深入解析spring AOP原理及源码

目录

• @EnableAspectJAutoProxy
• 找切面
• 代理对象的创建
• 代理方法的执行
• ExposeInvocationInterceptor#invoke
• 环绕通知的执行
• 前置通知的执行
• 后置通知的执行
• 返回后通知的执行
• 异常通知的执行

@EnableAspectJAutoProxy

@EnableAspectJAutoProxy注解用于开启AOP功能，那么这个注解底层到底做了什么呢？

查看@EnableAspectJAutoProxy的源码，发现它使用@Import注解向Spring容器中注入了一个类型为AspectJAutoProxyRegistrar的Bean：

class AspectJAutoProxyRegistrar implements ImportBeanDefinitionRegistrar {

    @Override
    public void registerBeanDefinitions(
            AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {

        // 注入一个bean名字为org.springframework.aop.config.internalAutoProxyCreator的AspectJAwareAdvisorAutoProxyCreator
        AopConfigUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary(registry);

        AnnotationAttributes enableAspectJAutoProxy =
                AnnotationConfigUtils.attributesFor(importingClassMetadata, EnableAspectJAutoProxy.class);
        if (enableAspectJAutoProxy != null) {
            if (enableAspectJAutoProxy.getBoolean("proxyTargetClass")) {
                // proxyTargetClass为true
                AopConfigUtils.forceAutoProxyCreatorToUseClassProxying(registry);
            }
            if (enableAspectJAutoProxy.getBoolean("exposeProxy")) {
                // exposeProxy为true
                AopConfigUtils.forceAutoProxyCreatorToExposeProxy(registry);
            }
        }
    }

}

AspectJAutoProxyRegistrar实现了ImportBeanDefinitionRegistrar接口，而ImportBeanDefinitionRegistrar是spring提供的扩展点之一，主要用来向容器中注入BeanDefinition，spring会根据BeanDefinion来生成Bean。

那么AspectJAutoProxyRegistrar到底向容器中注入了什么BeanDefinion呢？

org.springframework.aop.config.AopConfigUtils#registerAspectJAnnotationAutoProxyCreatorIfNecessary(org.springframework.beans.factory.support.BeanDefinitionRegistry)

@Nullable
public static BeanDefinition registerAspectJAnnotationAutoProxyCreatorIfNecessary(BeanDefinitionRegistry registry) {
    return registerAspectJAnnotationAutoProxyCreatorIfNecessary(registry, null);
}

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

@Nullable
private static BeanDefinition registerOrEscalateApcAsRequired(
        Class<?> cls, BeanDefinitionRegistry registry, @Nullable Object source) {

    Assert.notNull(registry, "BeanDefinitionRegistry must not be null");

    if (registry.containsBeanDefinition(AUTO_PROXY_CREATOR_BEAN_NAME)) {
        BeanDefinition apcDefinition = registry.getBeanDefinition(AUTO_PROXY_CREATOR_BEAN_NAME);
        if (!cls.getName().equals(apcDefinition.getBeanClassName())) {
            int currentPriority = findPriorityForClass(apcDefinition.getBeanClassName());
            int requiredPriority = findPriorityForClass(cls);
            if (currentPriority < requiredPriority) {
                apcDefinition.setBeanClassName(cls.getName());
            }
        }
        return null;
    }

    // 注入AspectJAwareAdvisorAutoProxyCreator
    RootBeanDefinition beanDefinition = new RootBeanDefinition(cls);
    beanDefinition.setSource(source);
    beanDefinition.getPropertyValues().add("order", Ordered.HIGHEST_PRECEDENCE);
    beanDefinition.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
    registry.registerBeanDefinition(AUTO_PROXY_CREATOR_BEAN_NAME, beanDefinition);
    return beanDefinition;
}

从源码可以发现AspectJAutoProxyRegistrar向容器中注入了一个类型为AnnotationAwareAspectJAutoProxyCreator的Bean。

