上文讲了spring是如何开启AOP的，简单点说就是将AnnotationAwareAspectJAutoProxyCreator这个类注册到容器中，因为这个类最终实现了BeanPostProcessor接口，并且在其postProcessAfterInitialization()方法中完成了AOP代理对象的创建，创建时机则是在bean的init方法被执行之后即bean初始化完成之后。postProcessAfterInitialization()方法是重点，本文及下一篇文章都是围绕着这个方法来的。
该方法是实现在其父类中的AbstractAutoProxyCreator，我们先来看一下其实现：
public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
if (bean != null) {
// 根据给定的bean的class和name构件key，格式：beanClassName_beanName
Object cacheKey = getCacheKey(bean.getClass(), beanName);
if (!this.earlyProxyReferences.containsKey(cacheKey)) {
// 如果它适合被代理，则在这里面生成代理类
return wrapIfNecessary(bean, beanName, cacheKey);
}
}
return bean;
}
protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
// 如果已经处理过则不再处理
if (beanName != null && this.targetSourcedBeans.containsKey(beanName)) {
return bean;
}
// 无需增强则直接返回
if (Boolean.FALSE.equals(this.advisedBeans.get(cacheKey))) {
return bean;
}
// 指定的bean类是否代表一个基础设施类，基础设施类不应被代理，或者配置了指定bean也不需要自动代理
if (isInfrastructureClass(bean.getClass()) || shouldSkip(bean.getClass(), beanName)) {
this.advisedBeans.put(cacheKey, Boolean.FALSE);
return bean;
}
// 如果存在Advice则创建代理
Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
// 如果获取到了增强则需要针对增强创建代理
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;
}
this.advisedBeans.put(cacheKey, Boolean.FALSE);
return bean;
}
在上面的函数中我们已经可以看到代理创建的雏形，当然，在真正开始之前还需要经过一系列判断，比如是否已经处理过或者是否是需要跳过的bean，而真正创建代理是从getAdvicesAndAdvisorsForBean开始的。
创建代理主要包含了两个步骤：
- 获取增强方法或者增强器；
- 根据获取的增强进行代理创建；
这里虽然看起来只有简单的两步，但是每一步中都有着大量复杂的逻辑。本文先来看看获取增强方法的实现逻辑。
protected Object[] getAdvicesAndAdvisorsForBean(Class beanClass, String beanName, TargetSource targetSource) {
List advisors = findEligibleAdvisors(beanClass, beanName);
if (advisors.isEmpty()) {
return DO_NOT_PROXY;
}
return advisors.toArray();
}
protected List<Advisor> findEligibleAdvisors(Class beanClass, String beanName) {
List<Advisor> candidateAdvisors = findCandidateAdvisors();
List<Advisor> eligibleAdvisors = findAdvisorsThatCanApply(candidateAdvisors, beanClass, beanName);
extendAdvisors(eligibleAdvisors);
if (!eligibleAdvisors.isEmpty()) {
eligibleAdvisors = sortAdvisors(eligibleAdvisors);
}
return eligibleAdvisors;
}
对于某个bean的增强方法的获取也是包含两个步骤的，获取所有的增强以及寻找所有增强中适用于bean的增强并应用，而findCandidateAdvisors()与findAdvisorsThatCanApply()便是做了这两件事情。这里如果无法找到对应的增强器则直接返回DO_NOT_PROXY，也就是null。
1. 获取增强器
这里我们分析的是注解方式的AOP，对于findCandidateAdvisors的实现其实是由AnnotationAwareAspectJAutoProxyCreator类来完成的，继续跟踪：
protected List<Advisor> findCandidateAdvisors() {
// 使用注解方式配置AOP的时候并不会丢弃对XML配置的支持，在这里会调用父类方法去加载配置文件中的AOP声明
List<Advisor> advisors = super.findCandidateAdvisors();
// Build Advisors for all AspectJ aspects in the bean factory.
