Spring AOP 源码解析三（代理对象的创建）

java框架Spring 阅读量

引言

上期我们对AOP核心概念及接口做了粗浅的分析，这期我们主要来探讨一下代理对象的创建过程。在开始之前，先问自己几个问题

Spring是如何帮我们去选择合适的Advice的？找到了又是通过何种方式创建代理对象的？

好了，现在我们开始分析代理对象的创建过程，首先，先来看一下AspectJAwareAdvisorAutoProxyCreator.class类的继承体系

image.png

可以看到AspectJAwareAdvisorAutoProxyCreator.class主要实现了这几个接口

  1. Aware Bean创建时注入一些容器属性等
  2. BeanPostProcessor IOC 扩展点
  3. AopInfrastructureBean 标识此类是AOP系统类，不能被代理

这里我们仅需要关注BeanPostProcessor接口的实现，因为代理的创建是在BeanPostProcessor接口的postProcessAfterInitialization方法执行时创建的，postProcessAfterInitialization方法的具体实现逻辑是在抽象父类AbstractAutoProxyCreator中实现的

入口

上文中提到AspectJAwareAdvisorAutoProxyCreator.class是BeanPostProcessor接口的实现，现在就让我们打开它的入口源码慢慢分析

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public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
	if (bean != null) {
		// 返回用做缓存的key键
		Object cacheKey = getCacheKey(bean.getClass(), beanName);
		if (!this.earlyProxyReferences.contains(cacheKey)) {
			return wrapIfNecessary(bean, beanName, cacheKey);
		}
	}
	return bean;
}
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protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
	if (beanName != null && this.targetSourcedBeans.contains(beanName)) {
		return bean;
	}
	if (Boolean.FALSE.equals(this.advisedBeans.get(cacheKey))) {
		return bean;
	}
	// 判断当前的Bean是否是AOP本身的系统类，是的话则跳过，不做代理操作
	if (isInfrastructureClass(bean.getClass()) || shouldSkip(bean.getClass(), beanName)) {
		this.advisedBeans.put(cacheKey, Boolean.FALSE);
		return bean;
	}

	// 为该Bean找到一个合适的 advisor
	Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
	// 返回不为空
	if (specificInterceptors != DO_NOT_PROXY) {
		this.advisedBeans.put(cacheKey, Boolean.TRUE);
		// 创建代理对象
		Object proxy = createProxy(
				bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));
		this.proxyTypes.put(cacheKey, proxy.getClass());
		return proxy;
	}
	
	// 没有找到合适的 advisor 则直接返回原始Bean
	this.advisedBeans.put(cacheKey, Boolean.FALSE);
	return bean;
}

看完wrapIfNecessary方法，这里就是创建代理对象的全过程。其实在这个方法中一共就做了两件事情:

  1. getAdvicesAndAdvisorsForBean方法 筛选合适的Advisor对象，其实就是为当前目标对象找到合适的通知(Advice)
  2. createProxy方法 根据筛选到的切面（Advisor）集合为目标对象创建代理

下面我们分别对这两件事情作分析

寻找合适的 Advisor

findEligibleAdvisors方法是在父类AbstractAdvisorAutoProxyCreator中，
且直接调用了findEligibleAdvisors 方法

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	List<Advisor> advisors = findEligibleAdvisors(beanClass, beanName);
	if (advisors.isEmpty()) {
		return DO_NOT_PROXY;
	}
	return advisors.toArray();
}
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protected List<Advisor> findEligibleAdvisors(Class<?> beanClass, String beanName) {
	// 找到容器中的全部通知
	List<Advisor> candidateAdvisors = findCandidateAdvisors();
	// 筛选合适通知对象集合
	List<Advisor> eligibleAdvisors = findAdvisorsThatCanApply(candidateAdvisors, beanClass, beanName);
	extendAdvisors(eligibleAdvisors);
	if (!eligibleAdvisors.isEmpty()) {
		eligibleAdvisors = sortAdvisors(eligibleAdvisors);
	}
	return eligibleAdvisors;
}

这里也是分开做了两件事情

  1. 首先从Spring 容器中获取到全部Advisors集合
  2. 执行筛选逻辑，获取符合条件的Advisors集合

找到Spring容器中全部 Advisor

这里是调用了BeanFactoryAdvisorRetrievalHelper.class的方法去获取到Spring 容器中全部Advisor集合的，这里需要注意BeanFactoryAdvisorRetrievalHelper.class类的初始化是在setBeanFactory方法中进行的

