前言
spring-aop技术是对oop(面向对象)的一个补充，其底层其实就是使用aspect+动态代理进行实现的，本篇文章将大概讨论下aop的核心实现流程
相关的核心概念
刚开始，先介绍下aop中比较核心的一些对象和概念，只要理解了这些，后面就可以很容易理解aop是怎么工作的了
Advisor接口
Advisor接口包含了Advice和Pointcut的定义，而这两个对象是我们进行代理的核心对象，举个例子你就明白了，如下
public class UserService{
public void a(){
}
public void b(){
}
}
假设有上面的类对象，现在有一段切入逻辑如下
public void method(){
"插入方法逻辑";
}
这是一段要插入到UserService类的a和b方法的逻辑，那么在这个场景下
Advice: 指的就是method这段逻辑
PonitCut: 假设只想要在a方法切入，不想在b方法切入，那么这个校验过程就是PonitCut的代表ProxyFactory对象
因为spring对于aop实现底层就是用代理实现的，所以底层会有这个对象进行管理代理对象，代理方法目前有两种，JdkDynamicAopProxy和ObjenesisCglibAopProxy(cglib)
这个对象主要就是用来获取代理对象的，其他的似乎没啥说的，因为核心逻辑还是在代理对象中进行实现的，下面以两个的其中一个来进行说明
JdkDynamicAopProxy
基于jdk的动态代理，Java基础扎实的人肯定都会知道他会实现一个InvocationHandler接口，该接口是代理对象最终会执行的入口，我们就来看看他的invoke接口具体干了啥，如下
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
Object oldProxy = null;
boolean setProxyContext = false;
//这里简单理解就是获取被代理的类，比如这里就是UserService
TargetSource targetSource = this.advised.targetSource;
Object target = null;
try {
//判断如果是以下这些方法时不需要进行代理
if (!this.equalsDefined && AopUtils.isEqualsMethod(method)) {
return equals(args[0]);
}
else if (!this.hashCodeDefined && AopUtils.isHashCodeMethod(method)) {
return hashCode();
}
else if (method.getDeclaringClass() == DecoratingProxy.class) {
return AopProxyUtils.ultimateTargetClass(this.advised);
}
else if (!this.advised.opaque && method.getDeclaringClass().isInterface() &&
method.getDeclaringClass().isAssignableFrom(Advised.class)) {
return AopUtils.invokeJoinpointUsingReflection(this.advised, method, args);
}
//下面真正进行代理逻辑的处理
Object retVal;
//是否把当前的代理对象绑定到当前线程，就是放到ThreadLocal中
if (this.advised.exposeProxy) {
oldProxy = AopContext.setCurrentProxy(proxy);
setProxyContext = true;
}
target = targetSource.getTarget();
Class<?> targetClass = (target != null ? target.getClass() : null);
// 获取所有关联的advisor对象，也就是上面刚说的很核心的对象
List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);
if (chain.isEmpty()) {
//如果为空表示没有任何的代理逻辑，直接返回即可
Object[] argsToUse = AopProxyUtils.adaptArgumentsIfNecessary(method, args);
retVal = AopUtils.invokeJoinpointUsingReflection(target, method, argsToUse);
}
else {
//如果不为空把数据封装为ReflectiveMethodInvocation对象，然后调用proceed方法，开始处理代理逻辑
MethodInvocation invocation =
new ReflectiveMethodInvocation(proxy, target, method, args, targetClass, chain);
// Proceed to the joinpoint through the interceptor chain.
retVal = invocation.proceed();
}
// 处理返回值，不是核心方法，，
Class<?> returnType = method.getReturnType();
if (retVal != null && retVal == target &&
returnType != Object.class && returnType.isInstance(proxy) &&
!RawTargetAccess.class.isAssignableFrom(method.getDeclaringClass())) {
retVal = proxy;
}
else if (retVal == null && returnType != Void.TYPE && returnType.isPrimitive()) {
throw new AopInvocationException(
"Null return value from advice does not match primitive return type for: " + method);
}
return retVal;
}
finally {
if (target != null && !targetSource.isStatic()) {
// Must have come from TargetSource.