advisors.addAll(this.aspectJAdvisorsBuilder.buildAspectJAdvisors());
return advisors;
}
AnnotationAwareAspectJAutoProxyCreator间接继承了AbstractAdvisorAutoProxyCreator，在实现获取增强的方法中除了保留父类的获取配置文件中定义的增强外，同时增加了获取Bean的注解增强的功能，这部分实现是由this.aspectJAdvisorsBuilder.buildAspectJAdvisors()来完成的。
这里其实可以自己先尝试想象一下解析思路，看看自己的实现与Spring是否有差别？我们可以先在自己头脑中尝试实现一下获取增强这个功能点，看看是否有思路。实际上，Spring在实现的时候主要分成了四步：
- 获取所有beanName，这一步骤中所有在beanFactory中注册的Bean都会被提取出来；
- 遍历所有beanName，并找出声明AspectJ注解的类，进行进一步的处理；
- 对标记为AspectJ注解的类进行增强器的提取；
- 将提取结果加入缓存；
我们来看看Spring如何实现对所有的类进行分析并提取Advisor：
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>();
// 获取所有的beanName
String[] beanNames =
BeanFactoryUtils.beanNamesForTypeIncludingAncestors(this.beanFactory, Object.class, true, false);
// 遍历所有的beanName找出对应的增强方法
for (String beanName : beanNames) {
// 不合法的bean则略过，由子类定义规则，默认返回true
if (!isEligibleBean(beanName)) {
continue;
}
// 获取对应的bean的类型
Class beanType = this.beanFactory.getType(beanName);
if (beanType == null) {
continue;
}
// 如果该bean上存在Aspect注解
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);
// 解析增强方法
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.EMPTY_LIST;
}
// 记录缓存
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;
}
到这里已经完成了Advisor的提取，在上面步骤中最为重要也最为复杂的就是增强器的获取，这一功能是委托给getAdvisors方法来实现(this.advisorFactory.getAdvisors(factory))。
public List<Advisor> getAdvisors(MetadataAwareAspectInstanceFactory maaif) {
// 获取标记为AspectJ的类及其名字，并验证
final Class<?> aspectClass = maaif.getAspectMetadata().getAspectClass();
final String aspectName = maaif.getAspectMetadata().getAspectName();
validate(aspectClass);
// We need to wrap the MetadataAwareAspectInstanceFactory with a decorator
// so that it will only instantiate once.
final MetadataAwareAspectInstanceFactory lazySingletonAspectInstanceFactory =
new LazySingletonAspectInstanceFactoryDecorator(maaif);
final List<Advisor> advisors = new LinkedList<Advisor>();
for (Method method : getAdvisorMethods(aspectClass)) {
// 获取增强器
Advisor advisor = getAdvisor(method, lazySingletonAspectInstanceFactory, advisors.size(), aspectName);
if (advisor != null) {
advisors.add(advisor);
}
}
// If it's a per target aspect, emit the dummy instantiating aspect.
if (!advisors.isEmpty() && lazySingletonAspectInstanceFactory.getAspectMetadata().isLazilyInstantiated()) {
Advisor instantiationAdvisor = new SyntheticInstantiationAdvisor(lazySingletonAspectInstanceFactory);
advisors.add(0, instantiationAdvisor);
}
// Find introduction fields.
for (Field field : aspectClass.getDeclaredFields()) {
Advisor advisor = getDeclareParentsAdvisor(field);
if (advisor != null) {
advisors.add(advisor);
}
}
return advisors;
}
上面函数中首先完成了对增强器的获取，包括获取注解以及根据注解生成增强的步骤，然后考虑到在配置中可能会将增强配置成延迟初始化，那么需要在首位加入同步实例化增强器以保证增强使用之前的实例化，最后是对DeclareParents注解的获取，这里着重分析对增强器的获取。
对普通增强器的获取逻辑是实现在getAdvisor方法中的，实现步骤包括对切点的注解的获取以及根据注解信息生成增强。
public Advisor getAdvisor(Method candidateAdviceMethod, MetadataAwareAspectInstanceFactory aif,
int declarationOrderInAspect, String aspectName) { validate(aif.getAspectMetadata().getAspectClass());
// 切点信息的获取
AspectJExpressionPointcut ajexp =
getPointcut(candidateAdviceMethod, aif.getAspectMetadata().getAspectClass());
if (ajexp == null) {
return null;
}
// 根据切点信息生成增强器
return new InstantiationModelAwarePointcutAdvisorImpl(
this, ajexp, aif, candidateAdviceMethod, declarationOrderInAspect, aspectName);
}
1.1 切点信息的获取
所谓获取切点信息就是指定注解的表达式信息的获取，如@Before("test()")。
private AspectJExpressionPointcut getPointcut(Method candidateAdviceMethod, Class<?> candidateAspectClass) {
// 获取方法上的注解
AspectJAnnotation<?> aspectJAnnotation =
AbstractAspectJAdvisorFactory.findAspectJAnnotationOnMethod(candidateAdviceMethod);
if (aspectJAnnotation == null) {
return null;
}
// 使用AspectJExpressionPointcut实例封装获取的信息
AspectJExpressionPointcut ajexp =
new AspectJExpressionPointcut(candidateAspectClass, new String[0], new Class[0]);
// 提取得到的注解中的表达式，如：
// @Pointcut("execution(* *.*test*(..))")中的execution(* *.*test*(..))")
