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前言
AOP，也就是面向切面程式設計，它可以將公共的程式碼抽離出來，動態的織入到目標類、目標方法中，大大提高我們程式設計的效率，也使程式變得更加優雅。如事務、操作日誌等都可以使用AOP實現。這種織入可以是在執行期動態生成代理物件實現，也可以在編譯期、類載入時期靜態織入到程式碼中。而Spring正是通過第一種方法實現，且在代理類的生成上也有兩種方式：JDK Proxy和CGLIB，預設當類實現了介面時使用前者，否則使用後者；另外Spring AOP只能實現對方法的增強。
正文
基本概念
AOP的術語很多，雖然不清楚術語我們也能很熟練地使用AOP，但是要理解分析原始碼，術語就需要深刻體會其含義。
- 增強（Advice）：就是我們想要額外增加的功能
- 目標物件（Target）：就是我們想要增強的目標類，如果沒有AOP，我們需要在每個目標物件中實現日誌、事務管理等非業務邏輯
- 連線點（JoinPoint）：程式執行時的特定時機，如方法執行前、後以及丟擲異常後等等。
- 切點（Pointcut）：連線點的導航，我們如何找到目標物件呢？切點的作用就在於此，在Spring中就是匹配表示式。
- 引介（Introduction）：引介是一種特殊的增強，它為類新增一些屬性和方法。這樣，即使一個業務類原本沒有實現某個介面，通過AOP的引介功能，我們可以動態地為該業務類新增介面的實現邏輯，讓業務類成為這個介面的實現類。
- 織入（Weaving）：即如何將增強新增到目標物件的連線點上，有動態（執行期生成代理）、靜態（編譯期、類載入時期）兩種方式。
- 代理（Proxy）：目標物件被織入增強後，就會產生一個代理物件，該物件可能是和原物件實現了同樣的一個介面（JDK），也可能是原物件的子類（CGLIB）。
- 切面（Aspect、Advisor）：切面由切點和增強組成，包含了這兩者的定義。
代理物件的建立
在熟悉了AOP術語後，下面就來看看Spring是如何建立代理物件的，是否還記得上一篇提到的AOP的入口呢？在AbstractAutowireCapableBeanFactory類的applyBeanPostProcessorsAfterInitialization方法中迴圈呼叫了BeanPostProcessor的postProcessAfterInitialization方法，其中一個就是我們建立代理物件的入口。這裡是Bean例項化完成去建立代理物件，理所當然應該這樣，但實際上在Bean例項化之前呼叫了一個resolveBeforeInstantiation方法，這裡實際上我們也是有機會可以提前建立代理物件的，這裡放到最後來分析，先來看主入口，進入到AbstractAutoProxyCreator類中：
public Object postProcessAfterInitialization(@Nullable Object bean, String beanName) {
if (bean != null) {
Object cacheKey = getCacheKey(bean.getClass(), beanName);
if (!this.earlyProxyReferences.contains(cacheKey)) {
return wrapIfNecessary(bean, beanName, cacheKey);
}
}
return bean;
}
protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
//建立當前bean的代理，如果這個bean有advice的話，重點看
// Create proxy if we have advice.
Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
//如果有切面，則生成該bean的代理
if (specificInterceptors != DO_NOT_PROXY) {
this.advisedBeans.put(cacheKey, Boolean.TRUE);
//把被代理物件bean例項封裝到SingletonTargetSource物件中
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;
}
先從快取中拿，沒有則呼叫wrapIfNecessary方法建立。在這個方法裡面主要看兩個地方：getAdvicesAndAdvisorsForBean和createProxy。簡單一句話概括就是先掃描後建立，問題是掃描什麼呢？你可以先結合上面的概念思考下，換你會怎麼做。進入到子類AbstractAdvisorAutoProxyCreator的getAdvicesAndAdvisorsForBean方法中：
protected Object[] getAdvicesAndAdvisorsForBean(
Class<?> beanClass, String beanName, @Nullable TargetSource targetSource) {
//找到合格的切面
List<Advisor> advisors = findEligibleAdvisors(beanClass, beanName);
if (advisors.isEmpty()) {
return DO_NOT_PROXY;
}
return advisors.toArray();
}
protected List<Advisor> findEligibleAdvisors(Class<?> beanClass, String beanName) {
//找到候選的切面,其實就是一個尋找有@Aspectj註解的過程，把工程中所有有這個註解的類封裝成Advisor返回
List<Advisor> candidateAdvisors = findCandidateAdvisors();
//判斷候選的切面是否作用在當前beanClass上面，就是一個匹配過程。現在就是一個匹配
List<Advisor> eligibleAdvisors = findAdvisorsThatCanApply(candidateAdvisors, beanClass, beanName);
extendAdvisors(eligibleAdvisors);
if (!eligibleAdvisors.isEmpty()) {
//對有@Order@Priority進行排序
eligibleAdvisors = sortAdvisors(eligibleAdvisors);
}
return eligibleAdvisors;
}
在findEligibleAdvisors方法中可以看到有兩個步驟，第一先找到所有的切面，即掃描所有帶有@Aspect註解的類，並將其中的切點（表示式）和增強封裝為切面，掃描完成後，自然是要判斷哪些切面能夠連線到當前Bean例項上。下面一步步來分析，首先是掃描過程，進入到AnnotationAwareAspectJAutoProxyCreator類中：
protected List<Advisor> findCandidateAdvisors() {
// 先通過父類AbstractAdvisorAutoProxyCreator掃描，這裡不重要
List<Advisor> advisors = super.findCandidateAdvisors();
// 主要看這裡
if (this.aspectJAdvisorsBuilder != null) {
advisors.addAll(this.aspectJAdvisorsBuilder.buildAspectJAdvisors());
}
return advisors;
}
這裡委託給了BeanFactoryAspectJAdvisorsBuilderAdapter類，並呼叫其父類的buildAspectJAdvisors方法建立切面物件：
public List<Advisor> buildAspectJAdvisors() {
List<String> aspectNames = this.aspectBeanNames;
if (aspectNames == null) {
synchronized (this) {
aspectNames = this.aspectBeanNames;
if (aspectNames == null) {
List<Advisor> advisors = new ArrayList<>();
aspectNames = new ArrayList<>();
//獲取spring容器中的所有bean的名稱BeanName
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;
}
//判斷類上是否有@Aspect註解
if (this.advisorFactory.isAspect(beanType)) {
aspectNames.add(beanName);
AspectMetadata amd = new AspectMetadata(beanType, beanName);
if (amd.getAjType().getPerClause().getKind() == PerClauseKind.SINGLETON) {
// 當@Aspect的value屬性為""時才會進入到這裡
// 建立獲取有@Aspect註解類的例項工廠，負責獲取有@Aspect註解類的例項
MetadataAwareAspectInstanceFactory factory =
new BeanFactoryAspectInstanceFactory(this.beanFactory, beanName);
//建立切面advisor物件
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 {
MetadataAwareAspectInstanceFactory factory =
new PrototypeAspectInstanceFactory(this.beanFactory, beanName);
this.aspectFactoryCache.put(beanName, factory);
advisors.addAll(this.advisorFactory.getAdvisors(factory));
}
}
}
this.aspectBeanNames = aspectNames;
return advisors;
}
}
}
return advisors;
}
這個方法裡面首先從IOC中拿到所有Bean的名稱，並迴圈判斷該類上是否帶有@Aspect註解，如果有則將BeanName和Bean的Class型別封裝到BeanFactoryAspectInstanceFactory中，並呼叫ReflectiveAspectJAdvisorFactory.getAdvisors建立切面物件：
public List<Advisor> getAdvisors(MetadataAwareAspectInstanceFactory aspectInstanceFactory) {
//從工廠中獲取有@Aspect註解的類Class
Class<?> aspectClass = aspectInstanceFactory.getAspectMetadata().getAspectClass();