那么AnnotationAwareAspectJAutoProxyCreator又是干什么的呢？

AnnotationAwareAspectJAutoProxyCreator主要实现了三个接口（由父类AbstractAutoProxyCreator实现）：

• 实现了BeanFactoryAware，内部持有BeanFactory的引用。
• 实现了SmartInstantiationAwareBeanPostProcessor(InstantiationAwareBeanPostProcessor).postProcessBeforeInstantiation，这个方法在bean的实例化（bean创建之前）之前执行。
• 实现了BeanPostProcessor.postProcessBeforeInitialization()，这个方法在bean的初始化之前（bean创建之后，属性被赋值之前）执行，BeanPostProcessor.postProcessAfterInitialization()在bean的初始化之后执行。

AnnotationAwareAspectJAutoProxyCreator的继承结构：

找切面

org.springframework.aop.framework.autoproxy.AbstractAdvisorAutoProxyCreator#findEligibleAdvisors

protected List<Advisor> findEligibleAdvisors(Class<?> beanClass, String beanName) {
	/**
	 * @see AnnotationAwareAspectJAutoProxyCreator#findCandidateAdvisors()
	 */
	// 获取容器中所有的切面Advisor
	// 这里返回的切面中的方法已经是有序的了，先按注解顺序（Around.class, Before.class, After.class, AfterReturning.class, AfterThrowing.class），再按方法名称
	List<Advisor> candidateAdvisors = findCandidateAdvisors();
	// 获取所有能够作用于当前Bean上的Advisor
	List<Advisor> eligibleAdvisors = findAdvisorsThatCanApply(candidateAdvisors, beanClass, beanName);
	/**
	 * @see AspectJAwareAdvisorAutoProxyCreator#extendAdvisors(java.util.List)
	 */
	// 往集合第一个位置加入了一个DefaultPointcutAdvisor
	extendAdvisors(eligibleAdvisors);
	if (!eligibleAdvisors.isEmpty()) {
		/**
		 * @see AspectJAwareAdvisorAutoProxyCreator#sortAdvisors(java.util.List)
		 */
		// 这里是对切面进行排序，例如有@Order注解或者实现了Ordered接口
		eligibleAdvisors = sortAdvisors(eligibleAdvisors);
	}
	return eligibleAdvisors;
}

org.springframework.aop.aspectj.annotation.AnnotationAwareAspectJAutoProxyCreator#findCandidateAdvisors

protected List<Advisor> findCandidateAdvisors() {
	// Add all the Spring advisors found according to superclass rules.
	// 获取容器中所有的切面Advisor
	List<Advisor> advisors = super.findCandidateAdvisors();
	// Build Advisors for all AspectJ aspects in the bean factory.
	if (this.aspectJAdvisorsBuilder != null) {
		// 这里还需要解析@Aspect注解，生成Advisor
		advisors.addAll(this.aspectJAdvisorsBuilder.buildAspectJAdvisors());
	}
	return advisors;
}

org.springframework.aop.support.AopUtils#findAdvisorsThatCanApply

public static List<Advisor> findAdvisorsThatCanApply(List<Advisor> candidateAdvisors, Class<?> clazz) {
	if (candidateAdvisors.isEmpty()) {
		return candidateAdvisors;
	}
	List<Advisor> eligibleAdvisors = new ArrayList<>();
	// InstantiationModelAwarePointcutAdvisorImpl
	for (Advisor candidate : candidateAdvisors) {
		if (candidate instanceof IntroductionAdvisor && canApply(candidate, clazz)) {
			// IntroductionAdvisor类型为引入切面，具体类型为DeclareParentsAdvisor
			eligibleAdvisors.add(candidate);
		}
	}
	boolean hasIntroductions = !eligibleAdvisors.isEmpty();
	for (Advisor candidate : candidateAdvisors) {
		if (candidate instanceof IntroductionAdvisor) {
			// already processed
			continue;
		}
		// PointCut中的ClassFilter.match 匹配类
		// PointCut中的MethodMatcher.match 匹配方法
		if (canApply(candidate, clazz, hasIntroductions)) {
			// @Aspect，类型为InstantiationModelAwarePointcutAdvisorImpl
			eligibleAdvisors.add(candidate);
		}
	}
	return eligibleAdvisors;
}

