Spring-AOP源码探讨-匹配bean通知

痛定思痛。 2022-02-26 13:20 203阅读 0赞

说明使用<aop:aspectj-autoproxy />声明式注解风格配置切面，其自动为spirng容器中符合条件的bean创建代理对象匹配通知(Advisor)内部流程。

配置：

<aop:aspectj-autoproxy />

aop命名空间中aspectj-autoproxy的解析类是AspectJAutoProxyBeanDefinitionParser。  
其解析方法：

public BeanDefinition parse(Element element, ParserContext parserContext) {
    		//向spring容器中注入代理对象构造器
    		AopNamespaceUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary(parserContext, element);
    		extendBeanDefinition(element, parserContext);
    		return null;
    	}

AopNamespaceUtils\#registerAspectJAnnotationAutoProxyCreatorIfNecessary方法：

public static void registerAspectJAnnotationAutoProxyCreatorIfNecessary(
    			ParserContext parserContext, Element sourceElement) {
    
    		BeanDefinition beanDefinition = AopConfigUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary(
    				parserContext.getRegistry(), parserContext.extractSource(sourceElement));
    		useClassProxyingIfNecessary(parserContext.getRegistry(), sourceElement);
    		registerComponentIfNecessary(beanDefinition, parserContext);
    	}

再看看AopConfigUtils\#registerAspectJAnnotationAutoProxyCreatorIfNecessary方法：

public static BeanDefinition registerAspectJAnnotationAutoProxyCreatorIfNecessary(BeanDefinitionRegistry registry,
    			@Nullable Object source) {
    
    		return registerOrEscalateApcAsRequired(AnnotationAwareAspectJAutoProxyCreator.class, registry, source);
    	}

由此可知，容器中代理对象构造器是AnnotationAwareAspectJAutoProxyCreator类。  
AnnotationAwareAspectJAutoProxyCreator是AbstractAutoProxyCreator类的子类，改类实现了BeanPostProcessor接口，是bean后置处理类，当容器完成bean的初始化后，调用后置处理器的postProcessAfterInitialization，此时构造器根据切点定义判断bean是否需要创建代理对象，是则创建，否则返回。

所以，先从AbstractAutoProxyCreator类入手，看看其postProcessAfterInitialization方法：

/**
     * Create a proxy with the configured interceptors if the bean is
     * identified as one to proxy by the subclass.
     * @see #getAdvicesAndAdvisorsForBean
     */
    @Override
    public Object postProcessAfterInitialization(@Nullable Object bean, String beanName) throws BeansException {
    	if (bean != null) {
    		Object cacheKey = getCacheKey(bean.getClass(), beanName);
    		if (!this.earlyProxyReferences.contains(cacheKey)) {
    			//决定是否创建代理类
    			return wrapIfNecessary(bean, beanName, cacheKey);
    		}
    	}
    	return bean;
    }

再看看wrapIfNecessary方法，省略一些无关代码：

/**
     * 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) {
    	...
    	// Create proxy if we have advice.
    	//获取目标bean的各种增强
    	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;
    	}
    
    	this.advisedBeans.put(cacheKey, Boolean.FALSE);
    	return bean;
    }

getAdvicesAndAdvisorsForBean方法在AbstractAdvisorAutoProxyCreator中实现：

protected Object[] getAdvicesAndAdvisorsForBean(
    		Class<?> beanClass, String beanName, @Nullable TargetSource targetSource) {
    	//查找匹配的Advisor
    	List<Advisor> advisors = findEligibleAdvisors(beanClass, beanName);
    	if (advisors.isEmpty()) {
    		return DO_NOT_PROXY;
    	}
    	return advisors.toArray();
    }

findEligibleAdvisors方法包含了3个方法，分别是查找、过滤、排序：

protected List<Advisor> findEligibleAdvisors(Class<?> beanClass, String beanName) {
    	//查找候选的增强
    	List<Advisor> candidateAdvisors = findCandidateAdvisors();
    	//匹配候选增强
    	List<Advisor> eligibleAdvisors = findAdvisorsThatCanApply(candidateAdvisors, beanClass, beanName);
    	extendAdvisors(eligibleAdvisors);
    	if (!eligibleAdvisors.isEmpty()) {
    		//对匹配增强进行排序
    		eligibleAdvisors = sortAdvisors(eligibleAdvisors);
    	}
    	return eligibleAdvisors;
    }

AnnotationAwareAspectJAutoProxyCreator类重写了父类的findCandidateAdvisors方法：

protected List<Advisor> findCandidateAdvisors() {
    	// Add all the Spring advisors found according to superclass rules.
    	List<Advisor> advisors = super.findCandidateAdvisors();
    	// Build Advisors for all AspectJ aspects in the bean factory.
    	if (this.aspectJAdvisorsBuilder != null) {
    		advisors.addAll(this.aspectJAdvisorsBuilder.buildAspectJAdvisors());
    	}
    	return advisors;
    }