//從工廠中獲取有@Aspect註解的類的名稱
String aspectName = aspectInstanceFactory.getAspectMetadata().getAspectName();
validate(aspectClass);
// 建立工廠的裝飾類，獲取例項只會獲取一次
MetadataAwareAspectInstanceFactory lazySingletonAspectInstanceFactory =
new LazySingletonAspectInstanceFactoryDecorator(aspectInstanceFactory);
List<Advisor> advisors = new ArrayList<>();
//這裡迴圈沒有@Pointcut註解的方法
for (Method method : getAdvisorMethods(aspectClass)) {
//非常重要重點看看
Advisor advisor = getAdvisor(method, lazySingletonAspectInstanceFactory, advisors.size(), aspectName);
if (advisor != null) {
advisors.add(advisor);
}
}
if (!advisors.isEmpty() && lazySingletonAspectInstanceFactory.getAspectMetadata().isLazilyInstantiated()) {
Advisor instantiationAdvisor = new SyntheticInstantiationAdvisor(lazySingletonAspectInstanceFactory);
advisors.add(0, instantiationAdvisor);
}
//判斷屬性上是否有引介註解，這裡可以不看
for (Field field : aspectClass.getDeclaredFields()) {
//判斷屬性上是否有DeclareParents註解，如果有返回切面
Advisor advisor = getDeclareParentsAdvisor(field);
if (advisor != null) {
advisors.add(advisor);
}
}
return advisors;
}
private List<Method> getAdvisorMethods(Class<?> aspectClass) {
final List<Method> methods = new ArrayList<>();
ReflectionUtils.doWithMethods(aspectClass, method -> {
// Exclude pointcuts
if (AnnotationUtils.getAnnotation(method, Pointcut.class) == null) {
methods.add(method);
}
});
methods.sort(METHOD_COMPARATOR);
return methods;
}
根據Aspect的Class拿到所有不帶@Pointcut註解的方法物件（為什麼是不帶@Pointcut註解的方法？仔細想想不難理解），另外要注意這裡對method進行了排序，看看這個METHOD_COMPARATOR比較器：
private static final Comparator<Method> METHOD_COMPARATOR;
static {
Comparator<Method> adviceKindComparator = new ConvertingComparator<>(
new InstanceComparator<>(
Around.class, Before.class, After.class, AfterReturning.class, AfterThrowing.class),
(Converter<Method, Annotation>) method -> {
AspectJAnnotation<?> annotation =
AbstractAspectJAdvisorFactory.findAspectJAnnotationOnMethod(method);
return (annotation != null ? annotation.getAnnotation() : null);
});
Comparator<Method> methodNameComparator = new ConvertingComparator<>(Method::getName);
METHOD_COMPARATOR = adviceKindComparator.thenComparing(methodNameComparator);
}
關注InstanceComparator建構函式引數，記住它們的順序，這就是AOP鏈式呼叫中同一個@Aspect類中Advice的執行順序。接著往下看，在getAdvisors方法中迴圈獲取到的methods，分別呼叫getAdvisor方法，也就是根據方法逐個去建立切面：
public Advisor getAdvisor(Method candidateAdviceMethod, MetadataAwareAspectInstanceFactory aspectInstanceFactory,
int declarationOrderInAspect, String aspectName) {
validate(aspectInstanceFactory.getAspectMetadata().getAspectClass());
//獲取pointCut物件，最重要的是從註解中獲取表示式
AspectJExpressionPointcut expressionPointcut = getPointcut(
candidateAdviceMethod, aspectInstanceFactory.getAspectMetadata().getAspectClass());
if (expressionPointcut == null) {
return null;
}
//建立Advisor切面類，這才是真正的切面類，一個切面類裡面肯定要有1、pointCut 2、advice
//這裡pointCut是expressionPointcut， advice 增強方法是 candidateAdviceMethod
return new InstantiationModelAwarePointcutAdvisorImpl(expressionPointcut, candidateAdviceMethod,
this, aspectInstanceFactory, declarationOrderInAspect, aspectName);
}
private static final Class<?>[] ASPECTJ_ANNOTATION_CLASSES = new Class<?>[] {
Pointcut.class, Around.class, Before.class, After.class, AfterReturning.class, AfterThrowing.class};
private AspectJExpressionPointcut getPointcut(Method candidateAdviceMethod, Class<?> candidateAspectClass) {
//從候選的增強方法裡面 candidateAdviceMethod 找有有註解
//Pointcut.class, Around.class, Before.class, After.class, AfterReturning.class, AfterThrowing.class
//並把註解資訊封裝成AspectJAnnotation物件
AspectJAnnotation<?> aspectJAnnotation =
AbstractAspectJAdvisorFactory.findAspectJAnnotationOnMethod(candidateAdviceMethod);
if (aspectJAnnotation == null) {
return null;
}
//建立一個PointCut類，並且把前面從註解裡面解析的表示式設定進去
AspectJExpressionPointcut ajexp =
new AspectJExpressionPointcut(candidateAspectClass, new String[0], new Class<?>[0]);
ajexp.setExpression(aspectJAnnotation.getPointcutExpression());
if (this.beanFactory != null) {
ajexp.setBeanFactory(this.beanFactory);
}
return ajexp;
}
之前就說過切面的定義，是切點和增強的組合，所以這裡首先通過getPointcut獲取到註解物件，然後new了一個Pointcut物件，並將表示式設定進去。然後在getAdvisor方法中最後new了一個InstantiationModelAwarePointcutAdvisorImpl物件：
public InstantiationModelAwarePointcutAdvisorImpl(AspectJExpressionPointcut declaredPointcut,
Method aspectJAdviceMethod, AspectJAdvisorFactory aspectJAdvisorFactory,
MetadataAwareAspectInstanceFactory aspectInstanceFactory, int declarationOrder, String aspectName) {
this.declaredPointcut = declaredPointcut;
this.declaringClass = aspectJAdviceMethod.getDeclaringClass();
this.methodName = aspectJAdviceMethod.getName();
this.parameterTypes = aspectJAdviceMethod.getParameterTypes();
this.aspectJAdviceMethod = aspectJAdviceMethod;
this.aspectJAdvisorFactory = aspectJAdvisorFactory;
this.aspectInstanceFactory = aspectInstanceFactory;
this.declarationOrder = declarationOrder;
this.aspectName = aspectName;
if (aspectInstanceFactory.getAspectMetadata().isLazilyInstantiated()) {
// Static part of the pointcut is a lazy type.
