（三）Spring 核心之面向切面编程（AOP）—

一. 前言

二. 代理的创建

2.1. 创建前准备

2.2. 获取所有的 Advisor

2.3. 创建代理的入口方法

2.4. 依据条件创建代理（JDK 或 CGLIB）

三. 动态代理要解决什么问题

3.1. 什么是代理

3.2. 什么是动态代理

四. 总结

一. 前言

前面两篇文章《（一）Spring 核心之面向切面编程（AOP）—— 配置及使用》和《（二）Spring 核心之面向切面编程（AOP）—— 切面的实现》主要介绍了 Spring AOP 原理解析的切面实现过程，如加载配置，将切面类的所有切面方法根据使用的注解生成对应 Advice，并将 Advice 连同切入点匹配器和切面类等信息一并封装到Advisor。本文在此基础上继续介绍postProcessAfterInitialization 的方法，即代理（JDK 代理和 CGLIB 代理）的创建过程。

二. 代理的创建

2.1. 创建前准备

创建代理的方法是 postProcessAfterInitialization，如果 Bean 被子类标识为代理，则使用配置的拦截器创建一个代理。

/*** 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) {if (bean != null) {Object cacheKey = getCacheKey(bean.getClass(), beanName);// 如果不是提前暴露的代理if (this.earlyProxyReferences.remove(cacheKey) != bean) {return wrapIfNecessary(bean, beanName, cacheKey);}}return bean;
}

wrapIfNecessary() 方法主要用于判断是否需要创建代理，如果 Bean 能够获取到 advisor 才需要创建代理：

/*** 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) {// 如果bean是通过TargetSource接口获取if (beanName != null && this.targetSourcedBeans.contains(beanName)) {return bean;}// 如果bean是切面类if (Boolean.FALSE.equals(this.advisedBeans.get(cacheKey))) {return bean;}// 如果是aop基础类？是否跳过？if (isInfrastructureClass(bean.getClass()) || shouldSkip(bean.getClass(), beanName)) {this.advisedBeans.put(cacheKey, Boolean.FALSE);return bean;}// 重点：获取所有advisor，如果没有获取到，那说明不要进行增强，也就不需要代理了。Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);if (specificInterceptors != DO_NOT_PROXY) {this.advisedBeans.put(cacheKey, Boolean.TRUE);// 重点：创建代理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;
}

2.2. 获取所有的 Advisor

我们看下获取所有 advisor 的方法 getAdvicesAndAdvisorsForBean()：

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

通过 findEligibleAdvisors() 方法获取 advisor，如果获取不到返回 DO_NOT_PROXY（不需要创建代理），findEligibleAdvisors() 方法如下：

/*** 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},* if there are no pointcuts or interceptors* @see #findCandidateAdvisors* @see #sortAdvisors* @see #extendAdvisors*/
protected List<Advisor> findEligibleAdvisors(Class<?> beanClass, String beanName) {// 和上文一样，获取所有切面类的切面方法生成AdvisorList<Advisor> candidateAdvisors = findCandidateAdvisors();// 找到这些Advisor中能够应用于beanClass的AdvisorList<Advisor> eligibleAdvisors = findAdvisorsThatCanApply(candidateAdvisors, beanClass, beanName);// 如果需要，交给子类拓展extendAdvisors(eligibleAdvisors);// 对Advisor排序if (!eligibleAdvisors.isEmpty()) {eligibleAdvisors = sortAdvisors(eligibleAdvisors);}return eligibleAdvisors;
}

获取所有切面类的切面方法生成 Advisor：

/*** Find all candidate Advisors to use in auto-proxying.* @return the List of candidate Advisors*/
protected List<Advisor> findCandidateAdvisors() {Assert.state(this.advisorRetrievalHelper != null, "No BeanFactoryAdvisorRetrievalHelper available");return this.advisorRetrievalHelper.findAdvisorBeans();
}

