按照自己的思路去研究Spring AOP源码【1】

目录

• 一个例子
• Spring AOP 原理
  • 从@EnableAspectJAutoProxy注解入手
  • 什么时候会创建代理对象？
  • 方法执行时怎么实现拦截的？
• 总结
• 问题
• 参考

一个例子

// 定义一个切面
package cn.eagle.li.source.aspect;

@Component
@Aspect
public class ServiceAspect {
	@Pointcut("execution(* cn.eagle.li.source.service.*.*(..))")
	public void pointCut() {
	}

	@Before("pointCut()")
	public void methodBefore() {
		System.out.println("===== Before =====");
	}

	@After("pointCut()")
	public void methodAfter() {
		System.out.println("===== After =====");
	}

	@AfterReturning("pointCut()")
	public void methodReturn() {
		System.out.println("===== AfterReturning =====");
	}

	@Around("pointCut()")
	public void doAround(ProceedingJoinPoint pjp) throws Throwable {
		System.out.println("===== Around before =====");
		pjp.proceed();
		System.out.println("===== Around after =====");
	}
}

// 一个接口
package cn.eagle.li.source.service;
public interface IService {
	void doService();
}

// 一个实现类
package cn.eagle.li.source.service.impl;
@Service
public class ServiceImpl implements IService {
	@Override
	public void doService() {
		System.out.println("do service");
	}
}

// Main类
@Configuration
@ComponentScan(basePackages = {"cn.eagle.li.source"})
@EnableAspectJAutoProxy(exposeProxy = true)
public class Main {
	public static void main(String[] args) {
		AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(Main.class);
		IService service = context.getBean(IService.class);
		service.doService();
	}
}

执行结果如下：

===== Around before =====
===== Before =====
do service
===== AfterReturning =====
===== After =====
===== Around after =====

Spring AOP 原理

从@EnableAspectJAutoProxy注解入手

下面是EnableAspectJAutoProxy注解类内容

@Import(AspectJAutoProxyRegistrar.class)
public @interface EnableAspectJAutoProxy {
        //代理的实现方式，true为CGLIB, false为JDK，默认false
	boolean proxyTargetClass() default false;

 	// 要不要暴露代理对象
	boolean exposeProxy() default false;
}

除了上面两个参数比较重要外，它还有一个Import注解，在Spring中，只要一种注解组合了另一种注解，它就具有该注解的功能，也就是这个注解拥有了@Import注解的功能。

@Import(AspectJAutoProxyRegistrar.class)

我们看到它import了AspectJAutoProxyRegistrar这个类，我们下面再看看这个类的内容，发现它主要的工作就是注册一个name为：

org.springframework.aop.config.internalAutoProxyCreator，类为AnnotationAwareAspectJAutoProxyCreator的BeanDefinition。

class AspectJAutoProxyRegistrar implements ImportBeanDefinitionRegistrar {
	@Override
	public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
		AopConfigUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary(registry);
             // ......
	}
}

AopConfigUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary(registry);

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

public abstract class AopConfigUtils {

  	public static final String AUTO_PROXY_CREATOR_BEAN_NAME =
			"org.springframework.aop.config.internalAutoProxyCreator";

        @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;
		}

		RootBeanDefinition beanDefinition = new RootBeanDefinition(cls);
		beanDefinition.setSource(source);
		beanDefinition.getPropertyValues().add("order", Ordered.HIGHEST_PRECEDENCE);
		beanDefinition.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);

                // 注册 BeanDefinition
                // AUTO_PROXY_CREATOR_BEAN_NAME = org.springframework.aop.config.internalAutoProxyCreator
                // beanDefinition = AnnotationAwareAspectJAutoProxyCreator.class
		registry.registerBeanDefinition(AUTO_PROXY_CREATOR_BEAN_NAME, beanDefinition);
		return beanDefinition;
	}
}

此时有一个疑问：那就是什么时候调用这个方法来加载AnnotationAwareAspectJAutoProxyCreator这个类的BeanDefinition呢？