Pointcut preInstantiationPointcut = Pointcuts.union(
aspectInstanceFactory.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, aspectInstanceFactory);
this.lazy = true;
}
else {
// A singleton aspect.
this.pointcut = this.declaredPointcut;
this.lazy = false;
//這個方法重點看看，建立advice物件
this.instantiatedAdvice = instantiateAdvice(this.declaredPointcut);
}
}
這個就是我們的切面類，在其構造方法的最後通過instantiateAdvice建立了Advice物件。注意這裡傳進來的declarationOrder引數，它就是迴圈method時的序號，其作用就是賦值給這裡的declarationOrder屬性以及Advice的declarationOrder屬性，在後面排序時就會通過這個序號來比較，因此Advice的執行順序是固定的，至於為什麼要固定，後面分析完AOP鏈式呼叫過程自然就明白了。
public Advice getAdvice(Method candidateAdviceMethod, AspectJExpressionPointcut expressionPointcut,
MetadataAwareAspectInstanceFactory aspectInstanceFactory, int declarationOrder, String aspectName) {
//獲取有@Aspect註解的類
Class<?> candidateAspectClass = aspectInstanceFactory.getAspectMetadata().getAspectClass();
validate(candidateAspectClass);
//找到candidateAdviceMethod方法上面的註解，並且包裝成AspectJAnnotation物件，這個物件中就有註解型別
AspectJAnnotation<?> aspectJAnnotation =
AbstractAspectJAdvisorFactory.findAspectJAnnotationOnMethod(candidateAdviceMethod);
if (aspectJAnnotation == null) {
return null;
}
AbstractAspectJAdvice springAdvice;
//根據不同的註解型別建立不同的advice類例項
switch (aspectJAnnotation.getAnnotationType()) {
case AtPointcut:
if (logger.isDebugEnabled()) {
logger.debug("Processing pointcut '" + candidateAdviceMethod.getName() + "'");
}
return null;
case AtAround:
//實現了MethodInterceptor介面
springAdvice = new AspectJAroundAdvice(
candidateAdviceMethod, expressionPointcut, aspectInstanceFactory);
break;
case AtBefore:
//實現了MethodBeforeAdvice介面，沒有實現MethodInterceptor介面
springAdvice = new AspectJMethodBeforeAdvice(
candidateAdviceMethod, expressionPointcut, aspectInstanceFactory);
break;
case AtAfter:
//實現了MethodInterceptor介面
springAdvice = new AspectJAfterAdvice(
candidateAdviceMethod, expressionPointcut, aspectInstanceFactory);
break;
case AtAfterReturning:
//實現了AfterReturningAdvice介面，沒有實現MethodInterceptor介面
springAdvice = new AspectJAfterReturningAdvice(
candidateAdviceMethod, expressionPointcut, aspectInstanceFactory);
AfterReturning afterReturningAnnotation = (AfterReturning) aspectJAnnotation.getAnnotation();
if (StringUtils.hasText(afterReturningAnnotation.returning())) {
springAdvice.setReturningName(afterReturningAnnotation.returning());
}
break;
case AtAfterThrowing:
//實現了MethodInterceptor介面
springAdvice = new AspectJAfterThrowingAdvice(
candidateAdviceMethod, expressionPointcut, aspectInstanceFactory);
AfterThrowing afterThrowingAnnotation = (AfterThrowing) aspectJAnnotation.getAnnotation();
if (StringUtils.hasText(afterThrowingAnnotation.throwing())) {
springAdvice.setThrowingName(afterThrowingAnnotation.throwing());
}
break;
default:
throw new UnsupportedOperationException(
"Unsupported advice type on method: " + candidateAdviceMethod);
}
// Now to configure the advice...
