Spring 集成 AspectJ的原理

今天试着用一下Spring + AspectJ的AOP，遇到了点问题，没有拦截到Bean的方法，比如controller里面的方法。网上搜索了大堆东西，都没有什么用了，而且看Spring官网的Reference也没有看出个什么问题。这种情况下只好去看源码分析一下了，首先我们使用的是注解的方式：

spring的beans.xml配置文件如下：

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:context="http://www.springframework.org/schema/context"
       xmlns:mvc="http://www.springframework.org/schema/mvc"
       xmlns:aop="http://www.springframework.org/schema/aop"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-4.2.xsd 
        http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd
        http://www.springframework.org/schema/mvc http://www.springframework.org/schema/mvc/spring-mvc.xsd
        http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop.xsd"
        >
    
    <context:component-scan base-package="org.mosmith.togethertravel" />
    ....

    <aop:aspectj-autoproxy />
</beans>

其实上面的的<aop:aspectj-autoroxy /> 对应了注解@EnableAspectJAutoProxy。从下面EnableAspectJAutoProxy的定义可以看到，注解导入了AspectJAutoProxyRegistrar配置类。

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Import(AspectJAutoProxyRegistrar.class)
public @interface EnableAspectJAutoProxy {

    /**
     * Indicate whether subclass-based (CGLIB) proxies are to be created as opposed
     * to standard Java interface-based proxies. The default is {@code false}.
     */
    boolean proxyTargetClass() default false;

}

AspectJAutoProxyRegistrar主要在BeanDefinitionRegistry里面注册了AnnotationAwareAspectJAutoProxyCreator。从AnnotationAwareAspectJAutoProxyCreator的继承结构可以看出AnnotationAwareAspectJAutoProxyCreator是一个BeanPostProcessor，类似EnableWebMvc，这个类用来整合AspectJ和Spring的IOC容器。

我们跳到继承结构上与AspectJ相关的annotation处理的相关方法，这个方法重写了父类的findCandidateadvisors（父类主要处理xml方式配置的advisor，这里添加了对注解声明的Adivisor的扫描，我们跳到BeanFactoryAspectJAdvisorsBuilder里面的buildAspectJAdvisors方法，从这个方法可以看到它从BeanFactory，也就是我们的Context里面取出所有的Bean，然后检查这个Bean是否有@Aspect注解，如果有的话则构建Advisor元数据，这里不直接构建Advisor实例，因为它由Spring IOC容器来管理。

public class AnnotationAwareAspectJAutoProxyCreator extends AspectJAwareAdvisorAutoProxyCreator {
        ...
    @Override
    protected List<Advisor> findCandidateAdvisors() {
        // Add all the Spring advisors found according to superclass rules.
        List<Advisor> advisors = super.findCandidateAdvisors();
        // Build Advisors for all AspectJ aspects in the bean factory.
        advisors.addAll(this.aspectJAdvisorsBuilder.buildAspectJAdvisors());
        return advisors;
    }
        ...
}

/**
 * Helper for retrieving @AspectJ beans from a BeanFactory and building
 * Spring Advisors based on them, for use with auto-proxying.
 *
 * @author Juergen Hoeller
 * @since 2.0.2
 * @see AnnotationAwareAspectJAutoProxyCreator
 */
public class BeanFactoryAspectJAdvisorsBuilder {

        ....
    /**
     * Look for AspectJ-annotated aspect beans in the current bean factory,
     * and return to a list of Spring AOP Advisors representing them.
     * <p>Creates a Spring Advisor for each AspectJ advice method.
     * @return the list of {@link org.springframework.aop.Advisor} beans
     * @see #isEligibleBean
     */
    public List<Advisor> buildAspectJAdvisors() {
        List<String> aspectNames = null;

