AOP是什么

面向切面的程序设计（Aspect-oriented programming，AOP，又译作面向方面的程序设计、剖面导向程序设计），是计算机科学中的一种程序设计思想，旨在将横切关注点与业务主体进行进一步分离，以提高程序代码的模块化程度。通过在现有代码基础上增加额外的通知（Advice）机制，能够对被声明为“切点（Pointcut）”的代码块进行统一管理与装饰。

怎么在Spring里使用AOP

在Spring里，AOP通过EnableAspectJAutoProxy注解开启。默认情况下，Spring会通过AopAutoConfiguration自动引入这个注解

@Configuration(proxyBeanMethods = false)
@ConditionalOnProperty(prefix = "spring.aop", name = "auto", havingValue = "true", matchIfMissing = true)
public class AopAutoConfiguration {

    @Configuration(proxyBeanMethods = false)
    @ConditionalOnClass(Advice.class)
    static class AspectJAutoProxyingConfiguration {

        @Configuration(proxyBeanMethods = false)
        @EnableAspectJAutoProxy(proxyTargetClass = false)
        @ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "false")
        static class JdkDynamicAutoProxyConfiguration {

        }

        @Configuration(proxyBeanMethods = false)
        @EnableAspectJAutoProxy(proxyTargetClass = true)
        @ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "true",
                matchIfMissing = true)
        static class CglibAutoProxyConfiguration {

        }

    }
}

可以看到，如果我们不主动设置spring.aop.auto=false。那么Spring默认会启用AOP。接下来，我们可以通过在类上标注Aspect即可使用AOP

package org.example.aspect;

@Aspect
@Component
public class SampleAspect {

    @Pointcut("execution(* org.example.xxx.*.*(..))")
    private void executionPointcut() {

    }

    @After(value = "executionPointcut()")
    public void doAfter() {

    }
}

源码分析

1. AOP初始化

1.1 初始化AspectJAutoProxyRegistrar

EnableAspectJAutoProxy通过Import注解引入了AspectJAutoProxyRegistrar

@Import(AspectJAutoProxyRegistrar.class)
public @interface EnableAspectJAutoProxy {
}

AspectJAutoProxyRegistrar实现了ImportBeanDefinitionRegistrar，Spring在初始化AopAutoConfiguration时把所有通过Import注解引入的ImportBeanDefinitionRegistrar实现类拿出来进行初始化，并调用其registerBeanDefinitions函数

class AspectJAutoProxyRegistrar implements ImportBeanDefinitionRegistrar {

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

        AopConfigUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary(registry);

}

1.2 初始化AnnotationAwareAspectJAutoProxyCreator

AspectJAutoProxyRegistrar 则在registerBeanDefinitions注册了一个AnnotationAwareAspectJAutoProxyCreator的BeanDefinition

public static BeanDefinition registerAspectJAnnotationAutoProxyCreatorIfNecessary(
            BeanDefinitionRegistry registry, @Nullable Object source) {

        return registerOrEscalateApcAsRequired(AnnotationAwareAspectJAutoProxyCreator.class, registry, source);
    }

AnnotationAwareAspectJAutoProxyCreator实现了BeanPostProcessor，Spring会在初始化普通Bean之前初始化所有BeanPostProcessor。

public abstract class AbstractApplicationContext extends DefaultResourceLoader
        implements ConfigurableApplicationContext {

        public void refresh() throws BeansException, IllegalStateException {
                    // 初始化BeanProcessor来拦截Bean的创建
                    registerBeanPostProcessors(beanFactory);

                    // 初始化所有剩下的非懒加载的Bean，比如我们写的Service
                    finishBeanFactoryInitialization(beanFactory);
        }
}

1.3 初始化切面方法跟切点

另外，AnnotationAwareAspectJAutoProxyCreator实现了InstantiationAwareBeanPostProcessor，Spring 会在Bean创建时调用其postProcessBeforeInstantiation方法对Bean进行处理。

在第一次调用该方法时，AnnotationAwareAspectJAutoProxyCreator会初始化切面

public abstract class AbstractAutoProxyCreator extends ProxyProcessorSupport
        implements SmartInstantiationAwareBeanPostProcessor, BeanFactoryAware {

    @Override
    public Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) {
        Object cacheKey = getCacheKey(beanClass, beanName);

        if (!StringUtils.hasLength(beanName) || !this.targetSourcedBeans.contains(beanName)) {
            //判断当前BeanName对应的Bean是否应该被代理
            //并将判断结果保存下来，避免后续的后处理方法重复计算
            //在第一次判断时，会在shouldSkip里扫描所有Bean进行切面初始化
            if (isInfrastructureClass(beanClass) || shouldSkip(beanClass, beanName)) {
                this.advisedBeans.put(cacheKey, Boolean.FALSE);
                return null;
            }
        }

