概述：
该篇文章主要讲解spring容器如何实现AOP思想，过程中涉及到bean后置工厂、aspect切面的处理、动态代理对象的生成等内容。
原理：
首先看下AnnotationAwareAspectJAutoProxyCreator的静态类图
看下spring容器启动过程中触发的该类的几个重要方法：
/**
* Create a proxy with the configured interceptors if the bean is
* identified as one to proxy by the subclass.
* 判断是否采用代理对象替换原对象
* @see #getAdvicesAndAdvisorsForBean
*/
@Override
public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
if (bean != null) {
Object cacheKey = getCacheKey(bean.getClass(), beanName);
if (!this.earlyProxyReferences.contains(cacheKey)) {
return wrapIfNecessary(bean, beanName, cacheKey);
}
}
return bean;
}
该方法合适被触发：bean工厂初始化bean时候initializeBean（），具体可以查看《spring的启动过程04-bean后置处理器》。
/**
* Wrap the given bean if necessary, i.e. if it is eligible for being proxied.
* @param bean the raw bean instance
* @param beanName the name of the bean
* @param cacheKey the cache key for metadata access
* @return a proxy wrapping the bean, or the raw bean instance as-is
*/
protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
if (beanName != null && this.targetSourcedBeans.contains(beanName)) {
return bean;
}
if (Boolean.FALSE.equals(this.advisedBeans.get(cacheKey))) {
return bean;
}
判断是否满足被代理的条件
if (isInfrastructureClass(bean.getClass()) || shouldSkip(bean.getClass(), beanName)) {
this.advisedBeans.put(cacheKey, Boolean.FALSE);
return bean;
}
// Create proxy if we have advice.
// 首先获取容器中存在的advice切面，然后判断该类满足哪些切面的要求
Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
if (specificInterceptors != DO_NOT_PROXY) {
this.advisedBeans.put(cacheKey, Boolean.TRUE);
// 创建代理对象
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;
}
下面看下spring容器如何获取所有的切面对象：
/** * Find all eligible Advisors for auto-proxying this class. * @param beanClass the clazz to find advisors for * @param beanName the name of the currently proxied bean * @return the empty List, not {@code null}, * if there are no pointcuts or interceptors * @see #findCandidateAdvisors * @see #sortAdvisors * @see #extendAdvisors */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;}findEligibleAdvisors方法首先获取容器中所有的Advisor实例，然后判断哪些Advisor实例满足该beanClass。
@Override调用工具类BeanFactoryAdvisorRetrievalHelper如下方法获取标准的spring Advisors。
protected List<Advisor> findCandidateAdvisors() {
// Add all the Spring advisors found according to superclass rules.
List<Advisor> advisors = super.findCandidateAdvisors();
// Build Advisors for all AspectJ aspects in the bean factory.
advisors.addAll(this.aspectJAdvisorsBuilder.buildAspectJAdvisors());
return advisors;
}
/**调用BeanFactoryAspectJAdvisorsBuilder类的如下工具类获取所有采用AspectJ注解的bean对象。
* Find all eligible Advisor beans in the current bean factory,
* ignoring FactoryBeans and excluding beans that are currently in creation.
* @return the list of {@link org.springframework.aop.Advisor} beans
* @see #isEligibleBean
*/
public List<Advisor> findAdvisorBeans() {
// Determine list of advisor bean names, if not cached already.
String[] advisorNames = null;
synchronized (this) {
advisorNames = this.cachedAdvisorBeanNames;
if (advisorNames == null) {
//获取所有实现Advisor接口的bean
advisorNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(
this.beanFactory, Advisor.class, true, false);
this.cachedAdvisorBeanNames = advisorNames;
}
}
if (advisorNames.length == 0) {
return new LinkedList<Advisor>();
}
List<Advisor> advisors = new LinkedList<Advisor>();
for (String name : advisorNames) {
if (isEligibleBean(name)) {
if (this.beanFactory.isCurrentlyInCreation(name)) {
}
else {
try {
advisors.add(this.beanFactory.getBean(name, Advisor.class));
}
catch (BeanCreationException ex) {
}
}
}
}
return advisors;
}
/**
* Look for AspectJ-annotated aspect beans in the current bean factory,
* and return to a list of Spring AOP Advisors representing them.
* <p>Creates a Spring Advisor for each AspectJ advice method.