ajexp.setExpression(aspectJAnnotation.getPointcutExpression());
return ajexp;
}
protected static AspectJAnnotation findAspectJAnnotationOnMethod(Method method) {
// 设置敏感的注解类
Class<? extends Annotation>[] classesToLookFor = new Class[] {
Before.class, Around.class, After.class, AfterReturning.class, AfterThrowing.class, Pointcut.class};
for (Class<? extends Annotation> c : classesToLookFor) {
AspectJAnnotation foundAnnotation = findAnnotation(method, c);
if (foundAnnotation != null) {
return foundAnnotation;
}
}
return null;
}
// 获取指定方法上的注解并使用AspectJAnnotation封装
private static <A extends Annotation> AspectJAnnotation<A> findAnnotation(Method method, Class<A> toLookFor) {
A result = AnnotationUtils.findAnnotation(method, toLookFor);
if (result != null) {
return new AspectJAnnotation<A>(result);
}
else {
return null;
}
}
1.2 根据切点信息生成增强
所有的增强都是由Advisor的实现类InstantiationModelAwarePointcutAdvisorImpl统一封装的。
public InstantiationModelAwarePointcutAdvisorImpl(AspectJAdvisorFactory af, AspectJExpressionPointcut ajexp,
MetadataAwareAspectInstanceFactory aif, Method method, int declarationOrderInAspect, String aspectName) { this.declaredPointcut = ajexp;
this.method = method;
this.atAspectJAdvisorFactory = af;
this.aspectInstanceFactory = aif;
this.declarationOrder = declarationOrderInAspect;
this.aspectName = aspectName; if (aif.getAspectMetadata().isLazilyInstantiated()) {
// Static part of the pointcut is a lazy type.
Pointcut preInstantiationPointcut =
Pointcuts.union(aif.getAspectMetadata().getPerClausePointcut(), this.declaredPointcut); // Make it dynamic: must mutate from pre-instantiation to post-instantiation state.
// If it's not a dynamic pointcut, it may be optimized out
// by the Spring AOP infrastructure after the first evaluation.
this.pointcut = new PerTargetInstantiationModelPointcut(this.declaredPointcut, preInstantiationPointcut, aif);
this.lazy = true;
}
else {
// A singleton aspect.
this.instantiatedAdvice = instantiateAdvice(this.declaredPointcut);
this.pointcut = declaredPointcut;
this.lazy = false;
}
}
在封装过程中只是简单地将信息封装在类的实例中，所有的信息单纯地赋值，在实例初始化的过程中还完成了对于增强器的初始化。因为不同的增强所体现的逻辑是不同的，比如@Before("test()")与@After("test()")标签的区别就是增强器增强的位置不同，所以就需要不同的增强器来完成不同的逻辑，而根据注解中的信息初始化对应的增强器就是在instantiateAdvice函数中实现的。
private Advice instantiateAdvice(AspectJExpressionPointcut pcut) {
return this.atAspectJAdvisorFactory.getAdvice(
this.method, pcut, this.aspectInstanceFactory, this.declarationOrder, this.aspectName);
}
public Advice getAdvice(Method candidateAdviceMethod, AspectJExpressionPointcut ajexp,
MetadataAwareAspectInstanceFactory aif, int declarationOrderInAspect, String aspectName) {
Class<?> candidateAspectClass = aif.getAspectMetadata().getAspectClass();
validate(candidateAspectClass);
AspectJAnnotation<?> aspectJAnnotation =
AbstractAspectJAdvisorFactory.findAspectJAnnotationOnMethod(candidateAdviceMethod);
if (aspectJAnnotation == null) {
return null;
}
// If we get here, we know we have an AspectJ method.