        //如果为AbstractAutoProxyCreator注入了自定义的TargetSourceCreator
        //则通过TargetSourceCreator创建的Bean都被被AOP代理
        //TargetSourceCreator默认为空
        TargetSource targetSource = getCustomTargetSource(beanClass, beanName);
        if (targetSource != null) {
            if (StringUtils.hasLength(beanName)) {
                this.targetSourcedBeans.add(beanName);
            }
            Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(beanClass, beanName, targetSource);
            Object proxy = createProxy(beanClass, beanName, specificInterceptors, targetSource);
            this.proxyTypes.put(cacheKey, proxy.getClass());
            return proxy;
        }

        return null;
    }
}

切面初始化

public class BeanFactoryAspectJAdvisorsBuilder {

        public List<Advisor> buildAspectJAdvisors() {
                List<String> aspectNames = this.aspectBeanNames;

                //如果还未进行初始化
                if (aspectNames == null) {
                    synchronized (this) {
                        aspectNames = this.aspectBeanNames;
                        if (aspectNames == null) {
                            List<Advisor> advisors = new ArrayList<>();
                            aspectNames = new ArrayList<>();

                            //拿到容器里所有的beanName
                            String[] beanNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(
                                    this.beanFactory, Object.class, true, false);
                            for (String beanName : beanNames) {

                                Class<?> beanType = this.beanFactory.getType(beanName, false);

                                //判断类上是否标注Aspect，以及判断该class是否已经被代码式的Aspectj处理过
                                if (this.advisorFactory.isAspect(beanType)) {
                                    aspectNames.add(beanName);
                                    AspectMetadata amd = new AspectMetadata(beanType, beanName);
                                    MetadataAwareAspectInstanceFactory factory =
                                                new BeanFactoryAspectInstanceFactory(this.beanFactory, beanName);

                                        //从类中拿到所有带有Before、Around等注解的方法，
                                        //将这些方法包装成MethodInterceptor放入Advisor，MethodInterceptor#invoke为增强方法的调用入口
                                        //将Advisor排好顺序组成List返回
                                        List<Advisor> classAdvisors = this.advisorFactory.getAdvisors(factory);

                                        this.advisorsCache.put(beanName, classAdvisors);
                                        advisors.addAll(classAdvisors);
                                }
                            }
                            this.aspectBeanNames = aspectNames;
                            return advisors;
                        }
                    }
                }
            }
}

Advisor排序

public class ReflectiveAspectJAdvisorFactory extends AbstractAspectJAdvisorFactory implements Serializable {
        private static final Comparator<Method> adviceMethodComparator;

        static {

            Comparator<Method> adviceKindComparator = new ConvertingComparator<>(
                    //按照注解顺序设置方法对应的advisor的顺序
                    //在AspectJAfterAdvice里，会先将请求继续向拦截器链后传播，
                    //对增强方法的调用是在后面的finnaly块里。所以这里的After顺序即使在AfterReturning前面也没关系
                    //另外，因为在finnly块里触发，所以即使后续的调用抛出了未捕获的异常，After指定的增强方法也会被执行
                    new InstanceComparator<>(
                            Around.class, Before.class, After.class, AfterReturning.class, AfterThrowing.class),

                    (Converter<Method, Annotation>) method -> {
                            //如果方法上没有标注上面的几个注解，则返回null，null会排在最后
                        AspectJAnnotation<?> ann = AbstractAspectJAdvisorFactory.findAspectJAnnotationOnMethod(method);
                        return (ann != null ? ann.getAnnotation() : null);
                    });
            Comparator<Method> methodNameComparator = new ConvertingComparator<>(Method::getName);
            adviceMethodComparator = adviceKindComparator.thenComparing(methodNameComparator);
        }
}

判断method是否属于切面方法

public class ReflectiveAspectJAdvisorFactory extends AbstractAspectJAdvisorFactory implements Serializable {
        public Advisor getAdvisor(Method candidateAdviceMethod, MetadataAwareAspectInstanceFactory aspectInstanceFactory,
                int declarationOrderInAspect, String aspectName) {

                //获取切点信息，如果candidateAdviceMethod不是切面方法，则返回null
                AspectJExpressionPointcut expressionPointcut = getPointcut(
                        candidateAdviceMethod, aspectInstanceFactory.getAspectMetadata().getAspectClass());
                if (expressionPointcut == null) {
                    return null;
                }

                return new InstantiationModelAwarePointcutAdvisorImpl(expressionPointcut, candidateAdviceMethod,
                        this, aspectInstanceFactory, declarationOrderInAspect, aspectName);
        }   

        private AspectJExpressionPointcut getPointcut(Method candidateAdviceMethod, Class<?> candidateAspectClass) {
                //在方法上查找Aspectj的相关注解（Around、After等）
                AspectJAnnotation<?> aspectJAnnotation =
                        AbstractAspectJAdvisorFactory.findAspectJAnnotationOnMethod(candidateAdviceMethod);
                if (aspectJAnnotation == null) {
                    return null;
                }

