AOP的底层原理:
1、创建AnnotationAwareAspectJAutoProxyCreator
在容器启动时会注册配置类，并调用refresh()方法，创建容器中的所有bean并装配bean的属性和功能等
public AnnotationConfigApplicationContext(Class<?>... annotatedClasses) {
this();
register(annotatedClasses);
refresh();
}容器刷新做的操作：
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// 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);
// Invoke factory processors registered as beans in the context.
invokeBeanFactoryPostProcessors(beanFactory);
// Register bean processors that intercept bean creation.
registerBeanPostProcessors(beanFactory);
// Initialize message source for this context.
initMessageSource();
// Initialize event multicaster for this context.
initApplicationEventMulticaster();
// Initialize other special beans in specific context subclasses.
onRefresh();
// Check for listener beans and register them.
registerListeners();
// Instantiate all remaining (non-lazy-init) singletons.
finishBeanFactoryInitialization(beanFactory);
// Last step: publish corresponding event.
finishRefresh();
}
catch (BeansException ex) {
if (logger.isWarnEnabled()) {
logger.warn("Exception encountered during context initialization - " +
"cancelling refresh attempt: " + ex);
}
// Destroy already created singletons to avoid dangling resources.
destroyBeans();
// Reset 'active' flag.
cancelRefresh(ex);
// Propagate exception to caller.
throw ex;
}
finally {
// Reset common introspection caches in Spring's core, since we
// might not ever need metadata for singleton beans anymore...
resetCommonCaches();
}
}
}有一处是注册bean的后置处理器(registerBeanPostProcessors(beanFactory);包括自定义的后置处理器和Spring自带的后置处理器)，用来拦截bean的创建：
public static void registerBeanPostProcessors(
ConfigurableListableBeanFactory beanFactory, AbstractApplicationContext applicationContext) {
String[] postProcessorNames = beanFactory.getBeanNamesForType(BeanPostProcessor.class, true, false);
// Register BeanPostProcessorChecker that logs an info message when
// a bean is created during BeanPostProcessor instantiation, i.e. when
// a bean is not eligible for getting processed by all BeanPostProcessors.
int beanProcessorTargetCount = beanFactory.getBeanPostProcessorCount() + 1 + postProcessorNames.length;
beanFactory.addBeanPostProcessor(new BeanPostProcessorChecker(beanFactory, beanProcessorTargetCount));
// Separate between BeanPostProcessors that implement PriorityOrdered,
// Ordered, and the rest.
List<BeanPostProcessor> priorityOrderedPostProcessors = new ArrayList<>();
List<BeanPostProcessor> internalPostProcessors = new ArrayList<>();
List<String> orderedPostProcessorNames = new ArrayList<>();
List<String> nonOrderedPostProcessorNames = new ArrayList<>();
for (String ppName : postProcessorNames) {
if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
priorityOrderedPostProcessors.add(pp);
if (pp instanceof MergedBeanDefinitionPostProcessor) {
internalPostProcessors.add(pp);
}
}
else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {
orderedPostProcessorNames.add(ppName);
}
else {
nonOrderedPostProcessorNames.add(ppName);
}
}
// First, register the BeanPostProcessors that implement PriorityOrdered.
sortPostProcessors(priorityOrderedPostProcessors, beanFactory);
registerBeanPostProcessors(beanFactory, priorityOrderedPostProcessors);
// Next, register the BeanPostProcessors that implement Ordered.
List<BeanPostProcessor> orderedPostProcessors = new ArrayList<>();
for (String ppName : orderedPostProcessorNames) {
BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
orderedPostProcessors.add(pp);
if (pp instanceof MergedBeanDefinitionPostProcessor) {
internalPostProcessors.add(pp);
}
}
sortPostProcessors(orderedPostProcessors, beanFactory);
registerBeanPostProcessors(beanFactory, orderedPostProcessors);
// Now, register all regular BeanPostProcessors.
List<BeanPostProcessor> nonOrderedPostProcessors = new ArrayList<>();
for (String ppName : nonOrderedPostProcessorNames) {
BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
nonOrderedPostProcessors.add(pp);
if (pp instanceof MergedBeanDefinitionPostProcessor) {
internalPostProcessors.add(pp);
}
}
registerBeanPostProcessors(beanFactory, nonOrderedPostProcessors);
// Finally, re-register all internal BeanPostProcessors.
sortPostProcessors(internalPostProcessors, beanFactory);
registerBeanPostProcessors(beanFactory, internalPostProcessors);
// Re-register post-processor for detecting inner beans as ApplicationListeners,
// moving it to the end of the processor chain (for picking up proxies etc).
beanFactory.addBeanPostProcessor(new ApplicationListenerDetector(applicationContext));
}该方法首先会获取到配置类中所有bean的后置处理器
给容器添加别的后置处理器类
优先注册实现了PriorityOrdered接口的BeanPostProcessor
再注册实现了Ordered接口的BeanPostProcessor
注册没实现优先级接口(Ordered)的BeanPostProcessor
注册BeanPostProcessor实际上就是有则从BeanFactory中获取，没有则创建BeanPostProcessor对象,并将其置于容器中,步骤：
1、创建bean实例
2、populateBean():给bean的各种属性赋值
3、initializeBean():初始化bean，bean的后置处理器就是在该方法的前后工作的，初始化时：
①调用invokeAwareMethods,处理Aware接口的方法回调
②调用applyBeanPostProcessorBeforeInitialization()方法，运行所有后置处理器的postProcessBeforeInitialization前置方法
③调用invokeInitMethods，执行bean的初始化
④调用applyBeanPostProcessorAfterInitialization()方法，运行所有后置处理器的postProcessAfterInitialization后置方法
4、把BeanPostProcessor注册到BeanFactory中：beanFactory.addBeanPostProcessor(beanPostProcessor);
以上是创建AnnotationAwareAspectJAutoProxyCreator的过程
2、完成BeanFactory的初始化工作，调用finishBeanFactoryInitialization(beanFactory)
①遍历获取容器中所有的bean，依次创建所有的bean，此处的创建bean是先从缓存中查询bean，查询不到才创建，以此保证单实例
②创建bean之前先调用resolveBeforeInstantiation()，希望后置处理器在此返回一个代理对象，如果能返回代理对象则使用该代理对象，否则继续，在此处会判断该Bean是否是InstantiationAwareBeanPostProcessor，如果是的话则会创建一个代理对象，这就是BeanPostProcessor和InstantiationAwareBeanPostProcessor的区别，BeanPostProcessor是在bean创建完成之后初始化的前后调用其前置和后置方法，而InstantiationAwareBeanPostProcessor是在创建bean之前调用的，而AnnotationAwareAspectJAutoProxyCreator父类实现了InstantiationAwareBeanPostProcessor接口，所以会返回一个代理对象，而此代理对象是在其他bean创建之前返回的，因此它可以将其方法穿插到其他bean的前后，而实现前置通知、后置通知等
③调用doCreateBean，真正的创建一个bean实例
创建bean实例
populateBean():给bean的各种属性赋值
initializeBean():初始化bean，bean的后置处理器就是在该方法的前后工作的，初始化时
调用invokeAwareMethods,处理Aware接口的方法回调
调用applyBeanPostProcessorBeforeInitialization()方法，运行所有后置处理器的postProcessBeforeInitialization前置方法
调用invokeInitMethods，执行bean的初始化
调用applyBeanPostProcessorAfterInitialization()方法，运行所有后置处理器的postProcessAfterInitialization后置方法