从下面这张图可以看到是在容器进行刷新的时候，调用invokeBeanFactoryPostProcessors这个方法来进行执行的，具体的执行过程就不展开了。

	public void refresh() throws BeansException, IllegalStateException {
		synchronized (this.startupShutdownMonitor) {
			StartupStep contextRefresh = this.applicationStartup.start("spring.context.refresh");

			// Prepare this context for refreshing.
			prepareRefresh();

			// Tell the subclass to refresh the internal bean factory.
			ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

			// Prepare the bean factory for use in this context.
			prepareBeanFactory(beanFactory);

			try {
				// Allows post-processing of the bean factory in context subclasses.
				postProcessBeanFactory(beanFactory);

				StartupStep beanPostProcess = this.applicationStartup.start("spring.context.beans.post-process");
				// Invoke factory processors registered as beans in the context.
                                // 在这里把beandefinition加载
				invokeBeanFactoryPostProcessors(beanFactory);
                                // ......
      }
    }

现在这个类的BeanDefinition准备好了，那什么时候会创建AnnotationAwareAspectJAutoProxyCreator这个实例，然后加载到容器里呢？

我们看一下这个类的继承关系，发现它是一个实现了BeanPostProcessor接口，也就是说它是一个后置处理器。在refresh()方法中有一个步骤是专门用来注册后置处理器的，也就是registerBeanPostProcessors()这个方法。

	public void refresh() throws BeansException, IllegalStateException {
		synchronized (this.startupShutdownMonitor) {
			StartupStep contextRefresh = this.applicationStartup.start("spring.context.refresh");

			// Prepare this context for refreshing.
			prepareRefresh();

			// Tell the subclass to refresh the internal bean factory.
			ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

			// Prepare the bean factory for use in this context.
			prepareBeanFactory(beanFactory);

			try {
				// Allows post-processing of the bean factory in context subclasses.
				postProcessBeanFactory(beanFactory);

				StartupStep beanPostProcess = this.applicationStartup.start("spring.context.beans.post-process");
				// Invoke factory processors registered as beans in the context.
				invokeBeanFactoryPostProcessors(beanFactory);

                                // Register bean processors that intercept bean creation.
                                // 在这里把BeanPostProcessor加载到容器中
				registerBeanPostProcessors(beanFactory);
                                // ......
      }
    }

在调试的过程中，发现在PostProcessorRegistrationDelegate这个类下的registerBeanPostProcessors()这个方法下会创建AnnotationAwareAspectJAutoProxyCreator，并把它注册到容器中，如下图。

PostProcessorRegistrationDelegate类下
public static void registerBeanPostProcessors(
			ConfigurableListableBeanFactory beanFactory, AbstractApplicationContext applicationContext) {
}

经过上面的一个讨论，我总结了一下它的一些关键触发路径，如下：

refresh() -> invokeBeanFactoryPostProcessors() -> AspectJAutoProxyRegistrar.registerBeanDefinitions()

name:"org.springframework.aop.config.internalAutoProxyCreator" beanDefinition:AnnotationAwareAspectJAutoProxyCreator -> registerBeanPostProcessors() -> 创建类实例并加载到容器中

什么时候会创建代理对象？

容器里有了AnnotationAwareAspectJAutoProxyCreator这个实例，它具体有什么用呢，它什么时候会生成代理对象呢？

protected Object initializeBean(String beanName, Object bean, @Nullable RootBeanDefinition mbd) {
		if (System.getSecurityManager() != null) {
			AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
				invokeAwareMethods(beanName, bean);
				return null;
			}, getAccessControlContext());
		}
		else {
			invokeAwareMethods(beanName, bean);
		}

		Object wrappedBean = bean;
		if (mbd == null || !mbd.isSynthetic()) {
			wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
		}

		try {
			invokeInitMethods(beanName, wrappedBean, mbd);
		}
		catch (Throwable ex) {
			throw new BeanCreationException(
					(mbd != null ? mbd.getResourceDescription() : null),
					beanName, "Invocation of init method failed", ex);
		}
		if (mbd == null || !mbd.isSynthetic()) {
			wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
		}

		return wrappedBean;
	}

大家都知道，Spring在创建一个类实例后，会对这个类进行初始化，然后会执行一系列的后置处理器，就在applyBeanPostProcessorsAfterInitialization()这个方法里面。

wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);