targetSource.releaseTarget(target);
}
if (setProxyContext) {
// 移除绑定关系
AopContext.setCurrentProxy(oldProxy);
}
}
}以上可以看到核心的处理流程，那么问题就来到了两个细节方法了，接下来分析下下面的问题
首先，来看下 this.advised.getInterceptorsAndDynamicInterceptionAdvice这个方法，这个方法是来获取所有的Advisor对象的，也可以理解为获取Advice对象，因为最终的代理逻辑就是在Advice里面的
public List<Object> getInterceptorsAndDynamicInterceptionAdvice(
Advised config, Method method, @Nullable Class<?> targetClass) {
AdvisorAdapterRegistry registry = GlobalAdvisorAdapterRegistry.getInstance();
//获取所有的Advisor，config其实就是ProxyFactory对象，在生成的时候已经绑定进来了
Advisor[] advisors = config.getAdvisors();
List<Object> interceptorList = new ArrayList<>(advisors.length);
Class<?> actualClass = (targetClass != null ? targetClass : method.getDeclaringClass());
Boolean hasIntroductions = null;
//循环处理
for (Advisor advisor : advisors) {
if (advisor instanceof PointcutAdvisor) {
// Add it conditionally.
PointcutAdvisor pointcutAdvisor = (PointcutAdvisor) advisor;
//粗筛，可以理解为对UserService类的筛选
if (config.isPreFiltered() || pointcutAdvisor.getPointcut().getClassFilter().matches(actualClass)) {
//方法匹配器，进一步对UserService类下面的方法进行筛选
MethodMatcher mm = pointcutAdvisor.getPointcut().getMethodMatcher();
boolean match;
if (mm instanceof IntroductionAwareMethodMatcher) {
if (hasIntroductions == null) {
hasIntroductions = hasMatchingIntroductions(advisors, actualClass);
}
match = ((IntroductionAwareMethodMatcher) mm).matches(method, actualClass, hasIntroductions);
}
else {
//是否匹配
match = mm.matches(method, actualClass);
}
if (match) {
//如果是的话会加入到这个逻辑里
//把数据包装为MethodInterceptor对象返回
MethodInterceptor[] interceptors = registry.getInterceptors(advisor);
if (mm.isRuntime()) {
// MethodMatcher是否配置了运行时还要校验，如果是的话到时具体执行的时候还会根据实时参数进一步筛选
for (MethodInterceptor interceptor : interceptors) {
interceptorList.add(new InterceptorAndDynamicMethodMatcher(interceptor, mm));
}
}
else {
interceptorList.addAll(Arrays.asList(interceptors));
}
}
}
}
else if (advisor instanceof IntroductionAdvisor) {
IntroductionAdvisor ia = (IntroductionAdvisor) advisor;
if (config.isPreFiltered() || ia.getClassFilter().matches(actualClass)) {
Interceptor[] interceptors = registry.getInterceptors(advisor);
interceptorList.addAll(Arrays.asList(interceptors));
}
}
else {
Interceptor[] interceptors = registry.getInterceptors(advisor);
interceptorList.addAll(Arrays.asList(interceptors));
}
}
return interceptorList;
}以上就是筛选匹配的Advisor对象，最终会统一封装为MethodInterceptor列表返回，可以进一步来看看这个，这里用了一个适配器模式，还是比较值得看看的
DefaultAdvisorAdapterRegistry#getInterceptors方法中进行处理的
@Override
public MethodInterceptor[] getInterceptors(Advisor advisor) throws UnknownAdviceTypeException {
List<MethodInterceptor> interceptors = new ArrayList<>(3);
//获取对应的Advice
Advice advice = advisor.getAdvice();
//如果是MethodInterceptor直接加入即可
if (advice instanceof MethodInterceptor) {
interceptors.add((MethodInterceptor) advice);
}
//如果不是会使用AdvisorAdapter对象进行对比并且加入
for (AdvisorAdapter adapter : this.adapters) {
if (adapter.supportsAdvice(advice)) {
interceptors.add(adapter.getInterceptor(advisor));
}
}
if (interceptors.isEmpty()) {
throw new UnknownAdviceTypeException(advisor.getAdvice());
}
return interceptors.toArray(new MethodInterceptor[0]);
}AdvisorAdapter是一种适配器，用于对原来不是MethodInterceptor的对象进行适配为MethodInterceptor对象，可以看看它的接口定义
/**
* Interface allowing extension to the Spring AOP framework to allow
* handling of new Advisors and Advice types.
