Spring源码-AOP

1、spring aop和aspectJ什么关系？

1. aop是编程思想，spring aop被aspectJ都是aop思想的具体实现。
2. spring aop为了不重复造轮子，通过一定的取舍选取了aspectJ中适合自己的注解。spring初期版本的aop只支持通过实现aop接口的方式来实现切面增强，后续版本增加支持XML配置方式，最新版本支持aspect的相关注解来定义切面。

2、spring aop的aspect注解引入

1. spring通过@EnableAspectJAutoProxy注解，采用@import+ImportBeanDefinitionRegistrar方式将AnnotationAwareAspectJAutoProxyCreator.class类注册到容器。
2. AnnotationAwareAspectJAutoProxyCreator实现了SmartInstantiationAwareBeanPostProcessor接口，通过重新BeanPostProcessor的postProcessBeforeInstantiation方法来做aop增强处理。

注：想要向spring容器注入自己的类对象除了这种方式外，还可以通过bean定义后置增强处理器BeanDefinitionRegistryPostProcessor进行注册。

class AspectJAutoProxyRegistrar implements ImportBeanDefinitionRegistrar {

	/**
	 * Register, escalate, and configure the AspectJ auto proxy creator based on the value
	 * of the @{@link EnableAspectJAutoProxy#proxyTargetClass()} attribute on the importing
	 * {@code @Configuration} class.
	 */
	@Override
	public void registerBeanDefinitions(
			AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {

		AopConfigUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary(registry);

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

}

AbstractAutoProxyCreator#postProcessBeforeInstantiation源码：

@SuppressWarnings("serial")
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)) {
			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.
		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;
	}
}