// Check that it's an AspectJ-annotated class
if (!isAspect(candidateAspectClass)) {
throw new AopConfigException("Advice must be declared inside an aspect type: " +
"Offending method '" + candidateAdviceMethod + "' in class [" +
candidateAspectClass.getName() + "]");
}
if (logger.isDebugEnabled()) {
logger.debug("Found AspectJ method: " + candidateAdviceMethod);
}
AbstractAspectJAdvice springAdvice;
// 根据不同的注解类型封装不同的增强器
switch (aspectJAnnotation.getAnnotationType()) {
case AtBefore:
springAdvice = new AspectJMethodBeforeAdvice(candidateAdviceMethod, ajexp, aif);
break;
case AtAfter:
springAdvice = new AspectJAfterAdvice(candidateAdviceMethod, ajexp, aif);
break;
case AtAfterReturning:
springAdvice = new AspectJAfterReturningAdvice(candidateAdviceMethod, ajexp, aif);
AfterReturning afterReturningAnnotation = (AfterReturning) aspectJAnnotation.getAnnotation();
if (StringUtils.hasText(afterReturningAnnotation.returning())) {
springAdvice.setReturningName(afterReturningAnnotation.returning());
}
break;
case AtAfterThrowing:
springAdvice = new AspectJAfterThrowingAdvice(candidateAdviceMethod, ajexp, aif);
AfterThrowing afterThrowingAnnotation = (AfterThrowing) aspectJAnnotation.getAnnotation();
if (StringUtils.hasText(afterThrowingAnnotation.throwing())) {
springAdvice.setThrowingName(afterThrowingAnnotation.throwing());
}
break;
case AtAround:
springAdvice = new AspectJAroundAdvice(candidateAdviceMethod, ajexp, aif);
break;
case AtPointcut:
if (logger.isDebugEnabled()) {
logger.debug("Processing pointcut '" + candidateAdviceMethod.getName() + "'");
}
return null;
default:
throw new UnsupportedOperationException(
"Unsupported advice type on method " + candidateAdviceMethod);
}
// Now to configure the advice...
springAdvice.setAspectName(aspectName);
springAdvice.setDeclarationOrder(declarationOrderInAspect);
String[] argNames = this.parameterNameDiscoverer.getParameterNames(candidateAdviceMethod);
if (argNames != null) {
springAdvice.setArgumentNamesFromStringArray(argNames);
}
springAdvice.calculateArgumentBindings();
return springAdvice;
}
从上面可以看到，Spring会根据不同的注解生成不同的增强器，例如AtBefore会对应AspectJMethodBeforeAdvice，在AspectJMethodBeforeAdvice中完成了增强方法的逻辑。本文尝试分析几个常用的增强器实现。
AspectJMethodBeforeAdvice
这是前置增强器，在Spring中，它会封装到MethodBeforeAdviceInterceptor类的内部，这个是一个拦截器，会被放在拦截器链中，在创建代理bean时会用到：
public class MethodBeforeAdviceInterceptor implements MethodInterceptor, Serializable {
private MethodBeforeAdvice advice;
/**
* Create a new MethodBeforeAdviceInterceptor for the given advice.
* @param advice the MethodBeforeAdvice to wrap
*/
public MethodBeforeAdviceInterceptor(MethodBeforeAdvice advice) {
Assert.notNull(advice, "Advice must not be null");
this.advice = advice;
}
public Object invoke(MethodInvocation mi) throws Throwable {
this.advice.before(mi.getMethod(), mi.getArguments(), mi.getThis() );
return mi.proceed();
}
}
其中的属性MethodBeforeAdvicce就代表着前置增强的AspectJMethodBeforeAdvice，跟踪其before()方法：
public void before(Method method, Object[] args, Object target) throws Throwable {
invokeAdviceMethod(getJoinPointMatch(), null, null);
}
protected Object invokeAdviceMethod(JoinPointMatch jpMatch, Object returnValue, Throwable ex) throws Throwable {
return invokeAdviceMethodWithGivenArgs(argBinding(getJoinPoint(), jpMatch, returnValue, ex));
}
protected Object invokeAdviceMethodWithGivenArgs(Object[] args) throws Throwable {
Object[] actualArgs = args;
if (this.aspectJAdviceMethod.getParameterTypes().length == 0) {
actualArgs = null;
}
try {
ReflectionUtils.makeAccessible(this.aspectJAdviceMethod);
// 激活增强方法
return this.aspectJAdviceMethod.invoke(this.aspectInstanceFactory.getAspectInstance(), actualArgs);
}
catch (IllegalArgumentException ex) {
throw new AopInvocationException("Mismatch on arguments to advice method [" +