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protected List<Advisor> findCandidateAdvisors() {
	return this.advisorRetrievalHelper.findAdvisorBeans();
}
  • BeanFactoryAdvisorRetrievalHelper的findAdvisorBeans方法
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public List<Advisor> findAdvisorBeans() {
	// Determine list of advisor bean names, if not cached already.
	String[] advisorNames = null;
	synchronized (this) {
		// 尝试从缓存中获取 Advisor 类型的Bean名称
		advisorNames = this.cachedAdvisorBeanNames;
		// 
		if (advisorNames == null) {
			// 从BeanFactory中获取 Advisor 类型的全部Bean名称集合，并设置缓存
			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>();
	// 遍历 Advisor类型 BeanName 集合
	for (String name : advisorNames) {
		// 判断是否合法，这里返回都是true
		if (isEligibleBean(name)) {
			// 判断该Bean是否正在创建中，创建中则直接跳过
			if (this.beanFactory.isCurrentlyInCreation(name)) {
				if (logger.isDebugEnabled()) {
					logger.debug("Skipping currently created advisor '" + name + "'");
				}
			}
			else {
				try {
					// 根据BeanName和Advisor类型从BeanFactory中获取Bean
					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("Skipping advisor '" + name +
										"' with dependency on currently created bean: " + 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;
}

findAdvisorBeans方法的的目的非常明确，就是找到容器中全部的Advisor

  1. 先尝试从缓存中获取Advisor集合
  2. 缓存中不存在，则执行获取逻辑
    1. 获取到了所有类型为Advisor的Bean的名称
    2. 根据获取到的BeanName获取Bean

为当前Bean匹配合适的 Advisor

在以上的步骤中，我们已经拿到了容器中所有的Advisor对象集合，也就是说我们已经拿到了容器中所有已配置的AOP切面。接下里的事情就是为当前的目标对象筛选出适合的Advisor集合，现在我们开始分析AbstractAdvisorAutoProxyCreator.class的findAdvisorsThatCanApply方法

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protected List<Advisor> findAdvisorsThatCanApply(
		List<Advisor> candidateAdvisors, Class<?> beanClass, String beanName) {

	ProxyCreationContext.setCurrentProxiedBeanName(beanName);
	try {
		// 调用 AopUtils.findAdvisorsThatCanApply 为当前Bean筛选合适的 Advisor
		return AopUtils.findAdvisorsThatCanApply(candidateAdvisors, beanClass);
	}
	finally {
		ProxyCreationContext.setCurrentProxiedBeanName(null);
	}
}

findAdvisorsThatCanApply方法里调用了工具类AOPUtils的findAdvisorsThatCanApply方法

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public static List<Advisor> findAdvisorsThatCanApply(List<Advisor> candidateAdvisors, Class<?> clazz) {
	if (candidateAdvisors.isEmpty()) {
		return candidateAdvisors;
	}
	List<Advisor> eligibleAdvisors = new LinkedList<Advisor>();
	// 先找出 IntroductionAdvisor 类型的Advisor
	for (Advisor candidate : candidateAdvisors) {
		if (candidate instanceof IntroductionAdvisor && canApply(candidate, clazz)) {
			eligibleAdvisors.add(candidate);
		}
	}
	boolean hasIntroductions = !eligibleAdvisors.isEmpty();
	// 再从余下的Advisor中继续匹配
	for (Advisor candidate : candidateAdvisors) {
		if (candidate instanceof IntroductionAdvisor) {
			// already processed
			continue;
		}
		if (canApply(candidate, clazz, hasIntroductions)) {
			eligibleAdvisors.add(candidate);
		}
	}
	return eligibleAdvisors;
}

以上代码逻辑会先筛选出IntroductionAdvisor类型的Advisor，再筛选余下的其他Advisor

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public static boolean canApply(Advisor advisor, Class<?> targetClass) {
	return canApply(advisor, targetClass, false);
}
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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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public static boolean canApply(Pointcut pc, Class<?> targetClass, boolean hasIntroductions) {
	Assert.notNull(pc, "Pointcut must not be null");
	// 先判断类型是否匹配
	if (!pc.getClassFilter().matches(targetClass)) {
		return false;
	}
	