springAdvice.setAspectName(aspectName);
springAdvice.setDeclarationOrder(declarationOrder);
String[] argNames = this.parameterNameDiscoverer.getParameterNames(candidateAdviceMethod);
if (argNames != null) {
springAdvice.setArgumentNamesFromStringArray(argNames);
}
//計算argNames和型別的對應關係
springAdvice.calculateArgumentBindings();
return springAdvice;
}
這裡邏輯很清晰，就是拿到方法上的註解型別，根據型別建立不同的增強Advice物件：AspectJAroundAdvice、AspectJMethodBeforeAdvice、AspectJAfterAdvice、AspectJAfterReturningAdvice、AspectJAfterThrowingAdvice。完成之後通過calculateArgumentBindings方法進行引數繫結，感興趣的可自行研究。這裡主要看看幾個Advice的繼承體系：
可以看到有兩個Advice是沒有實現MethodInterceptor介面的：AspectJMethodBeforeAdvice和AspectJAfterReturningAdvice。而MethodInterceptor有一個invoke方法，這個方法就是鏈式呼叫的核心方法，但那兩個沒有實現該方法的Advice怎麼處理呢？稍後會分析。
到這裡切面物件就建立完成了，接下來就是判斷當前建立的Bean例項是否和這些切面匹配以及對切面排序。匹配過程比較複雜，對理解主流程也沒什麼幫助，所以這裡就不展開分析，感興趣的自行分析（AbstractAdvisorAutoProxyCreator.findAdvisorsThatCanApply()）。下面看看排序的過程，回到AbstractAdvisorAutoProxyCreator.findEligibleAdvisors方法：
protected List<Advisor> findEligibleAdvisors(Class<?> beanClass, String beanName) {
//找到候選的切面,其實就是一個尋找有@Aspectj註解的過程，把工程中所有有這個註解的類封裝成Advisor返回
List<Advisor> candidateAdvisors = findCandidateAdvisors();
//判斷候選的切面是否作用在當前beanClass上面，就是一個匹配過程。。現在就是一個匹配
List<Advisor> eligibleAdvisors = findAdvisorsThatCanApply(candidateAdvisors, beanClass, beanName);
extendAdvisors(eligibleAdvisors);
if (!eligibleAdvisors.isEmpty()) {
//對有@Order@Priority進行排序
eligibleAdvisors = sortAdvisors(eligibleAdvisors);
}
return eligibleAdvisors;
}
sortAdvisors方法就是排序，但這個方法有兩個實現：當前類AbstractAdvisorAutoProxyCreator和子類AspectJAwareAdvisorAutoProxyCreator，應該走哪個呢？
通過類圖我們可以肯定是進入的AspectJAwareAdvisorAutoProxyCreator類，因為AnnotationAwareAspectJAutoProxyCreator的父類是它。
protected List<Advisor> sortAdvisors(List<Advisor> advisors) {
List<PartiallyComparableAdvisorHolder> partiallyComparableAdvisors = new ArrayList<>(advisors.size());
for (Advisor element : advisors) {
partiallyComparableAdvisors.add(
new PartiallyComparableAdvisorHolder(element, DEFAULT_PRECEDENCE_COMPARATOR));
}
List<PartiallyComparableAdvisorHolder> sorted = PartialOrder.sort(partiallyComparableAdvisors);
if (sorted != null) {
List<Advisor> result = new ArrayList<>(advisors.size());
for (PartiallyComparableAdvisorHolder pcAdvisor : sorted) {
result.add(pcAdvisor.getAdvisor());
}
return result;
}
else {
return super.sortAdvisors(advisors);
}
}
這裡排序主要是委託給PartialOrder進行的，而在此之前將所有的切面都封裝成了PartiallyComparableAdvisorHolder物件，注意傳入的DEFAULT_PRECEDENCE_COMPARATOR引數，這個就是比較器物件：
private static final Comparator<Advisor> DEFAULT_PRECEDENCE_COMPARATOR = new AspectJPrecedenceComparator();
所以我們直接看這個比較器的compare方法：
public int compare(Advisor o1, Advisor o2) {
int advisorPrecedence = this.advisorComparator.compare(o1, o2);
if (advisorPrecedence == SAME_PRECEDENCE && declaredInSameAspect(o1, o2)) {
advisorPrecedence = comparePrecedenceWithinAspect(o1, o2);
}
return advisorPrecedence;
}
private final Comparator<? super Advisor> advisorComparator;
public AspectJPrecedenceComparator() {
this.advisorComparator = AnnotationAwareOrderComparator.INSTANCE;
}
第一步先通過AnnotationAwareOrderComparator去比較，點進去看可以發現是對實現了PriorityOrdered和Ordered介面以及標記了Priority和Order註解的非同一個@Aspect類中的切面進行排序。這個和之前分析BeanFacotryPostProcessor類是一樣的原理。而對同一個@Aspect類中的切面排序主要是comparePrecedenceWithinAspect方法：
private int comparePrecedenceWithinAspect(Advisor advisor1, Advisor advisor2) {
boolean oneOrOtherIsAfterAdvice =
(AspectJAopUtils.isAfterAdvice(advisor1) || AspectJAopUtils.isAfterAdvice(advisor2));
int adviceDeclarationOrderDelta = getAspectDeclarationOrder(advisor1) - getAspectDeclarationOrder(advisor2);
if (oneOrOtherIsAfterAdvice) {
// the advice declared last has higher precedence
if (adviceDeclarationOrderDelta < 0) {
// advice1 was declared before advice2
// so advice1 has lower precedence
return LOWER_PRECEDENCE;
}
else if (adviceDeclarationOrderDelta == 0) {
return SAME_PRECEDENCE;
}
else {
return HIGHER_PRECEDENCE;
}
}
else {
// the advice declared first has higher precedence
if (adviceDeclarationOrderDelta < 0) {
// advice1 was declared before advice2
// so advice1 has higher precedence
return HIGHER_PRECEDENCE;
}
else if (adviceDeclarationOrderDelta == 0) {
return SAME_PRECEDENCE;
}
else {
return LOWER_PRECEDENCE;
}
}
}
private int getAspectDeclarationOrder(Advisor anAdvisor) {
AspectJPrecedenceInformation precedenceInfo =
AspectJAopUtils.getAspectJPrecedenceInformationFor(anAdvisor);
if (precedenceInfo != null) {
return precedenceInfo.getDeclarationOrder();
}
else {
return 0;
}
}
這裡就是通過precedenceInfo.getDeclarationOrder拿到在建立InstantiationModelAwarePointcutAdvisorImpl物件時設定的declarationOrder屬性，這就驗證了之前的說法（實際上這裡排序過程非常複雜，不是簡單的按照這個屬性進行排序）。
當上面的一切都進行完成後，就該建立代理物件了，回到AbstractAutoProxyCreator.wrapIfNecessary，看關鍵部分程式碼：
//如果有切面，則生成該bean的代理
if (specificInterceptors != DO_NOT_PROXY) {
this.advisedBeans.put(cacheKey, Boolean.TRUE);
//把被代理物件bean例項封裝到SingletonTargetSource物件中
Object proxy = createProxy(
bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));
this.proxyTypes.put(cacheKey, proxy.getClass());
return proxy;
}