找到这些 Advisor 中能够应用于 beanClass 的 Advisor：

/*** Determine the sublist of the {@code candidateAdvisors} list* that is applicable to the given class.* @param candidateAdvisors the Advisors to evaluate* @param clazz the target class* @return sublist of Advisors that can apply to an object of the given class* (may be the incoming List as-is)*/
public static List<Advisor> findAdvisorsThatCanApply(List<Advisor> candidateAdvisors, Class<?> clazz) {if (candidateAdvisors.isEmpty()) {return candidateAdvisors;}List<Advisor> eligibleAdvisors = new ArrayList<>();for (Advisor candidate : candidateAdvisors) {// 通过Introduction实现的adviceif (candidate instanceof IntroductionAdvisor && canApply(candidate, clazz)) {eligibleAdvisors.add(candidate);}}boolean hasIntroductions = !eligibleAdvisors.isEmpty();for (Advisor candidate : candidateAdvisors) {if (candidate instanceof IntroductionAdvisor) {// already processedcontinue;}// 是否能够应用于clazz的Adviceif (canApply(candidate, clazz, hasIntroductions)) {eligibleAdvisors.add(candidate);}}return eligibleAdvisors;
}

2.3. 创建代理的入口方法

获取所有 advisor 后，如果有 advisor，则说明需要增强，即需要创建代理，创建代理的方法如下：

/*** 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, @Nullable String beanName,@Nullable Object[] specificInterceptors, TargetSource targetSource) {if (this.beanFactory instanceof ConfigurableListableBeanFactory) {AutoProxyUtils.exposeTargetClass((ConfigurableListableBeanFactory) this.beanFactory, beanName, beanClass);}ProxyFactory proxyFactory = new ProxyFactory();proxyFactory.copyFrom(this);if (proxyFactory.isProxyTargetClass()) {// Explicit handling of JDK proxy targets (for introduction advice scenarios)if (Proxy.isProxyClass(beanClass)) {// Must allow for introductions; can't just set interfaces to the proxy's interfaces only.for (Class<?> ifc : beanClass.getInterfaces()) {proxyFactory.addInterface(ifc);}}}else {// No proxyTargetClass flag enforced, let's apply our default checks...if (shouldProxyTargetClass(beanClass, beanName)) {proxyFactory.setProxyTargetClass(true);}else {evaluateProxyInterfaces(beanClass, proxyFactory);}}Advisor[] advisors = buildAdvisors(beanName, specificInterceptors);proxyFactory.addAdvisors(advisors);proxyFactory.setTargetSource(targetSource);customizeProxyFactory(proxyFactory);proxyFactory.setFrozen(this.freezeProxy);if (advisorsPreFiltered()) {proxyFactory.setPreFiltered(true);}// Use original ClassLoader if bean class not locally loaded in overriding class loaderClassLoader classLoader = getProxyClassLoader();if (classLoader instanceof SmartClassLoader && classLoader != beanClass.getClassLoader()) {classLoader = ((SmartClassLoader) classLoader).getOriginalClassLoader();}return proxyFactory.getProxy(classLoader);
}

proxyFactory.getProxy(classLoader)：

/*** Create a new proxy according to the settings in this factory.* <p>Can be called repeatedly. Effect will vary if we've added* or removed interfaces. Can add and remove interceptors.* <p>Uses the given class loader (if necessary for proxy creation).* @param classLoader the class loader to create the proxy with* (or {@code null} for the low-level proxy facility's default)* @return the proxy object*/
public Object getProxy(@Nullable ClassLoader classLoader) {return createAopProxy().getProxy(classLoader);
}

2.4. 依据条件创建代理（JDK 或 CGLIB）

DefaultAopProxyFactory.createAopProxy：

@Override
public AopProxy createAopProxy(AdvisedSupport config) throws AopConfigException {if (!NativeDetector.inNativeImage() &&(config.isOptimize() || config.isProxyTargetClass() || hasNoUserSuppliedProxyInterfaces(config))) {Class<?> targetClass = config.getTargetClass();if (targetClass == null) {throw new AopConfigException("TargetSource cannot determine target class: " +"Either an interface or a target is required for proxy creation.");}if (targetClass.isInterface() || Proxy.isProxyClass(targetClass)) {return new JdkDynamicAopProxy(config);}return new ObjenesisCglibAopProxy(config);}else {return new JdkDynamicAopProxy(config);}
}

要点如下：

config.isOptimize() 是通过 optimize 设置，表示配置是自定义的，默认是 false；
config.isProxyTargetClass() 是通过 <aop:config proxy-target-class="true" /> 来配置的，表示优先使用 cglib 代理，默认是 false；
hasNoUserSuppliedProxyInterfaces(config) 表示是否目标类实现了接口。