                ...
        }
}

2. 生成代理对象

AbstractAutoProxyCreator实现了BeanPostProcessor，在创建Bean时，Spring会调用AbstractAutoProxyCreator#postProcessAfterInitialization对Bean进行处理

public abstract class AbstractAutoProxyCreator extends ProxyProcessorSupport
        implements SmartInstantiationAwareBeanPostProcessor, BeanFactoryAware {

        @Override
        public Object postProcessAfterInitialization(@Nullable Object bean, String beanName) {
            if (bean != null) {
                Object cacheKey = getCacheKey(bean.getClass(), beanName);
                if (this.earlyProxyReferences.remove(cacheKey) != bean) {
                    //对bean进行包装，返回代理bean
                    return wrapIfNecessary(bean, beanName, cacheKey);
                }
            }
            return bean;
        }

        protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
                //如果bean有TargetSourceCreator创建，说明已经被代理过了，直接返回
                if (StringUtils.hasLength(beanName) && this.targetSourcedBeans.contains(beanName)) {
                    return bean;
                }
                //拿出缓存的检测的结果进行判断
                if (Boolean.FALSE.equals(this.advisedBeans.get(cacheKey))) {
                    return bean;
                }
                //初步判断bean是否可以被代理
                if (isInfrastructureClass(bean.getClass()) || shouldSkip(bean.getClass(), beanName)) {
                    this.advisedBeans.put(cacheKey, Boolean.FALSE);
                    return bean;
                }

                //根据切点Point的表达式获得符合当前bean的所有advisor
                //如果当前bean不在切点的指向中，则返回DO_NOT_PROXY
                Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
                if (specificInterceptors != DO_NOT_PROXY) {
                    this.advisedBeans.put(cacheKey, Boolean.TRUE);
                    //创建代理对象，将所有advisor包装成DynamicAdvisedInterceptor，
                    //其intercept方法为所有增强方法的统一入口，这个类来自Spring
                    Object proxy = createProxy(
                            bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));
                    this.proxyTypes.put(cacheKey, proxy.getClass());
                    return proxy;
                }

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

选择代理对象的创建方式

public class DefaultAopProxyFactory implements AopProxyFactory, Serializable {  
        @Override
        public AopProxy createAopProxy(AdvisedSupport config) throws AopConfigException {
            if (!NativeDetector.inNativeImage() &&
                    (config.isOptimize() || config.isProxyTargetClass() || hasNoUserSuppliedProxyInterfaces(config))) {
                Class<?> targetClass = config.getTargetClass();

                //如果targetClass是接口类型或者是通过Proxy.getProxyClass生成的或者类名里包含$$Lambda
                //则使用JDK动态代理
                if (targetClass.isInterface() || Proxy.isProxyClass(targetClass) || ClassUtils.isLambdaClass(targetClass)) {
                    return new JdkDynamicAopProxy(config);
                }
                //使用cglib，ASM修改字节码的方式生产代理类
                return new ObjenesisCglibAopProxy(config);
            }
            else {
                return new JdkDynamicAopProxy(config);
            }
        }
}

3. 访问代理对象

DispatcherServlet 找到对应的实例跟方法后通过反射进行调用（前面Spring Mvc博文里已分析过）,此时会遍历代理对象上的所有MethodInterceptor,

public class ReflectiveMethodInvocation implements ProxyMethodInvocation, Cloneable {

        public Object proceed() throws Throwable {
            //如果拦截器遍历完了，则调用目标方法
            if (this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size() - 1) {
                return invokeJoinpoint();
            }

            //遍历所有增强器MethodInterceptor，
            //遍历方式是在MethodInterceptor里调用MethodInvocation#proceed
            //每次进入该方法，都会使currentInterceptorIndex增加1，从而达成遍历
            Object interceptorOrInterceptionAdvice =
                    this.interceptorsAndDynamicMethodMatchers.get(++this.currentInterceptorIndex);

            //调用MethodInterceptor#invoke
            return ((MethodInterceptor) interceptorOrInterceptionAdvice).invoke(this);
        }
}

增强方法实际执行的顺序图

Spring Aop增强方法调用顺序.png

接下来看看每个增强注解对应的MethodInterceptor处理类是怎么进行请求处理与传递的

1. AspectJAroundAdvice

public class AspectJAroundAdvice extends AbstractAspectJAdvice implements MethodInterceptor, Serializable {