注意這裡將被代理物件封裝成了一個SingletonTargetSource物件，它是TargetSource的實現類。
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);
}
//建立代理工廠
ProxyFactory proxyFactory = new ProxyFactory();
proxyFactory.copyFrom(this);
if (!proxyFactory.isProxyTargetClass()) {
if (shouldProxyTargetClass(beanClass, beanName)) {
//proxyTargetClass 是否對類進行代理，而不是對介面進行代理，設定為true時，使用CGLib代理。
proxyFactory.setProxyTargetClass(true);
}
else {
evaluateProxyInterfaces(beanClass, proxyFactory);
}
}
//把advice型別的增強包裝成advisor切面
Advisor[] advisors = buildAdvisors(beanName, specificInterceptors);
proxyFactory.addAdvisors(advisors);
proxyFactory.setTargetSource(targetSource);
customizeProxyFactory(proxyFactory);
////用來控制代理工廠被配置後，是否還允許修改代理的配置,預設為false
proxyFactory.setFrozen(this.freezeProxy);
if (advisorsPreFiltered()) {
proxyFactory.setPreFiltered(true);
}
//獲取代理例項
return proxyFactory.getProxy(getProxyClassLoader());
}
這裡通過ProxyFactory物件去建立代理例項，這是工廠模式的體現，但在建立代理物件之前還有幾個準備動作：需要判斷是JDK代理還是CGLIB代理以及通過buildAdvisors方法將擴充套件的Advice封裝成Advisor切面。準備完成則通過getProxy建立代理物件：
public Object getProxy(@Nullable ClassLoader classLoader) {
//根據目標物件是否有介面來判斷採用什麼代理方式，cglib代理還是jdk動態代理
return createAopProxy().getProxy(classLoader);
}
protected final synchronized AopProxy createAopProxy() {
if (!this.active) {
activate();
}
return getAopProxyFactory().createAopProxy(this);
}
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.");
}
if (targetClass.isInterface() || Proxy.isProxyClass(targetClass)) {
return new JdkDynamicAopProxy(config);
}
return new ObjenesisCglibAopProxy(config);
}
else {
return new JdkDynamicAopProxy(config);
}
}
首先通過配置拿到對應的代理類：ObjenesisCglibAopProxy和JdkDynamicAopProxy，然後再通過getProxy建立Bean的代理，這裡以JdkDynamicAopProxy為例：
public Object getProxy(@Nullable ClassLoader classLoader) {
//advised是代理工廠物件
Class<?>[] proxiedInterfaces = AopProxyUtils.completeProxiedInterfaces(this.advised, true);
findDefinedEqualsAndHashCodeMethods(proxiedInterfaces);
return Proxy.newProxyInstance(classLoader, proxiedInterfaces, this);
}
這裡的程式碼你應該不陌生了，就是JDK的原生API，newProxyInstance方法傳入的InvocationHandler物件是this，因此，最終AOP代理的呼叫就是從該類中的invoke方法開始。至此，代理物件的建立就完成了，下面來看下整個過程的時序圖：
小結
代理物件的建立過程整體來說並不複雜，首先找到所有帶有@Aspect註解的類，並獲取其中沒有@Pointcut註解的方法，迴圈建立切面，而建立切面需要切點和增強兩個元素，其中切點可簡單理解為我們寫的表示式，增強則是根據@Before、@Around、@After等註解建立的對應的Advice類。切面建立好後則需要迴圈判斷哪些切面能對當前的Bean例項的方法進行增強並排序，最後通過ProxyFactory建立代理物件。
AOP鏈式呼叫
熟悉JDK動態代理的都知道通過代理物件呼叫方法時，會進入到InvocationHandler物件的invoke方法，所以我們直接從JdkDynamicAopProxy的這個方法開始：
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
MethodInvocation invocation;
Object oldProxy = null;
boolean setProxyContext = false;
//從代理工廠中拿到TargetSource物件，該物件包裝了被代理例項bean
TargetSource targetSource = this.advised.targetSource;
Object target = null;
try {
//被代理物件的equals方法和hashCode方法是不能被代理的，不會走切面
.......
Object retVal;
// 可以從當前執行緒中拿到代理物件
if (this.advised.exposeProxy) {
// Make invocation available if necessary.
oldProxy = AopContext.setCurrentProxy(proxy);
setProxyContext = true;
}
//這個target就是被代理例項
target = targetSource.getTarget();
Class<?> targetClass = (target != null ? target.getClass() : null);
//從代理工廠中拿過濾器鏈 Object是一個MethodInterceptor型別的物件，其實就是一個advice物件
List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);
//如果該方法沒有執行鏈，則說明這個方法不需要被攔截，則直接反射呼叫
if (chain.isEmpty()) {
Object[] argsToUse = AopProxyUtils.adaptArgumentsIfNecessary(method, args);
retVal = AopUtils.invokeJoinpointUsingReflection(target, method, argsToUse);
}
else {
invocation = new ReflectiveMethodInvocation(proxy, target, method, args, targetClass, chain);
retVal = invocation.proceed();
}
// Massage return value if necessary.
Class<?> returnType = method.getReturnType();
if (retVal != null && retVal == target &&
returnType != Object.class && returnType.isInstance(proxy) &&
!RawTargetAccess.class.isAssignableFrom(method.getDeclaringClass())) {
retVal = proxy;
}
return retVal;
}
finally {
if (target != null && !targetSource.isStatic()) {
// Must have come from TargetSource.
targetSource.releaseTarget(target);
}
if (setProxyContext) {
// Restore old proxy.
AopContext.setCurrentProxy(oldProxy);
}
}
}
這段程式碼比較長，我刪掉了不關鍵的地方。首先來看this.advised.exposeProxy這個屬性，這在@EnableAspectJAutoProxy註解中可以配置，當為true時，會將該代理物件設定到當前執行緒的ThreadLocal物件中，這樣就可以通過AopContext.currentProxy拿到代理物件。這個有什麼用呢？我相信有經驗的Java開發都遇到過這樣一個BUG，在Service實現類中呼叫本類中的另一個方法時，事務不會生效，這是因為直接通過this呼叫就不會呼叫到代理物件的方法，而是原物件的，所以事務切面就沒有生效。因此這種情況下就可以從當前執行緒的ThreadLocal物件拿到代理物件，不過實際上直接使用@Autowired注入自己本身也可以拿到代理物件。
接下來就是通過getInterceptorsAndDynamicInterceptionAdvice拿到執行鏈，看看具體做了哪些事情：
public List<Object> getInterceptorsAndDynamicInterceptionAdvice(
Advised config, Method method, @Nullable Class<?> targetClass) {
AdvisorAdapterRegistry registry = GlobalAdvisorAdapterRegistry.getInstance();
//從代理工廠中獲得該被代理類的所有切面advisor，config就是代理工廠物件
Advisor[] advisors = config.getAdvisors();
List<Object> interceptorList = new ArrayList<>(advisors.length);
Class<?> actualClass = (targetClass != null ? targetClass : method.getDeclaringClass());
Boolean hasIntroductions = null;
for (Advisor advisor : advisors) {
//大部分走這裡
if (advisor instanceof PointcutAdvisor) {
// Add it conditionally.