代理对象的创建

代理对象的创建时机位于bean的初始化之后，因为代理对象内部还是需要去调用目标对象的方法，所以需要让目标对象实例化并完成初始化后才会创建代理对象。

org.springframework.aop.framework.autoproxy.AbstractAutoProxyCreator#postProcessAfterInitialization

public Object postProcessAfterInitialization(@Nullable Object bean, String beanName) {
	if (bean != null) {
		// 先从缓存中获取代理对象
		Object cacheKey = getCacheKey(bean.getClass(), beanName);
		if (this.earlyProxyReferences.remove(cacheKey) != bean) {
			// 按需生成代理对象
			return wrapIfNecessary(bean, beanName, cacheKey);
		}
	}
	return bean;
}

org.springframework.aop.framework.autoproxy.AbstractAutoProxyCreator#wrapIfNecessary

protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
	if (StringUtils.hasLength(beanName) && this.targetSourcedBeans.contains(beanName)) {
		return bean;
	}
	if (Boolean.FALSE.equals(this.advisedBeans.get(cacheKey))) {
		return bean;
	}
	if (isInfrastructureClass(bean.getClass()) || shouldSkip(bean.getClass(), beanName)) {
		this.advisedBeans.put(cacheKey, Boolean.FALSE);
		return bean;
	}

	// Create proxy if we have advice.
	/**
	 * @see AbstractAdvisorAutoProxyCreator#getAdvicesAndAdvisorsForBean(java.lang.Class, java.lang.String, org.springframework.aop.TargetSource)
	 */
	// 获取与当前Bean匹配的切面
	Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
	if (specificInterceptors != DO_NOT_PROXY) {
		this.advisedBeans.put(cacheKey, Boolean.TRUE);
		// 创建代理
		Object proxy = createProxy(
				bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));
		this.proxyTypes.put(cacheKey, proxy.getClass());
		return proxy;
	}

	// 缓存
	this.advisedBeans.put(cacheKey, Boolean.FALSE);
	return bean;
}

org.springframework.aop.framework.autoproxy.AbstractAutoProxyCreator#createProxy

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);
	}

	// 创建代理工厂
	ProxyFactory proxyFactory = new ProxyFactory();
	proxyFactory.copyFrom(this);

	if (!proxyFactory.isProxyTargetClass()) {
		// 进来说明proxyTargetClass=false，指定JDK代理
		if (shouldProxyTargetClass(beanClass, beanName)) {
			// 进来这里说明BD中有个属性preserveTargetClass=true，可以BD中属性设置的优先级最高
			proxyFactory.setProxyTargetClass(true);
		}
		else {
			// 这里会判断bean有没有实现接口，没有就只能使用CGlib
			evaluateProxyInterfaces(beanClass, proxyFactory);
		}
	}

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

	proxyFactory.setFrozen(this.freezeProxy);
	if (advisorsPreFiltered()) {
		proxyFactory.setPreFiltered(true);
	}

	// 使用JDK或者CGlib创建代理对象
	return proxyFactory.getProxy(getProxyClassLoader());
}

org.springframework.aop.framework.JdkDynamicAopProxy#getProxy(java.lang.ClassLoader)

public Object getProxy(@Nullable ClassLoader classLoader) {
	if (logger.isTraceEnabled()) {
		logger.trace("Creating JDK dynamic proxy: " + this.advised.getTargetSource());
	}
	Class<?>[] proxiedInterfaces = AopProxyUtils.completeProxiedInterfaces(this.advised, true);
	findDefinedEqualsAndHashCodeMethods(proxiedInterfaces);
	return Proxy.newProxyInstance(classLoader, proxiedInterfaces, this);
}

这里主要看JDK动态代理的实现，Proxy.newProxyInstance()的第三个参数为InvocationHandler，而这里传的是this，也就是当前的类肯定实现了InvocationHandler接口。

代理方法的执行

由于是JDK动态代理，那么代理方法的调用肯定会进入InvocationHandler.invoke()方法中，这里的InvocationHandler的实现类为org.springframework.aop.framework.JdkDynamicAopProxy。

org.springframework.aop.framework.JdkDynamicAopProxy#invoke

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
	Object oldProxy = null;
	boolean setProxyContext = false;