    @Override
    @Nullable
    public Object invoke(MethodInvocation mi) throws Throwable {
        ...
        ProxyMethodInvocation pmi = (ProxyMethodInvocation) mi;
        ProceedingJoinPoint pjp = lazyGetProceedingJoinPoint(pmi);
        JoinPointMatch jpm = getJoinPointMatch(pmi);
        //调用Around对应的增强方法，并将mi传如增强方法
        //mi持有了所有的拦截/增强器信息，通过Joinpoint#proceed实现请求的传递
        //所以，around增强方法里需要注意接收Joinpont的实例并调用其proceed方法
        return invokeAdviceMethod(pjp, jpm, null, null);
    }

}

2. MethodBeforeAdviceInterceptor

public class MethodBeforeAdviceInterceptor implements MethodInterceptor, BeforeAdvice, Serializable {

    private final MethodBeforeAdvice advice;

    public MethodBeforeAdviceInterceptor(MethodBeforeAdvice advice) {
        Assert.notNull(advice, "Advice must not be null");
        this.advice = advice;
    }

    @Override
    @Nullable
    public Object invoke(MethodInvocation mi) throws Throwable {
        //调用before增强方法
        this.advice.before(mi.getMethod(), mi.getArguments(), mi.getThis());
        //向后传递
        return mi.proceed();
    }

}

3. AspectJAfterAdvice

public class AspectJAfterAdvice extends AbstractAspectJAdvice
        implements MethodInterceptor, AfterAdvice, Serializable {

        public AspectJAfterAdvice(
                Method aspectJBeforeAdviceMethod, AspectJExpressionPointcut pointcut, AspectInstanceFactory aif) {

            super(aspectJBeforeAdviceMethod, pointcut, aif);
        }

        @Override
        @Nullable
        public Object invoke(MethodInvocation mi) throws Throwable {
            try {
                //先向后传递
                return mi.proceed();
            }
            finally {
                //执行After方法
                invokeAdviceMethod(getJoinPointMatch(), null, null);
            }
        }

}

4. AfterReturningAdviceInterceptor

public class AfterReturningAdviceInterceptor implements MethodInterceptor, AfterAdvice, Serializable {

    private final AfterReturningAdvice advice;

    public AfterReturningAdviceInterceptor(AfterReturningAdvice advice) {
        Assert.notNull(advice, "Advice must not be null");
        this.advice = advice;
    }

    @Override
    @Nullable
    public Object invoke(MethodInvocation mi) throws Throwable {

        //向后传递
        Object retVal = mi.proceed();
        //调用AfterReturning方法
        this.advice.afterReturning(retVal, mi.getMethod(), mi.getArguments(), mi.getThis());
        return retVal;
    }

}

5. AspectJAfterThrowingAdvice

public class AspectJAfterThrowingAdvice extends AbstractAspectJAdvice
        implements MethodInterceptor, AfterAdvice, Serializable {

        public AspectJAfterThrowingAdvice(
                Method aspectJBeforeAdviceMethod, AspectJExpressionPointcut pointcut, AspectInstanceFactory aif) {

            super(aspectJBeforeAdviceMethod, pointcut, aif);
        }

        @Override
        @Nullable
        public Object invoke(MethodInvocation mi) throws Throwable {
            try {
                //向后传递
                return mi.proceed();
            }
            catch (Throwable ex) {
                //判断增强器上定义的异常类型是否匹配
                if (shouldInvokeOnThrowing(ex)) {
                    //调用AfterThrowing方法
                    invokeAdviceMethod(getJoinPointMatch(), null, ex);
                }
                throw ex;
            }
        }

        /**
         * In AspectJ semantics, after throwing advice that specifies a throwing clause
         * is only invoked if the thrown exception is a subtype of the given throwing type.
         */
        private boolean shouldInvokeOnThrowing(Throwable ex) {
            return getDiscoveredThrowingType().isAssignableFrom(ex.getClass());
        }

}

6. ExposeInvocationInterceptor

public final class ExposeInvocationInterceptor implements MethodInterceptor, PriorityOrdered, Serializable {

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

        @Override
        @Nullable
        public Object invoke(MethodInvocation mi) throws Throwable {
            MethodInvocation oldInvocation = invocation.get();
            //将MethodInvocation绑定到当前线程的本地变量里，
            //从而实现在其他地方访问MethodInvocation
            //但不建议这样做，因为AOP对于被代理对象来说应该是无感知的，不应该产生这种依赖
            invocation.set(mi);
            try {
                return mi.proceed();
            }
            finally {
                invocation.set(oldInvocation);
            }
        }

}