PointcutAdvisor pointcutAdvisor = (PointcutAdvisor) advisor;
//如果切面的pointCut和被代理物件是匹配的，說明是切面要攔截的物件
if (config.isPreFiltered() || pointcutAdvisor.getPointcut().getClassFilter().matches(actualClass)) {
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 {
//接下來判斷方法是否是切面pointcut需要攔截的方法
match = mm.matches(method, actualClass);
}
//如果類和方法都匹配
if (match) {
//獲取到切面advisor中的advice，並且包裝成MethodInterceptor型別的物件
MethodInterceptor[] interceptors = registry.getInterceptors(advisor);
if (mm.isRuntime()) {
for (MethodInterceptor interceptor : interceptors) {
interceptorList.add(new InterceptorAndDynamicMethodMatcher(interceptor, mm));
}
}
else {
interceptorList.addAll(Arrays.asList(interceptors));
}
}
}
}
//如果是引介切面
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;
}
這也是個長方法，看關鍵的部分，因為之前我們建立的基本上都是InstantiationModelAwarePointcutAdvisorImpl物件，該類是PointcutAdvisor的實現類，所以會進入第一個if判斷裡，這裡首先進行匹配，看切點和當前物件以及該物件的哪些方法匹配，如果能匹配上，則呼叫getInterceptors獲取執行鏈：
private final List<AdvisorAdapter> adapters = new ArrayList<>(3);
public DefaultAdvisorAdapterRegistry() {
registerAdvisorAdapter(new MethodBeforeAdviceAdapter());
registerAdvisorAdapter(new AfterReturningAdviceAdapter());
registerAdvisorAdapter(new ThrowsAdviceAdapter());
}
public MethodInterceptor[] getInterceptors(Advisor advisor) throws UnknownAdviceTypeException {
List<MethodInterceptor> interceptors = new ArrayList<>(3);
Advice advice = advisor.getAdvice();
//如果是MethodInterceptor型別的，如：AspectJAroundAdvice
//AspectJAfterAdvice
//AspectJAfterThrowingAdvice
if (advice instanceof MethodInterceptor) {
interceptors.add((MethodInterceptor) advice);
}
//處理 AspectJMethodBeforeAdvice AspectJAfterReturningAdvice
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]);
}
這裡我們可以看到如果是MethodInterceptor的實現類，則直接新增到鏈中，如果不是，則需要通過介面卡去包裝後新增，剛好這裡有MethodBeforeAdviceAdapter和AfterReturningAdviceAdapter兩個介面卡對應上文兩個沒有實現MethodInterceptor介面的類。最後將Interceptors返回。
if (chain.isEmpty()) {
Object[] argsToUse = AopProxyUtils.adaptArgumentsIfNecessary(method, args);
retVal = AopUtils.invokeJoinpointUsingReflection(target, method, argsToUse);
}
else {
// We need to create a method invocation...
invocation = new ReflectiveMethodInvocation(proxy, target, method, args, targetClass, chain);
// Proceed to the joinpoint through the interceptor chain.
retVal = invocation.proceed();
}
返回到invoke方法後，如果執行鏈為空，說明該方法不需要被增強，所以直接反射呼叫原物件的方法（注意傳入的是TargetSource封裝的被代理物件）；反之，則通過ReflectiveMethodInvocation類進行鏈式呼叫，關鍵方法就是proceed：
private int currentInterceptorIndex = -1;
public Object proceed() throws Throwable {
//如果執行鏈中的advice全部執行完，則直接呼叫joinPoint方法，就是被代理方法
if (this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size() - 1) {
return invokeJoinpoint();
}
Object interceptorOrInterceptionAdvice =
this.interceptorsAndDynamicMethodMatchers.get(++this.currentInterceptorIndex);
if (interceptorOrInterceptionAdvice instanceof InterceptorAndDynamicMethodMatcher) {
InterceptorAndDynamicMethodMatcher dm =
(InterceptorAndDynamicMethodMatcher) interceptorOrInterceptionAdvice;
Class<?> targetClass = (this.targetClass != null ? this.targetClass : this.method.getDeclaringClass());
if (dm.methodMatcher.matches(this.method, targetClass, this.arguments)) {
return dm.interceptor.invoke(this);
}
else {
return proceed();
}
}
else {
//呼叫MethodInterceptor中的invoke方法
return ((MethodInterceptor) interceptorOrInterceptionAdvice).invoke(this);
}
}
這個方法的核心就在兩個地方：invokeJoinpoint和interceptorOrInterceptionAdvice.invoke(this)。當增強方法呼叫完後就會通過前者呼叫到被代理的方法，否則則是依次呼叫Interceptor的invoke方法。下面就分別看看每個Interceptor是怎麼實現的。
- AspectJAroundAdvice
public Object invoke(MethodInvocation mi) throws Throwable {
if (!(mi instanceof ProxyMethodInvocation)) {
throw new IllegalStateException("MethodInvocation is not a Spring ProxyMethodInvocation: " + mi);
}
ProxyMethodInvocation pmi = (ProxyMethodInvocation) mi;
ProceedingJoinPoint pjp = lazyGetProceedingJoinPoint(pmi);
JoinPointMatch jpm = getJoinPointMatch(pmi);
return invokeAdviceMethod(pjp, jpm, null, null);
}
- MethodBeforeAdviceInterceptor -> AspectJMethodBeforeAdvice
public Object invoke(MethodInvocation mi) throws Throwable {
this.advice.before(mi.getMethod(), mi.getArguments(), mi.getThis());
return mi.proceed();
}
public void before(Method method, Object[] args, @Nullable Object target) throws Throwable {
invokeAdviceMethod(getJoinPointMatch(), null, null);
}
- AspectJAfterAdvice
public Object invoke(MethodInvocation mi) throws Throwable {
try {
return mi.proceed();
}
finally {
invokeAdviceMethod(getJoinPointMatch(), null, null);
}
}
- AfterReturningAdviceInterceptor -> AspectJAfterReturningAdvice
public Object invoke(MethodInvocation mi) throws Throwable {
Object retVal = mi.proceed();
this.advice.afterReturning(retVal, mi.getMethod(), mi.getArguments(), mi.getThis());