	TargetSource targetSource = this.advised.targetSource;
	Object target = null;

	try {
		if (!this.equalsDefined && AopUtils.isEqualsMethod(method)) {
			// The target does not implement the equals(Object) method itself.
			return equals(args[0]);
		}
		else if (!this.hashCodeDefined && AopUtils.isHashCodeMethod(method)) {
			// The target does not implement the hashCode() method itself.
			return hashCode();
		}
		else if (method.getDeclaringClass() == DecoratingProxy.class) {
			// There is only getDecoratedClass() declared -> dispatch to proxy config.
			return AopProxyUtils.ultimateTargetClass(this.advised);
		}
		else if (!this.advised.opaque && method.getDeclaringClass().isInterface() &&
				method.getDeclaringClass().isAssignableFrom(Advised.class)) {
			// Service invocations on ProxyConfig with the proxy config...
			return AopUtils.invokeJoinpointUsingReflection(this.advised, method, args);
		}

		Object retVal;

		if (this.advised.exposeProxy) {
			// Make invocation available if necessary.
			oldProxy = AopContext.setCurrentProxy(proxy);
			setProxyContext = true;
		}

		// Get as late as possible to minimize the time we "own" the target,
		// in case it comes from a pool.
		target = targetSource.getTarget(); // 目标对象
		Class<?> targetClass = (target != null ? target.getClass() : null); // 目标对象的类型

		// Get the interception chain for this method.
		// 这里会对方法进行匹配，因为不是目标对象中的所有方法都需要增强
		List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);

		// Check whether we have any advice. If we don't, we can fallback on direct
		// reflective invocation of the target, and avoid creating a MethodInvocation.
		if (chain.isEmpty()) {
			// We can skip creating a MethodInvocation: just invoke the target directly
			// Note that the final invoker must be an InvokerInterceptor so we know it does
			// nothing but a reflective operation on the target, and no hot swapping or fancy proxying.
			// 没有匹配的切面，直接通过反射调用目标对象的目标方法
			Object[] argsToUse = AopProxyUtils.adaptArgumentsIfNecessary(method, args);
			retVal = AopUtils.invokeJoinpointUsingReflection(target, method, argsToUse);
		}
		else {
			// We need to create a method invocation...
			MethodInvocation invocation =
					new ReflectiveMethodInvocation(proxy, target, method, args, targetClass, chain);
			// Proceed to the joinpoint through the interceptor chain.
			/**
			 * @see ReflectiveMethodInvocation#proceed()
			 */
			// 这里才是增强的调用，重点，火炬的传递
			retVal = invocation.proceed();
		}

		// Massage return value if necessary.
		Class<?> returnType = method.getReturnType();
		if (retVal != null && retVal == target &&
				returnType != Object.class && returnType.isInstance(proxy) &&
				!RawTargetAccess.class.isAssignableFrom(method.getDeclaringClass())) {
			// Special case: it returned "this" and the return type of the method
			// is type-compatible. Note that we can't help if the target sets
			// a reference to itself in another returned object.
			retVal = proxy;
		}
		else if (retVal == null && returnType != Void.TYPE && returnType.isPrimitive()) {
			throw new AopInvocationException(
					"Null return value from advice does not match primitive return type for: " + method);
		}
		return retVal;
	}
	finally {
		if (target != null && !targetSource.isStatic()) {
			// Must have come from TargetSource.
			targetSource.releaseTarget(target);
		}
		if (setProxyContext) {
			// Restore old proxy.
			AopContext.setCurrentProxy(oldProxy);
		}
	}
}

org.springframework.aop.framework.ReflectiveMethodInvocation#proceed

public Object proceed() throws Throwable {
	// We start with an index of -1 and increment early.
	if (this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size() - 1) {
		// 执行到最后一个Advice，才会到这里执行目标方法
		return invokeJoinpoint();
	}

	Object interceptorOrInterceptionAdvice =
			this.interceptorsAndDynamicMethodMatchers.get(++this.currentInterceptorIndex);
	if (interceptorOrInterceptionAdvice instanceof InterceptorAndDynamicMethodMatcher) {
		// Evaluate dynamic method matcher here: static part will already have
		// been evaluated and found to match.
		// dm.isRuntime()=true的走这
		InterceptorAndDynamicMethodMatcher dm =
				(InterceptorAndDynamicMethodMatcher) interceptorOrInterceptionAdvice;
		Class<?> targetClass = (this.targetClass != null ? this.targetClass : this.method.getDeclaringClass());
		if (dm.methodMatcher.matches(this.method, targetClass, this.arguments)) {
			return dm.interceptor.invoke(this);
		}
		else {
			// Dynamic matching failed.
			// Skip this interceptor and invoke the next in the chain.
			return proceed();
		}
	}
	else {
		// It's an interceptor, so we just invoke it: The pointcut will have
		// been evaluated statically before this object was constructed.
		// 走这
		return ((MethodInterceptor) interceptorOrInterceptionAdvice).invoke(this);
	}
}

interceptorsAndDynamicMethodMatchers中第一个advice为org.springframework.aop.interceptor.ExposeInvocationInterceptor。

