在：https://www.jianshu.com/p/366c3aad046f
分析了整个IOC生成bean实例的流程，实际上，spring aop也是在IOC的基础上去实现的。
在doCreateBean()方法的后半部分，也就是initializeBean方法中，会通过BeanPostProcessor对Bean做加强：
protected Object initializeBean(final String beanName, final Object bean, @Nullable RootBeanDefinition mbd) {
if (System.getSecurityManager() != null) {
AccessController.doPrivileged((PrivilegedAction这里会通过BeanPostProcessor，对bean做进一步的增强，而AOP的核心，就是一个BeanPostProcessor：
image.png
AbstractAutoProxyCreator 之 getEarlyBeanReference
在IOC中，为了解决单例模式下循环依赖的问题，spring会将实例化的bean提前暴露出来：
protected Object getEarlyBeanReference(String beanName, RootBeanDefinition mbd, Object bean) {
Object exposedObject = bean;
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof SmartInstantiationAwareBeanPostProcessor) {
SmartInstantiationAwareBeanPostProcessor ibp = (SmartInstantiationAwareBeanPostProcessor) bp;
exposedObject = ibp.getEarlyBeanReference(exposedObject, beanName);
}
}
}
return exposedObject;
}
简单来说就是通过SmartInstantiationAwareBeanPostProcessor.getEarlyBeanReference方法获取增强后的bean，而AbstractAutoProxyCreator同样是SmartInstantiationAwareBeanPostProcessor的实现类，其中实现了getEarlyBeanReference方法：
@Override
public Object getEarlyBeanReference(Object bean, String beanName) {
Object cacheKey = getCacheKey(bean.getClass(), beanName);
this.earlyProxyReferences.put(cacheKey, bean);
return wrapIfNecessary(bean, beanName, cacheKey);
}
这里面会将bean放入缓存中，然后通过wrapIfNecessary对bean进行AOP包装，并将包装后的bean返回出去，wrapIfNecessary的逻辑我们后边再说。
AbstractAutoProxyCreator 之 postProcessAfterInitialization
getEarlyBeanReference是为了解决提前暴露的问题，而postProcessAfterInitialization则是AOP正常流程的入口：
@Override
public Object postProcessAfterInitialization(@Nullable Object bean, String beanName) {
if (bean != null) {
Object cacheKey = getCacheKey(bean.getClass(), beanName);
// 如果earlyProxyReferences中没有bean 则 wrapIfNecessary
if (this.earlyProxyReferences.remove(cacheKey) != bean) {
return wrapIfNecessary(bean, beanName, cacheKey);
}
}
return bean;
}
可以看到，如果earlyProxyReferences中不含有改bean， 则remove一定会返null 从而执行wrapIfNecessary对bean进行包装。
AOP包装流程
protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
// 关于targetSourcedBeans 先不去管它 有兴趣可以自己查下
if (StringUtils.hasLength(beanName) && this.targetSourcedBeans.contains(beanName)) {
return bean;
}
// 不需要创建 直接返回
if (Boolean.FALSE.equals(this.advisedBeans.get(cacheKey))) {
return bean;
}
// 这里shouldSkip也有加载所有advisor的作用
// 有兴趣可以去AspectJAwareAdvisorAutoProxyCreator类看下实现
if (isInfrastructureClass(bean.getClass()) || shouldSkip(bean.getClass(), beanName)) {
this.advisedBeans.put(cacheKey, Boolean.FALSE);
return bean;
}
// 上面做了一些不需要创建代理的情况的判断
// 返回匹配当前 bean 的所有的 advisor、advice、interceptor
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;
}
// 标识不需要代理逻辑
this.advisedBeans.put(cacheKey, Boolean.FALSE);
return bean;
}
这里会通过createProxy 方法创建代理对象：
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ProxyFactory 实例
ProxyFactory proxyFactory = new ProxyFactory();
proxyFactory.copyFrom(this);
if (!proxyFactory.isProxyTargetClass()) {
if (shouldProxyTargetClass(beanClass, beanName)) {
proxyFactory.setProxyTargetClass(true);
}
else {
evaluateProxyInterfaces(beanClass, proxyFactory);
}
}
// 返回匹配了当前bean的Advisor
Advisor[] advisors = buildAdvisors(beanName, specificInterceptors);
proxyFactory.addAdvisors(advisors);
proxyFactory.setTargetSource(targetSource);
customizeProxyFactory(proxyFactory);
proxyFactory.setFrozen(this.freezeProxy);
if (advisorsPreFiltered()) {
proxyFactory.setPreFiltered(true);
}
return proxyFactory.getProxy(getProxyClassLoader());
}
这里主要就是创建ProxyFactory并且通过proxyFactory.getProxy生成代理对象；
public Object getProxy(@Nullable ClassLoader classLoader) {
return createAopProxy().getProxy(classLoader);
}
protected final synchronized AopProxy createAopProxy() {
if (!this.active) {
activate();
}
return getAopProxyFactory().createAopProxy(this);
}
@Override
public AopProxy createAopProxy(AdvisedSupport config) throws AopConfigException {
if (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.");
}
// 如果要代理的类本身就是接口，则使用 JDK 动态代理
if (targetClass.isInterface() || Proxy.isProxyClass(targetClass)) {
return new JdkDynamicAopProxy(config);
}
return new ObjenesisCglibAopProxy(config);
}
else {
// 如果有接口，会跑到这个分支
return new JdkDynamicAopProxy(config);
}
}
大体上就是 实现了接口，则使用JDK动态代理，否则使用cglib动态代理。
后面两个具体实现代理就不去细说了，有兴趣的可以了解下jdk动态代理跟cglib是如何实现的。
参考：
https://www.javadoop.com/post/spring-aop-source