return retVal;
}
public void afterReturning(@Nullable Object returnValue, Method method, Object[] args, @Nullable Object target) throws Throwable {
if (shouldInvokeOnReturnValueOf(method, returnValue)) {
invokeAdviceMethod(getJoinPointMatch(), returnValue, null);
}
}
- AspectJAfterThrowingAdvice
public Object invoke(MethodInvocation mi) throws Throwable {
try {
return mi.proceed();
}
catch (Throwable ex) {
if (shouldInvokeOnThrowing(ex)) {
invokeAdviceMethod(getJoinPointMatch(), null, ex);
}
throw ex;
}
}
這裡的呼叫順序是怎樣的呢？其核心就是通過proceed方法控制流程，每執行完一個Advice就會回到proceed方法中呼叫下一個Advice。可以思考一下，怎麼才能讓呼叫結果滿足如下圖的執行順序。
以上就是AOP的鏈式呼叫過程，但是這只是只有一個切面類的情況，如果有多個@Aspect類呢，這個呼叫過程又是怎樣的？其核心思想和“棧”一樣，就是“先進後出，後進先出”。
AOP擴充套件知識
一、自定義全域性攔截器Interceptor
在上文建立代理物件的時候有這樣一個方法：
protected Advisor[] buildAdvisors(@Nullable String beanName, @Nullable Object[] specificInterceptors) {
//自定義MethodInterceptor.拿到setInterceptorNames方法注入的Interceptor物件
Advisor[] commonInterceptors = resolveInterceptorNames();
List<Object> allInterceptors = new ArrayList<>();
if (specificInterceptors != null) {
allInterceptors.addAll(Arrays.asList(specificInterceptors));
if (commonInterceptors.length > 0) {
if (this.applyCommonInterceptorsFirst) {
allInterceptors.addAll(0, Arrays.asList(commonInterceptors));
}
else {
allInterceptors.addAll(Arrays.asList(commonInterceptors));
}
}
}
Advisor[] advisors = new Advisor[allInterceptors.size()];
for (int i = 0; i < allInterceptors.size(); i++) {
//對自定義的advice要進行包裝，把advice包裝成advisor物件，切面物件
advisors[i] = this.advisorAdapterRegistry.wrap(allInterceptors.get(i));
}
return advisors;
}
這個方法的作用就在於我們可以擴充套件我們自己的Interceptor，首先通過resolveInterceptorNames方法獲取到通過setInterceptorNames方法設定的Interceptor，然後呼叫DefaultAdvisorAdapterRegistry.wrap方法將其包裝為DefaultPointcutAdvisor物件並返回：
public Advisor wrap(Object adviceObject) throws UnknownAdviceTypeException {
if (adviceObject instanceof Advisor) {
return (Advisor) adviceObject;
}
if (!(adviceObject instanceof Advice)) {
throw new UnknownAdviceTypeException(adviceObject);
}
Advice advice = (Advice) adviceObject;
if (advice instanceof MethodInterceptor) {
return new DefaultPointcutAdvisor(advice);
}
for (AdvisorAdapter adapter : this.adapters) {
if (adapter.supportsAdvice(advice)) {
return new DefaultPointcutAdvisor(advice);
}
}
throw new UnknownAdviceTypeException(advice);
}
public DefaultPointcutAdvisor(Advice advice) {
this(Pointcut.TRUE, advice);
}
需要注意DefaultPointcutAdvisor構造器裡面傳入了一個Pointcut.TRUE，表示這種擴充套件的Interceptor是全域性的攔截器。下面來看看如何使用：
public class MyMethodInterceptor implements MethodInterceptor {
@Override
public Object invoke(MethodInvocation invocation) throws Throwable {
System.out.println("自定義攔截器");
return invocation.proceed();
}
}
首先寫一個類實現MethodInterceptor 介面，在invoke方法中實現我們的攔截邏輯，然後通過下面的方式測試，只要UserService 有AOP攔截就會發現自定義的MyMethodInterceptor也生效了。
public void costomInterceptorTest() {
AnnotationAwareAspectJAutoProxyCreator bean = applicationContext.getBean(AnnotationAwareAspectJAutoProxyCreator.class);
bean.setInterceptorNames("myMethodInterceptor ");
UserService userService = applicationContext.getBean(UserService.class);
userService.queryUser("dark");
}
但是如果換個順序，像下面這樣：
public void costomInterceptorTest() {
UserService userService = applicationContext.getBean(UserService.class);
AnnotationAwareAspectJAutoProxyCreator bean = applicationContext.getBean(AnnotationAwareAspectJAutoProxyCreator.class);
bean.setInterceptorNames("myMethodInterceptor ");
userService.queryUser("dark");
}
這時自定義的全域性攔截器就沒有作用了，這是為什麼呢？因為當執行getBean的時候，如果有切面匹配就會通過ProxyFactory去建立代理物件，注意Interceptor是存到這個Factory物件中的，而這個物件和代理物件是一一對應的，因此呼叫getBean時，還沒有myMethodInterceptor這個物件，自定義攔截器就沒有效果了，也就是說要想自定義攔截器生效，就必須在代理物件生成之前註冊進去。
二、迴圈依賴三級快取存在的必要性
在上一篇文章我分析了Spring是如何通過三級快取來解決迴圈依賴的問題的，但你是否考慮過第三級快取為什麼要存在？我直接將bean存到二級不就行了麼，為什麼還要存一個ObjectFactory物件到第三級快取中？這個在學習了AOP之後就很清楚了，因為我們在@Autowired物件時，想要注入的不一定是Bean本身，而是想要注入一個修改過後的物件，如代理物件。在AbstractAutowireCapableBeanFactory.getEarlyBeanReference方法中迴圈呼叫了SmartInstantiationAwareBeanPostProcessor.getEarlyBeanReference方法，AbstractAutoProxyCreator物件就實現了該方法：
public Object getEarlyBeanReference(Object bean, String beanName) {
Object cacheKey = getCacheKey(bean.getClass(), beanName);
if (!this.earlyProxyReferences.contains(cacheKey)) {
this.earlyProxyReferences.add(cacheKey);
}
// 建立代理物件
return wrapIfNecessary(bean, beanName, cacheKey);
}
因此，當我們想要對循壞依賴的Bean做出修改時，就可以像AOP這樣做。
三、如何在Bean建立之前提前建立代理物件
Spring的代理物件基本上都是在Bean例項化完成之後建立的，但在文章開始我就說過，Spring也提供了一個機會在建立Bean物件之前就建立代理物件，在AbstractAutowireCapableBeanFactory.resolveBeforeInstantiation方法中：
protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) {
Object bean = null;
if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) {
// Make sure bean class is actually resolved at this point.