先调用父类AbstractAdvisorAutoProxyCreator的findCandidateAdvisors方法：

protected List<Advisor> findCandidateAdvisors() {
    	Assert.state(this.advisorRetrievalHelper != null, "No BeanFactoryAdvisorRetrievalHelper available");
    	return this.advisorRetrievalHelper.findAdvisorBeans();
    }

其通过BeanFactoryAdvisorRetrievalHelper类查找容器中Advisor接口类型的实现类(省略无关代码)：

public List<Advisor> findAdvisorBeans() {
    	// Determine list of advisor bean names, if not cached already.
    	String[] advisorNames = null;
    	synchronized (this) {
    		advisorNames = this.cachedAdvisorBeanNames;
    		if (advisorNames == null) {
    			// Do not initialize FactoryBeans here: We need to leave all regular beans
    			// uninitialized to let the auto-proxy creator apply to them!
    			//查找容器中Advisor的实现类
    			advisorNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(
    					this.beanFactory, Advisor.class, true, false);
    			this.cachedAdvisorBeanNames = advisorNames;
    		}
    	}
    	if (advisorNames.length == 0) {
    		return new LinkedList<>();
    	}
    
    	List<Advisor> advisors = new LinkedList<>();
    	for (String name : advisorNames) {
    		if (isEligibleBean(name)) {
    			if (this.beanFactory.isCurrentlyInCreation(name)) {
    				if (logger.isDebugEnabled()) {
    					logger.debug("Skipping currently created advisor '" + name + "'");
    				}
    			}
    			else {
    				try {
    					//添加到集合后返回
    					advisors.add(this.beanFactory.getBean(name, Advisor.class));
    				}
    				catch (BeanCreationException ex) {
    					...
    					throw ex;
    				}
    			}
    		}
    	}
    	return advisors;
    }

AnnotationAwareAspectJAutoProxyCreator类调用完父类的findCandidateAdvisors方法后，再通过BeanFactoryAspectJAdvisorsBuilderAdapter类构建匹配的Advisor类，实际上就是解析容器中标记@Aspect的类，将类中@Before、@After等注解的方法包装成相应的InstantiationModelAwarePointcutAdvisorImpl类实例：

public List<Advisor> buildAspectJAdvisors() {
    	List<String> aspectNames = this.aspectBeanNames;
    
    	if (aspectNames == null) {
    		synchronized (this) {
    			aspectNames = this.aspectBeanNames;
    			if (aspectNames == null) {
    				List<Advisor> advisors = new LinkedList<>();
    				aspectNames = new LinkedList<>();
    				String[] beanNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(
    						this.beanFactory, Object.class, true, false);
    				for (String beanName : beanNames) {
    					if (!isEligibleBean(beanName)) {
    						continue;
    					}
    					// We must be careful not to instantiate beans eagerly as in this case they
    					// would be cached by the Spring container but would not have been weaved.
    					Class<?> beanType = this.beanFactory.getType(beanName);
    					if (beanType == null) {
    						continue;
    					}
    					//判断类是否有@Aspect注解
    					if (this.advisorFactory.isAspect(beanType)) {
    						aspectNames.add(beanName);
    						AspectMetadata amd = new AspectMetadata(beanType, beanName);
    						if (amd.getAjType().getPerClause().getKind() == PerClauseKind.SINGLETON) {
    							MetadataAwareAspectInstanceFactory factory =
    									new BeanFactoryAspectInstanceFactory(this.beanFactory, beanName);
    							//将类中@Before、@After等注解的方法包装成相应的Advisor类实例
    							List<Advisor> classAdvisors = this.advisorFactory.getAdvisors(factory);
    							if (this.beanFactory.isSingleton(beanName)) {
    								this.advisorsCache.put(beanName, classAdvisors);
    							}
    							else {
    								this.aspectFactoryCache.put(beanName, factory);
    							}
    							advisors.addAll(classAdvisors);
    						}
    						else {
    							// Per target or per this.
    							if (this.beanFactory.isSingleton(beanName)) {
    								throw new IllegalArgumentException("Bean with name '" + beanName +
    										"' is a singleton, but aspect instantiation model is not singleton");
    							}
    							MetadataAwareAspectInstanceFactory factory =
    									new PrototypeAspectInstanceFactory(this.beanFactory, beanName);
    							this.aspectFactoryCache.put(beanName, factory);
    							advisors.addAll(this.advisorFactory.getAdvisors(factory));
    						}
    					}
    				}
    				this.aspectBeanNames = aspectNames;
    				return advisors;
    			}
    		}
    	}
    
    
    	return advisors;
    }

this.advisorFactory.getAdvisors(factory)如下：

private List<Method> getAdvisorMethods(Class<?> aspectClass) {
    	final List<Method> methods = new LinkedList<>();
    	ReflectionUtils.doWithMethods(aspectClass, method -> {
    		// Exclude pointcuts
    		if (AnnotationUtils.getAnnotation(method, Pointcut.class) == null) {
    			methods.add(method);
    		}
    	});
    	//排序，按照方法名进行自然排序
    	methods.sort(METHOD_COMPARATOR);
    	return methods;
    }