*
* <p>Implementing objects can create AOP Alliance Interceptors from
* custom advice types, enabling these advice types to be used
* in the Spring AOP framework, which uses interception under the covers.
*
* <p>There is no need for most Spring users to implement this interface;
* do so only if you need to introduce more Advisor or Advice types to Spring.
*
* @author Rod Johnson
*/
public interface AdvisorAdapter {
/**
* Does this adapter understand this advice object? Is it valid to
* invoke the {@code getInterceptors} method with an Advisor that
* contains this advice as an argument?
* @param advice an Advice such as a BeforeAdvice
* @return whether this adapter understands the given advice object
* @see #getInterceptor(org.springframework.aop.Advisor)
* @see org.springframework.aop.BeforeAdvice
*/
boolean supportsAdvice(Advice advice);
/**
* Return an AOP Alliance MethodInterceptor exposing the behavior of
* the given advice to an interception-based AOP framework.
* <p>Don't worry about any Pointcut contained in the Advisor;
* the AOP framework will take care of checking the pointcut.
* @param advisor the Advisor. The supportsAdvice() method must have
* returned true on this object
* @return an AOP Alliance interceptor for this Advisor. There's
* no need to cache instances for efficiency, as the AOP framework
* caches advice chains.
*/
MethodInterceptor getInterceptor(Advisor advisor);
}
只有两个方法，一个是否支持，一个获取到MethodInterceptor对象，接下来可以看看其中一个实现类，如下
class MethodBeforeAdviceAdapter implements AdvisorAdapter, Serializable {
@Override
public boolean supportsAdvice(Advice advice) {
//是否为MethodBeforeAdvice类型的
return (advice instanceof MethodBeforeAdvice);
}
@Override
public MethodInterceptor getInterceptor(Advisor advisor) {
MethodBeforeAdvice advice = (MethodBeforeAdvice) advisor.getAdvice();
//把advice适配为 MethodInterceptor 对象
return new MethodBeforeAdviceInterceptor(advice);
}
}这种设计模式很巧妙，很多地方都会用到这个技术，比如gateway也会有相应的使用
AnnotationAwareAspectJAutoProxyCreator
自动处理aop逻辑的处理器，它实现了BeanPostProcessor接口，也就是说，所有bean的创建都会经过这个类，它的大概逻辑如下
1 .bean创建的时候经常这个处理器
2. 查找当前bean匹配的Advisor对象，分为两种方式，一种是直接从ioc容器中获取Advisor对象，一种是通过对@Aspect进行解析里面的方法
3. 如果没有配置的就直接返回
4. 如果有一个及以上匹配的Advisor就使用ProxyFactory进行创建代理对象
5. 最终整个流程就被串起来了
到此，aop的核心流程就已经说完了，最后的问题，AnnotationAwareAspectJAutoProxyCreator这个类是如何加载到ioc容器的，其实很简单，我们在使用aop的时候，会标记以下的注解以启用aop功能 @EnableAspectJAutoProxy，这个注解会往容器中导入AspectJAutoProxyRegistrar对象，而AspectJAutoProxyRegistrar对象又会注册AnnotationAwareAspectJAutoProxyCreator对象，最终，整个流程就串起来了
结语
整个流程其实并不复杂，只要理解了核心的几个对象，加上些许的Ioc容器工作过程，就可以理解了，源码的入口其实就是在AnnotationAwareAspectJAutoProxyCreator这个对象的后置处理方法中进行的，主要是这个方法AbstractAutoProxyCreator#wrapIfNecessary进行了包装，当然了这个还会涉及到循环依赖问题，这里就不说了
最后再放一张我自己画的简单流程图，如下。。。