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
Class<?> targetType = determineTargetType(beanName, mbd);
if (targetType != null) {
bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);
if (bean != null) {
bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);
}
}
}
mbd.beforeInstantiationResolved = (bean != null);
}
return bean;
}
protected Object applyBeanPostProcessorsBeforeInstantiation(Class<?> beanClass, String beanName) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
Object result = ibp.postProcessBeforeInstantiation(beanClass, beanName);
if (result != null) {
return result;
}
}
}
return null;
}
主要是InstantiationAwareBeanPostProcessor.postProcessBeforeInstantiation方法中，這裡又會進入到AbstractAutoProxyCreator類中：
public Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) {
TargetSource targetSource = getCustomTargetSource(beanClass, beanName);
if (targetSource != null) {
if (StringUtils.hasLength(beanName)) {
this.targetSourcedBeans.add(beanName);
}
Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(beanClass, beanName, targetSource);
Object proxy = createProxy(beanClass, beanName, specificInterceptors, targetSource);
this.proxyTypes.put(cacheKey, proxy.getClass());
return proxy;
}
return null;
}
protected TargetSource getCustomTargetSource(Class<?> beanClass, String beanName) {
// We can't create fancy target sources for directly registered singletons.
if (this.customTargetSourceCreators != null &&
this.beanFactory != null && this.beanFactory.containsBean(beanName)) {
for (TargetSourceCreator tsc : this.customTargetSourceCreators) {
TargetSource ts = tsc.getTargetSource(beanClass, beanName);
if (ts != null) {
return ts;
}
}
}
// No custom TargetSource found.
return null;
}
看到這裡大致應該明白了，先是獲取到一個自定義的TargetSource物件，然後建立代理物件，所以我們首先需要自己實現一個TargetSource類，這裡直接繼承一個抽象類，getTarget方法則返回原始物件：
public class MyTargetSource extends AbstractBeanFactoryBasedTargetSource {
@Override
public Object getTarget() throws Exception {
return getBeanFactory().getBean(getTargetBeanName());
}
}
但這還不夠，上面首先判斷了customTargetSourceCreators!=null，而這個屬性是個陣列，可以通過下面這個方法設定進來：
public void setCustomTargetSourceCreators(TargetSourceCreator... targetSourceCreators) {
this.customTargetSourceCreators = targetSourceCreators;
}
所以我們還要實現一個TargetSourceCreator類，同樣繼承一個抽象類實現，並只對userServiceImpl物件進行攔截：
public class MyTargetSourceCreator extends AbstractBeanFactoryBasedTargetSourceCreator {
@Override
protected AbstractBeanFactoryBasedTargetSource createBeanFactoryBasedTargetSource(Class<?> beanClass, String beanName) {
if (getBeanFactory() instanceof ConfigurableListableBeanFactory) {
if(beanName.equalsIgnoreCase("userServiceImpl")) {
return new MyTargetSource();
}
}
return null;
}
}
createBeanFactoryBasedTargetSource方法是在AbstractBeanFactoryBasedTargetSourceCreator.getTargetSource中呼叫的，而getTargetSource就是在上面getCustomTargetSource中呼叫的。以上工作做完後，還需要將其設定到AnnotationAwareAspectJAutoProxyCreator物件中，因此需要我們注入這個物件：
@Configuration
public class TargetSourceCreatorBean {
@Autowired
private BeanFactory beanFactory;
@Bean
public AnnotationAwareAspectJAutoProxyCreator annotationAwareAspectJAutoProxyCreator() {
AnnotationAwareAspectJAutoProxyCreator creator = new AnnotationAwareAspectJAutoProxyCreator();
MyTargetSourceCreator myTargetSourceCreator = new MyTargetSourceCreator();
myTargetSourceCreator.setBeanFactory(beanFactory);
creator.setCustomTargetSourceCreators(myTargetSourceCreator);
return creator;
}
}
這樣，當我們通過getBean獲取userServiceImpl的物件時，就會優先生成代理物件，然後在呼叫執行鏈的過程中再通過TargetSource.getTarget獲取到被代理物件。但是，為什麼我們在getTarget方法中呼叫getBean就能拿到被代理物件呢？
繼續探究，通過斷點我發現從getTarget進入時，在resolveBeforeInstantiation方法中返回的bean就是null了，而getBeanPostProcessors方法返回的Processors中也沒有了AnnotationAwareAspectJAutoProxyCreator物件，也就是沒有進入到AbstractAutoProxyCreator.postProcessBeforeInstantiation方法中，所以不會再次獲取到代理物件，那AnnotationAwareAspectJAutoProxyCreator物件是在什麼時候移除的呢？
帶著問題，我開始反推，發現在AbstractBeanFactoryBasedTargetSourceCreator類中有這樣一個方法buildInternalBeanFactory：
protected DefaultListableBeanFactory buildInternalBeanFactory(ConfigurableBeanFactory containingFactory) {
DefaultListableBeanFactory internalBeanFactory = new DefaultListableBeanFactory(containingFactory);
// Required so that all BeanPostProcessors, Scopes, etc become available.
internalBeanFactory.copyConfigurationFrom(containingFactory);
// Filter out BeanPostProcessors that are part of the AOP infrastructure,
// since those are only meant to apply to beans defined in the original factory.
internalBeanFactory.getBeanPostProcessors().removeIf(beanPostProcessor ->
beanPostProcessor instanceof AopInfrastructureBean);
return internalBeanFactory;
}
在這裡移除掉了所有AopInfrastructureBean的子類，而AnnotationAwareAspectJAutoProxyCreator就是其子類，那這個方法是在哪裡呼叫的呢？繼續反推：
protected DefaultListableBeanFactory getInternalBeanFactoryForBean(String beanName) {
synchronized (this.internalBeanFactories) {
DefaultListableBeanFactory internalBeanFactory = this.internalBeanFactories.get(beanName);
if (internalBeanFactory == null) {
internalBeanFactory = buildInternalBeanFactory(this.beanFactory);
this.internalBeanFactories.put(beanName, internalBeanFactory);
}
return internalBeanFactory;
}
}
public final TargetSource getTargetSource(Class<?> beanClass, String beanName) {
AbstractBeanFactoryBasedTargetSource targetSource =
createBeanFactoryBasedTargetSource(beanClass, beanName);
// 建立完targetSource後就移除掉AopInfrastructureBean型別的BeanPostProcessor物件，如AnnotationAwareAspectJAutoProxyCreator
DefaultListableBeanFactory internalBeanFactory = getInternalBeanFactoryForBean(beanName);
......
return targetSource;
}
至此，關於TargetSource介面擴充套件的原理就搞明白了。
總結
本篇篇幅比較長，主要搞明白Spring代理物件是如何建立的以及AOP鏈式呼叫過程，而後面的擴充套件則是對AOP以及Bean建立過程中一些疑惑的補充，可根據實際情況學習掌握。