ExposeInvocationInterceptor#invoke

org.springframework.aop.interceptor.ExposeInvocationInterceptor#invoke

private static final ThreadLocal<MethodInvocation> invocation =
 new NamedThreadLocal<>("Current AOP method invocation");

public Object invoke(MethodInvocation mi) throws Throwable {
	MethodInvocation oldInvocation = invocation.get();
	invocation.set(mi);
	try {
		return mi.proceed();
	}
	finally {
		invocation.set(oldInvocation);
	}
}

ExposeInvocationInterceptor#invoke，只干了一件事就是将MethodInvocation加入到了ThreadLocal中，这样后续可以在其他地方使用ExposeInvocationInterceptor#currentInvocation获取到MethodInvocation，而MethodInvocation中封装了目标对象，目标方法，方法参数等信息。

环绕通知的执行

org.springframework.aop.aspectj.AspectJAroundAdvice#invoke

public Object invoke(MethodInvocation mi) throws Throwable {
	if (!(mi instanceof ProxyMethodInvocation)) {
		throw new IllegalStateException("MethodInvocation is not a Spring ProxyMethodInvocation: " + mi);
	}
	ProxyMethodInvocation pmi = (ProxyMethodInvocation) mi;
	ProceedingJoinPoint pjp = lazyGetProceedingJoinPoint(pmi);
	JoinPointMatch jpm = getJoinPointMatch(pmi);
	return invokeAdviceMethod(pjp, jpm, null, null);
}

这里会去调用环绕通知的增强方法，而环绕通知的增强方法中会执行proceedingJoinPoint.proceed()，这样就会调用下一个MethodInterceptor–>MethodBeforeAdviceInterceptor。

前置通知的执行

org.springframework.aop.framework.adapter.MethodBeforeAdviceInterceptor

public Object invoke(MethodInvocation mi) throws Throwable {
	this.advice.before(mi.getMethod(), mi.getArguments(), mi.getThis());
	return mi.proceed();
}

这里又会调用MethodInvocation.proceed()传递给下一个MethodInterceptor。

后置通知的执行

org.springframework.aop.aspectj.AspectJAfterAdvice#invoke

public Object invoke(MethodInvocation mi) throws Throwable {
	try {
		return mi.proceed();
	}
	finally {
		invokeAdviceMethod(getJoinPointMatch(), null, null);
	}
}

先执行MethodInvocation.proceed()，最后在finally块中调用后置通知的增强，不管目标方法有没有抛出异常，finally代码块中的代码都会执行，也就是不管目标方法有没有抛出异常，后置通知都会执行。

返回后通知的执行

org.springframework.aop.framework.adapter.AfterReturningAdviceInterceptor#invoke

public Object invoke(MethodInvocation mi) throws Throwable {
	Object retVal = mi.proceed();
	this.advice.afterReturning(retVal, mi.getMethod(), mi.getArguments(), mi.getThis());
	return retVal;
}

先执行MethodInvocation.proceed()，然后再执行返回后通知的增强。

异常通知的执行

org.springframework.aop.aspectj.AspectJAfterThrowingAdvice#invoke

public Object invoke(MethodInvocation mi) throws Throwable {
	try {
		return mi.proceed();
	}
	catch (Throwable ex) {
		if (shouldInvokeOnThrowing(ex)) {
			invokeAdviceMethod(getJoinPointMatch(), null, ex);
		}
		throw ex;
	}
}

先执行MethodInvocation.proceed()，如果目标方法抛出了异常就会执行异常通知的增强，然后抛出异常，所以这时返回后通知的增强就不会执行了。

总结各种通知的执行顺序：

Around begin // 环绕通知开始
Before // 前置通知
UserServiceImpl
 // 目标方法的执行
AfterReturning
 // 返回后通知
After
 // 后置通知
Around end // 环绕通知结束

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