它会依次地去调用每一个后置处理器，当然也包括了我们刚刚注册的AnnotationAwareAspectJAutoProxyCreator这个后置处理器，具体的执行方法就是postProcessAfterInitialization();

	public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
			throws BeansException {

		Object result = existingBean;
		for (BeanPostProcessor processor : getBeanPostProcessors()) {
			Object current = processor.postProcessAfterInitialization(result, beanName);
			if (current == null) {
				return result;
			}
			result = current;
		}
		return result;
	}

我们来看一下AnnotationAwareAspectJAutoProxyCreator类的postProcessAfterInitialization这个方法，发现如果没有包装过，就把它包装一下。

	public Object postProcessAfterInitialization(@Nullable Object bean, String beanName) {
		if (bean != null) {
			Object cacheKey = getCacheKey(bean.getClass(), beanName);
			if (this.earlyProxyReferences.remove(cacheKey) != bean) {
				return wrapIfNecessary(bean, beanName, cacheKey);
			}
		}
		return bean;
	}

return wrapIfNecessary(bean, beanName, cacheKey);

我们看到它会获取一些适配这个类的Advices和Advisors，如果不为null，就创建一个代理对象。

	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.
		Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
                // DO_NOT_PROXY = null;
		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;
		}

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

Object proxy = createProxy(
					bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));

protected Object createProxy(Class<?> beanClass, @Nullable String beanName,
			@Nullable Object[] specificInterceptors, TargetSource targetSource) {
                // ......
		return proxyFactory.getProxy(targetClassLoader);
	}

	public Object getProxy(@Nullable ClassLoader classLoader) {
		return createAopProxy().getProxy(classLoader);
	}

Spring AOP有两种代理类，Jdk代理类和Cglib代理类，具体要看proxyTargetClass这个配置项和这个类是否实现了接口。

	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.");
			}
                        // 如果目标类是一个接口 || 或者目标类是一个代理类
			if (targetClass.isInterface() || Proxy.isProxyClass(targetClass)) {
				return new JdkDynamicAopProxy(config);
			}
			return new ObjenesisCglibAopProxy(config);
		}
		else {
			return new JdkDynamicAopProxy(config);
		}
	}

我们以JDK动态代理为例，发现它就是用Java的api进行创建的，代理类JdkDynamicAopProxy这个类

final class JdkDynamicAopProxy implements AopProxy, InvocationHandler, Serializable {
	@Override
	public Object getProxy(@Nullable ClassLoader classLoader) {
		if (logger.isTraceEnabled()) {
			logger.trace("Creating JDK dynamic proxy: " + this.advised.getTargetSource());
		}
		return Proxy.newProxyInstance(classLoader, this.proxiedInterfaces, this);
	}
}

到现在，我们知道，当创建一个bean实例后，在它初始化后，会调用每个后置处理器的初始化后的方法

当调用AOP的后置处理器的时候，会根据有没有适配它的advice和advitor来创建代理类

创建代理类的时候，有两种选择，一是JDK代理类，二是Cglib代理类

如果proxyTargetClass为false或者这个类没有实现接口的话，就选择Cglib代理类，否则选择JDK代理类。

方法执行时怎么实现拦截的？

现在我们获取一个bean时，获取的就是它的代理类，那在调用其方法时，哪些符合的通知是怎么一步步执行的呢？

我们以JDK代理类为例，我们都知道，在JDK动态代理中，实际执行的时候是执行的代理类的invoke方法，所以看一下它的具体内容：

final class JdkDynamicAopProxy implements AopProxy, InvocationHandler, Serializable {

	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;

                        // 如果设置了exposeProxy参数
                        // 这里暴露了代理对象
                        // AopContext.currentProxy() 获得
			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();
			Class<?> targetClass = (target != null ? target.getClass() : null);

			// Get the interception chain for this method.
			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.
				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.
				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);
			}
		}
	}
}

总的看下来，主要关心以下内容：

// 如果设置了exposeProxy参数
// 这里暴露了代理对象
// AopContext.currentProxy() 获得
if (this.advised.exposeProxy) {
  // Make invocation available if necessary.
  // 这里是用ThreadLocal实现的
  oldProxy = AopContext.setCurrentProxy(proxy);
  setProxyContext = true;
}