查找完候选的Advisor后，接着匹配合适的Advisor：

protected List<Advisor> findAdvisorsThatCanApply(
    		List<Advisor> candidateAdvisors, Class<?> beanClass, String beanName) {
    
    	ProxyCreationContext.setCurrentProxiedBeanName(beanName);
    	try {
    		return AopUtils.findAdvisorsThatCanApply(candidateAdvisors, beanClass);
    	}
    	finally {
    		ProxyCreationContext.setCurrentProxiedBeanName(null);
    	}
    }

AopUtils\#findAdvisorsThatCanApply(candidateAdvisors, beanClass)：

public static List<Advisor> findAdvisorsThatCanApply(List<Advisor> candidateAdvisors, Class<?> clazz) {
    	if (candidateAdvisors.isEmpty()) {
    		return candidateAdvisors;
    	}
    	List<Advisor> eligibleAdvisors = new LinkedList<>();
    	for (Advisor candidate : candidateAdvisors) {
    		if (candidate instanceof IntroductionAdvisor && canApply(candidate, clazz)) {
    			eligibleAdvisors.add(candidate);
    		}
    	}
    	boolean hasIntroductions = !eligibleAdvisors.isEmpty();
    	for (Advisor candidate : candidateAdvisors) {
    		if (candidate instanceof IntroductionAdvisor) {
    			// already processed
    			continue;
    		}
    		//判断是否匹配
    		if (canApply(candidate, clazz, hasIntroductions)) {
    			eligibleAdvisors.add(candidate);
    		}
    	}
    	return eligibleAdvisors;
    }

再看看canApply(candidate, clazz, hasIntroductions)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) {
    		//InstantiationModelAwarePointcutAdvisorImpl是其子类
    		PointcutAdvisor pca = (PointcutAdvisor) advisor;
    		return canApply(pca.getPointcut(), targetClass, hasIntroductions);
    	}
    	else {
    		// It doesn't have a pointcut so we assume it applies.
    		return true;
    	}
    }

再看看canApply(pca.getPointcut(), targetClass, hasIntroductions)方法：

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<>();
    	if (!Proxy.isProxyClass(targetClass)) {
    		classes.add(ClassUtils.getUserClass(targetClass));
    	}
    	classes.addAll(ClassUtils.getAllInterfacesForClassAsSet(targetClass));
    
    	for (Class<?> clazz : classes) {
    		Method[] methods = ReflectionUtils.getAllDeclaredMethods(clazz);
    		for (Method method : methods) {
    			//匹配类的方法与切点是否匹配
    			if (introductionAwareMethodMatcher != null ?
    					introductionAwareMethodMatcher.matches(method, targetClass, hasIntroductions) :
    					methodMatcher.matches(method, targetClass)) {
    				return true;
    			}
    		}
    	}
    
    	return false;
    }

切点如何匹配以后有找时间再深入。

匹配完成后，将Advisor进行排序：

/**
     * Sort the rest by AspectJ precedence. If two pieces of advice have
     * come from the same aspect they will have the same order.
     * Advice from the same aspect is then further ordered according to the
     * following rules:
     * <ul>
     * <li>if either of the pair is after advice, then the advice declared
     * last gets highest precedence (runs last)</li>
     * <li>otherwise the advice declared first gets highest precedence (runs first)</li>
     * </ul>
     * <p><b>Important:</b> Advisors are sorted in precedence order, from highest
     * precedence to lowest. "On the way in" to a join point, the highest precedence
     * advisor should run first. "On the way out" of a join point, the highest precedence
     * advisor should run last.
     */
    @Override
    @SuppressWarnings("unchecked")
    protected List<Advisor> sortAdvisors(List<Advisor> advisors) {
    	List<PartiallyComparableAdvisorHolder> partiallyComparableAdvisors = new ArrayList<>(advisors.size());
    	for (Advisor element : advisors) {
    		partiallyComparableAdvisors.add(
    				new PartiallyComparableAdvisorHolder(element, DEFAULT_PRECEDENCE_COMPARATOR));
    	}
    	List<PartiallyComparableAdvisorHolder> sorted = PartialOrder.sort(partiallyComparableAdvisors);
    	if (sorted != null) {
    		List<Advisor> result = new ArrayList<>(advisors.size());
    		for (PartiallyComparableAdvisorHolder pcAdvisor : sorted) {
    			result.add(pcAdvisor.getAdvisor());
    		}
    		return result;
    	}
    	else {
    		return super.sortAdvisors(advisors);
    	}
    }

注：在前一步解析@Before、@After时，同类的Advisor按照方法名进行自然排序，这里排序是将后置通知排到集合前，例如After排到Before前面。

参考

> [https://www.imooc.com/article/43378][https_www.imooc.com_article_43378]

[https_www.imooc.com_article_43378]: https://www.imooc.com/article/43378