简单的 Spring Aop 原理剖析

刺骨的言语ヽ痛彻心扉 2022-05-26 06:36 207阅读 0赞

spring aop  
aop 的全称是Aspect-Oriented Programming（面向切面编程），怎么理解面向切面这句话？可以这么理解：每一个类的一个方法是一个点，同时处理多个点也就  
可以理解为处理一个面，这个面包含多个点。概念部分也就更明显了，需要被处理的点叫做Pointcut（切点），处理的动作叫做Advice（通知），将切面和点连接在一起的策略叫做Advisor（通知器）。

![这里写图片描述][70]

这里呢，只看一个包org.springframework.aop，直觉告诉我，这是最核心的包。把aop的源码导进eclipse看看，从aop包中看到，Pointcut、Advice、Proxy、Method出现的次数很多

![这里写图片描述][70 1]

他们之间的关系怎样，导进powerdesigner 在整理一下，看看这些类之间的关系，其它的不做说明，我只看aop3个最主要的内容，Pointcut、Advice、Advisor。

**Filter、Method**

![这里写图片描述][70 2]

**Advice**

![这里写图片描述][70 3]

**Advisor**

![这里写图片描述][70 4]

*\*其它*

![这里写图片描述][70 5]

以上4张图，仅仅是描述aop的架构，但是，具体怎么与IOC中bean相结合，这点卡了很久，突破口是代理！spring的代理主要有两种，一种是jdk动态代理，一种是CGLIB，根据我的分析，spring用的应该是jdk代理，因为接口太多了，通过接口实现类达到代理的效果。在org.springframework.aop.framework.autoproxy包中，有一个AbstractAutoProxyCreator.java类，从注释上看，aop的代理动作是通过拦截器完成的。而继承AbstractAutoProxyCreator的子类也可以用自己的策略按照类型、名称自动代理bean。

![这里写图片描述][70 6]  
![这里写图片描述][70 7]

看看该类里面的postProcessAfterInitialization方法：

/** * 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;
        }

结合注释，从缓存中找bean的代理，实在找不到了再wrapIfNecessary创建一个代理

/** * 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) {
                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;
        }

结合注释，成功创建的代理对象都会将beanName加入到targetSourceBeans中，从代码可以看到，是用createProxy方法创建的代理

/** * Create an AOP proxy for the given bean. * @param beanClass the class of the bean * @param beanName the name of the bean * @param specificInterceptors the set of interceptors that is * specific to this bean (may be empty, but not null) * @param targetSource the TargetSource for the proxy, * already pre-configured to access the bean * @return the AOP proxy for the bean * @see #buildAdvisors */
        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);
        }

从源码中可以追踪到代理的创建时在ProxyFactory中创建的，在追踪，看看ProxyFactory

![这里写图片描述][70 8]

这里不多说，但是从源码看到Interfaces 和 class两个关键字，可以推算，ProxyFactory一定是根据代理的方式决定采用哪种代理方式，基于Interfaces的是jdk动态代理，基于class的是cglib代理方式。  
再回到createProxy中来，不管ProxyFactory如何创建的代理，最后加入了advisors

Advisor[] advisors = buildAdvisors(beanName, specificInterceptors);

advisors前面已经说了，是切面，联合切点和通知的地方，所以上面的代码就是将切点放进切面当中！

至此，spring 自动代理bean大概的脉络就清晰了，切面连接切点就通了。至于切面和通知是怎么处理到一起的？  
还是回到AbstractAutoProxyCreator.java中来，看到这里有个getAdvicesAndAdvisorsForBean方法，进入他的子类看看是怎么实现的：

@Override
        protected Object[] getAdvicesAndAdvisorsForBean(Class beanClass, String beanName, TargetSource targetSource) {
            List advisors = findEligibleAdvisors(beanClass, beanName);
            if (advisors.isEmpty()) {
                return DO_NOT_PROXY;
            }
            return advisors.toArray();
        }

进入findEligibleAdvisors看看是怎么织入的

/**
         * Find all eligible Advisors for auto-proxying this class.
         * @param beanClass the clazz to find advisors for
         * @param beanName the name of the currently proxied bean
         * @return the empty List, not <code>null</code>,
         * if there are no pointcuts or interceptors
         * @see #findCandidateAdvisors
         * @see #sortAdvisors
         * @see #extendAdvisors
         */
        protected List<Advisor> findEligibleAdvisors(Class beanClass, String beanName) {
            List<Advisor> candidateAdvisors = findCandidateAdvisors();
            List<Advisor> eligibleAdvisors = findAdvisorsThatCanApply(candidateAdvisors, beanClass, beanName);
            extendAdvisors(eligibleAdvisors);
            if (!eligibleAdvisors.isEmpty()) {
                eligibleAdvisors = sortAdvisors(eligibleAdvisors);
            }
            return eligibleAdvisors;
        }

没什么收获，进入findCandidateAdvisors看看，就在该方法下面，在进入findAdvisorBeans：

/** * Find all eligible Advisor beans in the current bean factory, * ignoring FactoryBeans and excluding beans that are currently in creation. * @return the list of {@link org.springframework.aop.Advisor} beans * @see #isEligibleBean */
        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!
                    advisorNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(
                            this.beanFactory, Advisor.class, true, false);
                    this.cachedAdvisorBeanNames = advisorNames;
                }
            }
            if (advisorNames.length == 0) {
                return new LinkedList<Advisor>();
            }
    
            List<Advisor> advisors = new LinkedList<Advisor>();
            for (String name : advisorNames) {
                if (isEligibleBean(name) && !this.beanFactory.isCurrentlyInCreation(name)) {
                    try {
                        advisors.add(this.beanFactory.getBean(name, Advisor.class));
                    }
                    catch (BeanCreationException ex) {
                        Throwable rootCause = ex.getMostSpecificCause();
                        if (rootCause instanceof BeanCurrentlyInCreationException) {
                            BeanCreationException bce = (BeanCreationException) rootCause;
                            if (this.beanFactory.isCurrentlyInCreation(bce.getBeanName())) {
                                if (logger.isDebugEnabled()) {
                                    logger.debug("Ignoring currently created advisor '" + name + "': " + ex.getMessage());
                                }
                                // Ignore: indicates a reference back to the bean we're trying to advise.
                                // We want to find advisors other than the currently created bean itself.
                                continue;
                            }
                        }
                        throw ex;
                    }
                }
            }
            return advisors;
        }

一大堆不知道怎么看，分点看即可：

（1）获取advisorNames，看看有没有合适的EligibleBean  
（2）遍历advisorNames，将bean加入advisor（通知+切面）

至此就完了，很简单的流程，绕来绕去我自己也看不太懂，没法画流程图，也不知道是谁执行的aop，这方便以后有时间再慢慢看。

[70]: /images/20220526/8c461af91c75431ea4a2ead97ffd046d.png
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