	// 获取切点方法匹配器
	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) {
		// 获取当前目标class的所有方法
		Method[] methods = ReflectionUtils.getAllDeclaredMethods(clazz);
		for (Method method : methods) {
			// 调用 methodMatcher.matches 判断当前方法是否匹配该切点
			if ((introductionAwareMethodMatcher != null &&
					introductionAwareMethodMatcher.matches(method, targetClass, hasIntroductions)) ||
					methodMatcher.matches(method, targetClass)) {
				return true;
			}
		}
	}

	return false;
}

canApply是用于筛选核心方法，在上期分析连接点Pointcut接口时，我们知道Pointcut持有ClassFilter和MethodMatcher对象，具体的筛选逻辑就是由它们完成的，至于在它们具体的macthes方法是是如何实现筛选的，这里我也没有进行过深入分析，感兴趣的可以在其实现类AspectJExpressionPointcut.class中继续查看

生成代理对象

在上文的分析结束后，我们已经筛选出来了合适的 Advisor，Advisor持有Advice(通知)，接下来的操作就是根据我们配置的Advice为目标对象创建代理对象了

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protected Object createProxy(
		Class<?> beanClass, String beanName, Object[] specificInterceptors, TargetSource targetSource) {

	if (this.beanFactory instanceof ConfigurableListableBeanFactory) {
		AutoProxyUtils.exposeTargetClass((ConfigurableListableBeanFactory) this.beanFactory, beanName, beanClass);
	}
	
	// 首先创建一个代理创建工厂类，之后的操作都是为此工厂配置属性
	ProxyFactory proxyFactory = new ProxyFactory();
	proxyFactory.copyFrom(this);
	// 判断配置属性proxy-target-class  是否等于False
	if (!proxyFactory.isProxyTargetClass()) {
		// 判断目标 BeanDefinition是否配置preserveTargetClass 为 ture，是的话配置CGLIB动态代理
		if (shouldProxyTargetClass(beanClass, beanName)) {
			proxyFactory.setProxyTargetClass(true);
		}
		else {
			// 设置目标类接口到proxyFactory，如果没有实现接口则使用CGLIB代理
			evaluateProxyInterfaces(beanClass, proxyFactory);
		}
	}

	Advisor[] advisors = buildAdvisors(beanName, specificInterceptors);
	// proxyFactory 设置 advisors
	proxyFactory.addAdvisors(advisors);
	// 设置目标对象资源
	proxyFactory.setTargetSource(targetSource);
	customizeProxyFactory(proxyFactory);

	proxyFactory.setFrozen(this.freezeProxy);
	if (advisorsPreFiltered()) {
		proxyFactory.setPreFiltered(true);
	}
	
	// 创建代理
	return proxyFactory.getProxy(getProxyClassLoader());

创建代理对象前，会新建一个代理工厂创建类，并为此工厂类配置相关属性，例如proxy-target-class的配置，虽然默认配置是false会使用JDK动态代理，但如果没有实现接口，也会自动设置proxy-target-class为true使用CGLIB创建代理对象

完成ProxyFactory的配置之后，就可以通过它创建代理对象了

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/**
 * 创建代理对象
 */
public Object getProxy(ClassLoader classLoader) {
	return createAopProxy().getProxy(classLoader);
}

调用父类ProxyCreatorSupport的createAopProxy方法，获取到代理创建工厂，工厂类(DefaultAopProxyFactory)是在父类的构造方法中创建的

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protected final synchronized AopProxy createAopProxy() {
	if (!this.active) {
		activate();
	}
	// 获取代理创建工厂类 （DefaultAopProxyFactory.class）创建代理对象
	return getAopProxyFactory().createAopProxy(this);
}

最终在DefaultAopProxyFactory工厂类createAopProxy中创建代理对象

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public AopProxy createAopProxy(AdvisedSupport config) throws AopConfigException {
	// 这里主要是判断 proxy-target-class 属性是否为true。默认是false
	if (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);
		}
		// 使用CGLIB动态代理
		return new ObjenesisCglibAopProxy(config);
	}
	else {
		// 默认使用JDK动态打理
		return new JdkDynamicAopProxy(config);
	}
}

最终的代理的创建是在ObjenesisCglibAopProxy或JdkDynamicAopProxy中完成的，至于更具体的创建逻辑，因为这一系列的源码分析只是为了能够对AOP的整体逻辑有清晰的认识，所以这里就不做更详细的分析了。

尾言

本篇文章分析了AOP代理创建的整个过程，纵观整篇文章，篇幅有限，其中还有很多的点没有详细展开分析，只是粗略的分析了AOP代理的创建过程，惭愧 … 。分析有误的地方，希望大家可以指出。

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