// 创建拦截器链
List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);
if (chain.isEmpty()) {
  Object[] argsToUse = AopProxyUtils.adaptArgumentsIfNecessary(method, args);
  // 直接执行本方法
  retVal = AopUtils.invokeJoinpointUsingReflection(target, method, argsToUse);
}
else {
  MethodInvocation invocation =
    new ReflectiveMethodInvocation(proxy, target, method, args, targetClass, chain);
  retVal = invocation.proceed();
}

这个执行链就是一个数组，然后一个一个向下执行，根据之前的例子，这个生成的执行链如下：

AspectJAroundAdvice MethodBeforeAdviceInterceptor AspectJAfterAdvice AspectJAfterReturningAdvice

最终包装成ReflectiveMethodInvocation这个类，调用其proceed()方法执行，如下：

public class ReflectiveMethodInvocation implements ProxyMethodInvocation, Cloneable {

	public Object proceed() throws Throwable {
		// 拦截器链中的最后一个拦截器执行完
		if (this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size() - 1) {
                        // 执行目标方法
			return invokeJoinpoint();
		}

                // 每次执行新的拦截器，下标+1
		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.
			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.
                        // 上面的例子每次都会这个方法
			return ((MethodInterceptor) interceptorOrInterceptionAdvice).invoke(this);
		}
	}
}

public class AspectJAroundAdvice extends AbstractAspectJAdvice implements MethodInterceptor, Serializable {
	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);
                // 调用通知的方法
		return invokeAdviceMethod(pjp, jpm, null, null);
	}
}

public class MethodBeforeAdviceInterceptor implements MethodInterceptor, BeforeAdvice, Serializable {
	public Object invoke(MethodInvocation mi) throws Throwable {
                // 调用通知方法
		this.advice.before(mi.getMethod(), mi.getArguments(), mi.getThis());
                // 下一个拦截器
		return mi.proceed();
	}
}

public class AspectJAfterAdvice extends AbstractAspectJAdvice
		implements MethodInterceptor, AfterAdvice, Serializable {
	public Object invoke(MethodInvocation mi) throws Throwable {
		try {
                        // 下一个拦截器
			return mi.proceed();
		}
		finally {
                        // 调用通知方法
			invokeAdviceMethod(getJoinPointMatch(), null, null);
		}
	}
}

public class AfterReturningAdviceInterceptor implements MethodInterceptor, AfterAdvice, Serializable {
		implements MethodInterceptor, AfterAdvice, Serializable {
	public Object invoke(MethodInvocation mi) throws Throwable {
                // 下一个拦截器
		Object retVal = mi.proceed();
                // 调用通知方法
		this.advice.afterReturning(retVal, mi.getMethod(), mi.getArguments(), mi.getThis());
		return retVal;
	}
}

===== Around before =====
===== Before =====
do service
===== AfterReturning =====
===== After =====
===== Around after =====

从上面可以看出，代理类执行的时候，就是将符合的advice排序得到一个数组，然后依次的进行执行。

总结

1. 从EnableAspectJAutoProxy注解入手，Spring会注册AnnotationAwareAspectJAutoProxyCreator这个类
2. AnnotationAwareAspectJAutoProxyCreator是个BeanPostProcessor，在初始化后，会调用后置处理器，生成代理对象
3. 在类执行方法时，会生成一个数组，然后将所有相关的通知一个一个的执行

问题

我看看到上面的顺序是 AspectJAroundAdvice MethodBeforeAdviceInterceptor AspectJAfterAdvice AspectJAfterReturningAdvice

但是好多文章都说是AspectJAfterReturningAdvice AspectJAfterAdvice AspectJAroundAdvice MethodBeforeAdviceInterceptor

不同的执行顺序会对你的程序会有不同的影响

后续将会写一篇文章专门介绍这个原因

参考

Spring AOP

SpringAop源码分析(基于注解) 一

SpringAop源码分析(基于注解) 三

SpringAop源码分析(基于注解)四

Spring AOP执行顺序