        synchronized (this) {
            aspectNames = this.aspectBeanNames;
            if (aspectNames == null) {
                List<Advisor> advisors = new LinkedList<Advisor>();
                aspectNames = new LinkedList<String>();
                String[] beanNames =
                        BeanFactoryUtils.beanNamesForTypeIncludingAncestors(this.beanFactory, Object.class, true, false);
                for (String beanName : beanNames) {
                    if (!isEligibleBean(beanName)) {
                        continue;
                    }
                    // We must be careful not to instantiate beans eagerly as in this
                    // case they would be cached by the Spring container but would not
                    // have been weaved
                    Class<?> beanType = this.beanFactory.getType(beanName);
                    if (beanType == null) {
                        continue;
                    }
                    if (this.advisorFactory.isAspect(beanType)) {
                        aspectNames.add(beanName);
                        AspectMetadata amd = new AspectMetadata(beanType, beanName);
                        if (amd.getAjType().getPerClause().getKind() == PerClauseKind.SINGLETON) {
                            MetadataAwareAspectInstanceFactory factory =
                                    new BeanFactoryAspectInstanceFactory(this.beanFactory, beanName);
                            List<Advisor> classAdvisors = this.advisorFactory.getAdvisors(factory);
                            if (this.beanFactory.isSingleton(beanName)) {
                                this.advisorsCache.put(beanName, classAdvisors);
                            }
                            else {
                                this.aspectFactoryCache.put(beanName, factory);
                            }
                            advisors.addAll(classAdvisors);
                        }
                        else {
                            // Per target or per this.
                            if (this.beanFactory.isSingleton(beanName)) {
                                throw new IllegalArgumentException("Bean with name '" + beanName +
                                        "' is a singleton, but aspect instantiation model is not singleton");
                            }
                            MetadataAwareAspectInstanceFactory factory =
                                    new PrototypeAspectInstanceFactory(this.beanFactory, beanName);
                            this.aspectFactoryCache.put(beanName, factory);
                            advisors.addAll(this.advisorFactory.getAdvisors(factory));
                        }
                    }
                }
                this.aspectBeanNames = aspectNames;
                return advisors;
            }
        }

        if (aspectNames.isEmpty()) {
            return Collections.emptyList();
        }
        List<Advisor> advisors = new LinkedList<Advisor>();
        for (String aspectName : aspectNames) {
            List<Advisor> cachedAdvisors = this.advisorsCache.get(aspectName);
            if (cachedAdvisors != null) {
                advisors.addAll(cachedAdvisors);
            }
            else {
                MetadataAwareAspectInstanceFactory factory = this.aspectFactoryCache.get(aspectName);
                advisors.addAll(this.advisorFactory.getAdvisors(factory));
            }
        }
        return advisors;
    }
    ....
}

我们再去看一下Bean在实例化的时候，Spring通过getAdvicesAndAdvisorsForBean方法取出拦截此bean的Advisor Bean，然后创建代理，然后返回代理，这样子就可以拦截到这个bean的方法调用了。但通过调试发现这里并不是AspectJ实现拦截的地方，AspectJ实现拦截的地方在AbstractAutoProxyCreator.postProcessAfterInitialization方法，它调用wrapIfNecessary方法来对Bean进行Wrap，Wrapper类是利用cglib动态生成的子类。

@SuppressWarnings("serial")
public abstract class AbstractAutoProxyCreator extends ProxyProcessorSupport
        implements SmartInstantiationAwareBeanPostProcessor, BeanFactoryAware {
    ....
    @Override
    public Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) throws BeansException {
        Object cacheKey = getCacheKey(beanClass, beanName);

        if (beanName == null || !this.targetSourcedBeans.contains(beanName)) {
            if (this.advisedBeans.containsKey(cacheKey)) {
                return null;
            }
            if (isInfrastructureClass(beanClass) || shouldSkip(beanClass, beanName)) {
                this.advisedBeans.put(cacheKey, Boolean.FALSE);
                return null;
            }
        }

        // Create proxy here if we have a custom TargetSource.
        // Suppresses unnecessary default instantiation of the target bean:
        // The TargetSource will handle target instances in a custom fashion.
        if (beanName != null) {
            TargetSource targetSource = getCustomTargetSource(beanClass, beanName);
            if (targetSource != null) {
                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;
    }
    ....


    /**
     * 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(Object bean, String beanName) throws BeansException {
        if (bean != null) {
            Object cacheKey = getCacheKey(bean.getClass(), beanName);
            if (!this.earlyProxyReferences.contains(cacheKey)) {
                return wrapIfNecessary(bean, beanName, cacheKey);
            }
        }
        return bean;
    }

    ....
}