this.aspectJAdviceMethod + "]; pointcut expression [" +
this.pointcut.getPointcutExpression() + "]", ex);
}
catch (InvocationTargetException ex) {
throw ex.getTargetException();
}
}
invokeAdviceMethodWithGivenArgs方法中的aspectJAdviceMethod正是对应于前置增强的方法，在这里实现了调用。
AspectJAfterAdvice
后置增强与前置增强有稍许不一致的地方。回顾上面讲过的前置增强，大致的结构是在拦截器链中放置MethodBeforeAdviceInterceptor，并在MethodBeforeAdviceInterceptor中又放置了AspectJMethodBeforeAdvice，在调用inovke时首先串联调用。但是在后置增强的时候却是不一样的，没有提供如MethodBeforeAdviceInterceptor的中间类，而是直接实现MethodInterceptor接口，并在拦截器链中使用了中间的AspectAfterAdvice。
public class AspectJAfterAdvice extends AbstractAspectJAdvice implements MethodInterceptor, AfterAdvice {
public AspectJAfterAdvice(Method aspectJBeforeAdviceMethod, AspectJExpressionPointcut pointcut, AspectInstanceFactory aif) {
super(aspectJBeforeAdviceMethod, pointcut, aif);
}
public Object invoke(MethodInvocation mi) throws Throwable {
try {
return mi.proceed();
}
finally {
// 激活增强方法
invokeAdviceMethod(getJoinPointMatch(), null, null);
}
}
}
2. 寻找匹配的增强器
前面已经把所有增强器的获取分析完了，但是对于所有增强来说，并不一定都适用于当前的Bean，还要挑选出适合的增强器，也就是满足我们配置的通配符的增强器。这部分的具体实现在findAdvisorsThatCanApply中。
protected List<Advisor> findAdvisorsThatCanApply(
List<Advisor> candidateAdvisors, Class beanClass, String beanName) { ProxyCreationContext.setCurrentProxiedBeanName(beanName);
try {
// 过滤已经得到的advisors
return AopUtils.findAdvisorsThatCanApply(candidateAdvisors, beanClass);
}
finally {
ProxyCreationContext.setCurrentProxiedBeanName(null);
}
} public static List<Advisor> findAdvisorsThatCanApply(List<Advisor> candidateAdvisors, Class<?> clazz) {
if (candidateAdvisors.isEmpty()) {
return candidateAdvisors;
}
List<Advisor> eligibleAdvisors = new LinkedList<Advisor>();
for (Advisor candidate : candidateAdvisors) {
if (candidate instanceof IntroductionAdvisor && canApply(candidate, clazz)) {
eligibleAdvisors.add(candidate);
}
}
boolean hasIntroductions = !eligibleAdvisors.isEmpty();
for (Advisor candidate : candidateAdvisors) {
if (candidate instanceof IntroductionAdvisor) {
// already processed
continue;
}
if (canApply(candidate, clazz, hasIntroductions)) {
eligibleAdvisors.add(candidate);
}
}
return eligibleAdvisors;
}
findAdvisorsThatCanApply函数的主要功能是寻找所有增强器中适用于当前class的增强器，而对于真正的匹配其实是在canApply中实现的。
public static boolean canApply(Advisor advisor, Class<?> targetClass, boolean hasIntroductions) {
if (advisor instanceof IntroductionAdvisor) {
return ((IntroductionAdvisor) advisor).getClassFilter().matches(targetClass);
}
else if (advisor instanceof PointcutAdvisor) {
PointcutAdvisor pca = (PointcutAdvisor) advisor;
return canApply(pca.getPointcut(), targetClass, hasIntroductions);
}
else {
// It doesn't have a pointcut so we assume it applies.
return true;
}
}
public static boolean canApply(Pointcut pc, Class<?> targetClass, boolean hasIntroductions) {
Assert.notNull(pc, "Pointcut must not be null");
if (!pc.getClassFilter().matches(targetClass)) {
return false;
}
MethodMatcher methodMatcher = pc.getMethodMatcher();
IntroductionAwareMethodMatcher introductionAwareMethodMatcher = null;
if (methodMatcher instanceof IntroductionAwareMethodMatcher) {
introductionAwareMethodMatcher = (IntroductionAwareMethodMatcher) methodMatcher;
}
Set<Class> classes = new LinkedHashSet<Class>(ClassUtils.getAllInterfacesForClassAsSet(targetClass));
classes.add(targetClass);
for (Class<?> clazz : classes) {
Method[] methods = clazz.getMethods();
for (Method method : methods) {
if ((introductionAwareMethodMatcher != null &&
introductionAwareMethodMatcher.matches(method, targetClass, hasIntroductions)) ||
methodMatcher.matches(method, targetClass)) {
return true;
}
}
}
return false;
}
这里的逻辑也不是很复杂，无非就是根据advisor中封装的classFilter来判断是否match对应的类。
3. 总结
本文主要学习了Spring AOP核心实现原理中的对增强方法的获取，主要包含两个步骤：
- 获取所有增强；
- 寻找所有增强中适用于bean的增强；
下文会聚焦在AOP核心实现原理的另一部分--代理的创建。