransactionAttribute computeTransactionAttribute(Method method, Class<?> targetClass) {
		// Don't allow no-public methods as required.
		
		// 非public方法事务不生效。
		if (allowPublicMethodsOnly() && !Modifier.isPublic(method.getModifiers())) {
			return null;
		}

		// Ignore CGLIB subclasses - introspect the actual user class.
		Class<?> userClass = ClassUtils.getUserClass(targetClass);
		// The method may be on an interface, but we need attributes from the target class.
		// If the target class is null, the method will be unchanged.
		
		// 获取真实的方法(有接口方法的,要获取实现类上的那个方法)
		Method specificMethod = ClassUtils.getMostSpecificMethod(method, userClass);
		// If we are dealing with method with generic parameters, find the original method.
		specificMethod = BridgeMethodResolver.findBridgedMethod(specificMethod);

		// First try is the method in the target class.
		//首先查看实现类方法上是否有@Transactional注解
		TransactionAttribute txAttr = findTransactionAttribute(specificMethod);
		if (txAttr != null) {
			return txAttr;
		}

		// Second try is the transaction attribute on the target class.
		//其次查看实现类方法所在类上是否有@Transactional注解
		txAttr = findTransactionAttribute(specificMethod.getDeclaringClass());
		if (txAttr != null && ClassUtils.isUserLevelMethod(method)) {
			return txAttr;
		}

		if (specificMethod != method) {
			// Fallback is to look at the original method.
			//还不行去看接口方法上是否有@Transactional注解
			txAttr = findTransactionAttribute(method);
			if (txAttr != null) {
				return txAttr;
			}
			// Last fallback is the class of the original method.
			//最后看接口上是否有@Transactional注解
			txAttr = findTransactionAttribute(method.getDeclaringClass());
			if (txAttr != null && ClassUtils.isUserLevelMethod(method)) {
				return txAttr;
			}
		}

		return null;
	}
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由此:我们们也终于知道,@Transactional注解的查找顺序,实现类方法--》实现类--》接口方法--》接口 我们在这四个地方添加@Transactional注解都会使事务生效。

在这四个地方任一一个地方找到了@Transactional注解,说明 BeanFactoryTransactionAttributeSourceAdvisor 适用于当前类,getAdvicesAndAdvisorsForBean 的返回不为空。接下来就可以创建动态代理了。

2.代理的生成

// Create proxy if we have advice.
		Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
		if (specificInterceptors != DO_NOT_PROXY) {
			this.advisedBeans.put(cacheKey, Boolean.TRUE);
			Object proxy = createProxy(
					bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));
			this.proxyTypes.put(cacheKey, proxy.getClass());
			return proxy;
		}
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createProxy()方法,在 spring源码系列8:AOP源码解析之代理的创建 一节我们一节分析过了。

最终的结果就是通过 CglibAopProxy. getProxy()返回代理对象 或者 JdkDynamicAopProxy. getProxy()返回代理对象

总结:

spring事务,就是在spring AOP基础上实现的。 通过定义一个适用于事务的Advisor(Advice+Pointcut)完美的套用AOP的东西,实现了事务。

我们看看这个公式:

  1. AOP = 动态代理+ Advised(Adisor+TargetSource);
  2. Adisor = Advice+Pointcut

试想,如果我们可不可以利用这个公式也能创建出一个类似事务的东西呢?

原文  https://juejin.im/post/5dafa987f265da5b6a16aef8
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