* @return the list of {@link org.springframework.aop.Advisor} beans
* @see #isEligibleBean
*/
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spring容器中所有的bean
String[] beanNames =
BeanFactoryUtils.beanNamesForTypeIncludingAncestors(this.beanFactory, Object.class, true, false);
for (String beanName : beanNames) {
if (!isEligibleBean(beanName)) {
continue;
}
Class<?> beanType = this.beanFactory.getType(beanName);
if (beanType == null) {
continue;
}
// 判断bean是否为Aspect切面
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);
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.
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.emptyList();
}
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;
}
@Overridepublic boolean isAspect(Class<?> clazz) {return (hasAspectAnnotation(clazz) && !compiledByAjc(clazz));}private boolean hasAspectAnnotation(Class<?> clazz) {return (AnnotationUtils.findAnnotation(clazz, Aspect.class) != null);}
调用ReflectiveAspectJAdvisorFactory具体的方法获取Advisors
@Override以上代码展示了spring如何获取容器上下文中所有的切面对象，下面看下如何获取具体的某个bean对象适配的切面对象集合。
public List<Advisor> getAdvisors(MetadataAwareAspectInstanceFactory aspectInstanceFactory) {
Class<?> aspectClass = aspectInstanceFactory.getAspectMetadata().getAspectClass();
String aspectName = aspectInstanceFactory.getAspectMetadata().getAspectName();
validate(aspectClass);
// We need to wrap the MetadataAwareAspectInstanceFactory with a decorator
// so that it will only instantiate once.
MetadataAwareAspectInstanceFactory lazySingletonAspectInstanceFactory =
new LazySingletonAspectInstanceFactoryDecorator(aspectInstanceFactory);
List<Advisor> advisors = new LinkedList<Advisor>();
for (Method method : getAdvisorMethods(aspectClass)) {
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;
}
@Override
public Advisor getAdvisor(Method candidateAdviceMethod, MetadataAwareAspectInstanceFactory aspectInstanceFactory,
int declarationOrderInAspect, String aspectName) {
validate(aspectInstanceFactory.getAspectMetadata().getAspectClass());
AspectJExpressionPointcut expressionPointcut = getPointcut(
candidateAdviceMethod, aspectInstanceFactory.getAspectMetadata().getAspectClass());
if (expressionPointcut == null) {
return null;
}
return new InstantiationModelAwarePointcutAdvisorImpl(expressionPointcut, candidateAdviceMethod,
this, aspectInstanceFactory, declarationOrderInAspect, aspectName);
}
private AspectJExpressionPointcut getPointcut(Method candidateAdviceMethod, Class<?> candidateAspectClass) {
AspectJAnnotation<?> aspectJAnnotation =
AbstractAspectJAdvisorFactory.findAspectJAnnotationOnMethod(candidateAdviceMethod);
if (aspectJAnnotation == null) {
return null;
}
AspectJExpressionPointcut ajexp =
new AspectJExpressionPointcut(candidateAspectClass, new String[0], new Class<?>[0]);
ajexp.setExpression(aspectJAnnotation.getPointcutExpression());
return ajexp;
}
/**
* Find and return the first AspectJ annotation on the given method
* (there <i>should</i> only be one anyway...)
*/
@SuppressWarnings("unchecked")
protected static AspectJAnnotation<?> findAspectJAnnotationOnMethod(Method method) {
Class<?>[] classesToLookFor = new Class<?>[] {
Before.class, Around.class, After.class, AfterReturning.class, AfterThrowing.class, Pointcut.class};
for (Class<?> c : classesToLookFor) {
AspectJAnnotation<?> foundAnnotation = findAnnotation(method, (Class<Annotation>) c);
if (foundAnnotation != null) {
return foundAnnotation;
}
}
return null;
}
/**
* Search the given candidate Advisors to find all Advisors that
* can apply to the specified bean.
* @param candidateAdvisors the candidate Advisors
* @param beanClass the target's bean class
* @param beanName the target's bean name
* @return the List of applicable Advisors
* @see ProxyCreationContext#getCurrentProxiedBeanName()
*/
protected List<Advisor> findAdvisorsThatCanApply(
List<Advisor> candidateAdvisors, Class<?> beanClass, String beanName) {
ProxyCreationContext.setCurrentProxiedBeanName(beanName);
try {
return AopUtils.findAdvisorsThatCanApply(candidateAdvisors, beanClass);
}
finally {
ProxyCreationContext.setCurrentProxiedBeanName(null);
}
}
/** * Determine the sublist of the {@code candidateAdvisors} list * that is applicable to the given class. * @param candidateAdvisors the Advisors to evaluate * @param clazz the target class * @return sublist of Advisors that can apply to an object of the given class * (may be the incoming List as-is) */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 processedcontinue;}if (canApply(candidate, clazz, hasIntroductions)) {eligibleAdvisors.add(candidate);}}return eligibleAdvisors;}
/** * Can the given advisor apply at all on the given class? * <p>This is an important test as it can be used to optimize out a advisor for a class. * This version also takes into account introductions (for IntroductionAwareMethodMatchers). * @param advisor the advisor to check * @param targetClass class we're testing * @param hasIntroductions whether or not the advisor chain for this bean includes * any introductions * @return whether the pointcut can apply on any method */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;}}
/** * Can the given pointcut apply at all on the given class? * <p>This is an important test as it can be used to optimize * out a pointcut for a class. * @param pc the static or dynamic pointcut to check * @param targetClass the class to test * @param hasIntroductions whether or not the advisor chain * for this bean includes any introductions * @return whether the pointcut can apply on any method */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);//遍历所有方法和切面表达式进行匹配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;}
不同的切面对象对应不同的表达式。上述过程表明了spring是如何匹配某个类是否匹配具体的切面表达式。
下面看下如何创建代理对象：
/**
* Create an AOP proxy for the given bean.
* @param beanClass the class of the bean
* @param beanName the name of the bean
* @param specificInterceptors the set of interceptors that is
* specific to this bean (may be empty, but not null)
* @param targetSource the TargetSource for the proxy,
* already pre-configured to access the bean
* @return the AOP proxy for the bean
* @see #buildAdvisors
*/
protected Object createProxy(
Class<?> beanClass, String beanName, Object[] specificInterceptors, TargetSource targetSource) {
if (this.beanFactory instanceof ConfigurableListableBeanFactory) {
AutoProxyUtils.exposeTargetClass((ConfigurableListableBeanFactory) this.beanFactory, beanName, beanClass);
}
ProxyFactory proxyFactory = new ProxyFactory();
proxyFactory.copyFrom(this);
if (!proxyFactory.isProxyTargetClass()) {
if (shouldProxyTargetClass(beanClass, beanName)) {
proxyFactory.setProxyTargetClass(true);
}
else {
evaluateProxyInterfaces(beanClass, proxyFactory);
}
}
Advisor[] advisors = buildAdvisors(beanName, specificInterceptors);
for (Advisor advisor : advisors) {
proxyFactory.addAdvisor(advisor);
}
proxyFactory.setTargetSource(targetSource);
customizeProxyFactory(proxyFactory);
proxyFactory.setFrozen(this.freezeProxy);
if (advisorsPreFiltered()) {
proxyFactory.setPreFiltered(true);
}
//DefaultAopProxyFactory，采用代理工厂创建代理对象
return proxyFactory.getProxy(getProxyClassLoader());
}
DefaultAopProxyFactory
@Override
public AopProxy createAopProxy(AdvisedSupport config) throws AopConfigException {
// Optimize为true，proxyTargetClass为true，bean没有实现任何接口的
if (config.isOptimize() || config.isProxyTargetClass() || hasNoUserSuppliedProxyInterfaces(config)) {
Class<?> targetClass = config.getTargetClass();
if (targetClass == null) {
}
// 目标对象为接口或者本事为代理对象
if (targetClass.isInterface() || Proxy.isProxyClass(targetClass)) {
return new JdkDynamicAopProxy(config);
}
return new ObjenesisCglibAopProxy(config);
}
else {
return new JdkDynamicAopProxy(config);
}
}
spring拥有自己的机制来规定哪些bean需要采用proxy进行包装。
1.spring容器内置标准的切面逻辑，如事务、缓存等。
2.spring提供AspectJ注解方式，用户可以自定义切面，如request拦截器、事务拦截器、缓存拦截器等。
3.spring通过bean后置处理器机制在initializeBean过程中执行postProcessBeforeInstantiation拦截器，拦截某个具体的bean对象适配的切面。
4.通过JDK或者CGLIB进行代理对象的创建。
5.该代理对象依然被spring容器管理且beanName不变。
6.如果对代理对象进行特殊处理如利用反射机制进行属性字段赋值，需要拿到目标对象进行操作。