• 【Spring源码分析】AOP源码解析(下篇)


    AspectJAwareAdvisorAutoProxyCreator及为Bean生成代理时机分析

    上篇文章说了,org.springframework.aop.aspectj.autoproxy.AspectJAwareAdvisorAutoProxyCreator这个类是Spring提供给开发者的AOP的核心类,就是AspectJAwareAdvisorAutoProxyCreator完成了【类/接口-->代理】的转换过程,首先我们看一下AspectJAwareAdvisorAutoProxyCreator的层次结构:

    这里最值得注意的一点是最左下角的那个方框,我用几句话总结一下:

    1. AspectJAwareAdvisorAutoProxyCreator是BeanPostProcessor接口的实现类
    2. postProcessBeforeInitialization方法与postProcessAfterInitialization方法实现在父类AbstractAutoProxyCreator
    3. postProcessBeforeInitialization方法是一个空实现
    4. 逻辑代码在postProcessAfterInitialization方法中

    基于以上的分析,将Bean生成代理的时机已经一目了然了:在每个Bean初始化之后,如果需要,调用AspectJAwareAdvisorAutoProxyCreator中的postProcessBeforeInitialization为Bean生成代理

    代理对象实例化----判断是否为<bean>生成代理

    上文分析了Bean生成代理的时机是在每个Bean初始化之后,下面把代码定位到Bean初始化之后,先是AbstractAutowireCapableBeanFactory的initializeBean方法进行初始化:

     1 protected Object initializeBean(final String beanName, final Object bean, RootBeanDefinition mbd) {
     2     if (System.getSecurityManager() != null) {
     3         AccessController.doPrivileged(new PrivilegedAction<Object>() {
     4             public Object run() {
     5                 invokeAwareMethods(beanName, bean);
     6                 return null;
     7             }
     8         }, getAccessControlContext());
     9     }
    10     else {
    11         invokeAwareMethods(beanName, bean);
    12     }
    13     
    14     Object wrappedBean = bean;
    15     if (mbd == null || !mbd.isSynthetic()) {
    16         wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
    17     }
    18 
    19     try {
    20     invokeInitMethods(beanName, wrappedBean, mbd);
    21     }
    22     catch (Throwable ex) {
    23         throw new BeanCreationException(
    24                 (mbd != null ? mbd.getResourceDescription() : null),
    25                 beanName, "Invocation of init method failed", ex);
    26     }
    27 
    28     if (mbd == null || !mbd.isSynthetic()) {
    29         wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
    30     }
    31     return wrappedBean;
    32 }

    初始化之前是第16行的applyBeanPostProcessorsBeforeInitialization方法,初始化之后即29行的applyBeanPostProcessorsAfterInitialization方法:

     1 public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
     2         throws BeansException {
     3 
     4     Object result = existingBean;
     5     for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {
     6         result = beanProcessor.postProcessAfterInitialization(result, beanName);
     7         if (result == null) {
     8             return result;
     9         }
    10     }
    11     return result;
    12 }

    这里调用每个BeanPostProcessor的postProcessBeforeInitialization方法。按照之前的分析,看一下AbstractAutoProxyCreator的postProcessAfterInitialization方法实现:

    1 public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
    2     if (bean != null) {
    3         Object cacheKey = getCacheKey(bean.getClass(), beanName);
    4         if (!this.earlyProxyReferences.contains(cacheKey)) {
    5             return wrapIfNecessary(bean, beanName, cacheKey);
    6         }
    7     }
    8     return bean;
    9 }

    跟一下第5行的方法wrapIfNecessary:

     1 protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
     2     if (this.targetSourcedBeans.contains(beanName)) {
     3         return bean;
     4     }
     5     if (this.nonAdvisedBeans.contains(cacheKey)) {
     6         return bean;
     7     }
     8     if (isInfrastructureClass(bean.getClass()) || shouldSkip(bean.getClass(), beanName)) {
     9         this.nonAdvisedBeans.add(cacheKey);
    10         return bean;
    11     }
    12 
    13     // Create proxy if we have advice.
    14     Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
    15     if (specificInterceptors != DO_NOT_PROXY) {
    16         this.advisedBeans.add(cacheKey);
    17         Object proxy = createProxy(bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));
    18         this.proxyTypes.put(cacheKey, proxy.getClass());
    19         return proxy;
    20     }
    21 
    22     this.nonAdvisedBeans.add(cacheKey);
    23     return bean;
    24 }

    第2行~第11行是一些不需要生成代理的场景判断,这里略过。首先我们要思考的第一个问题是:哪些目标对象需要生成代理因为配置文件里面有很多Bean,肯定不能对每个Bean都生成代理,因此需要一套规则判断Bean是不是需要生成代理,这套规则就是第14行的代码getAdvicesAndAdvisorsForBean:

    1 protected List<Advisor> findEligibleAdvisors(Class beanClass, String beanName) {
    2     List<Advisor> candidateAdvisors = findCandidateAdvisors();
    3     List<Advisor> eligibleAdvisors = findAdvisorsThatCanApply(candidateAdvisors, beanClass, beanName);
    4     extendAdvisors(eligibleAdvisors);
    5     if (!eligibleAdvisors.isEmpty()) {
    6         eligibleAdvisors = sortAdvisors(eligibleAdvisors);
    7     }
    8     return eligibleAdvisors;
    9 }

    顾名思义,方法的意思是为指定class寻找合适的Advisor。

    第2行代码,寻找候选Advisors,根据上文的配置文件,有两个候选Advisor,分别是<aop:aspect>节点下的<aop:before>和<aop:after>这两个,这两个在XML解析的时候已经被转换生成了RootBeanDefinition。

    跳过第3行的代码,先看下第4行的代码extendAdvisors方法,之后再重点看一下第3行的代码。第4行的代码extendAdvisors方法作用是向候选Advisor链的开头(也就是List.get(0)的位置)添加一个org.springframework.aop.support.DefaultPointcutAdvisor

    第3行代码,根据候选Advisors,寻找可以使用的Advisor,跟一下方法实现:

     1 public static List<Advisor> findAdvisorsThatCanApply(List<Advisor> candidateAdvisors, Class<?> clazz) {
     2     if (candidateAdvisors.isEmpty()) {
     3         return candidateAdvisors;
     4     }
     5     List<Advisor> eligibleAdvisors = new LinkedList<Advisor>();
     6     for (Advisor candidate : candidateAdvisors) {
     7         if (candidate instanceof IntroductionAdvisor && canApply(candidate, clazz)) {
     8             eligibleAdvisors.add(candidate);
     9         }
    10     }
    11     boolean hasIntroductions = !eligibleAdvisors.isEmpty();
    12     for (Advisor candidate : candidateAdvisors) {
    13         if (candidate instanceof IntroductionAdvisor) {
    14             // already processed
    15             continue;
    16         }
    17         if (canApply(candidate, clazz, hasIntroductions)) {
    18             eligibleAdvisors.add(candidate);
    19         }
    20     }
    21     return eligibleAdvisors;
    22 }

    整个方法的主要判断都围绕canApply展开方法:

     1 public static boolean canApply(Advisor advisor, Class<?> targetClass, boolean hasIntroductions) {
     2     if (advisor instanceof IntroductionAdvisor) {
     3         return ((IntroductionAdvisor) advisor).getClassFilter().matches(targetClass);
     4     }
     5     else if (advisor instanceof PointcutAdvisor) {
     6         PointcutAdvisor pca = (PointcutAdvisor) advisor;
     7         return canApply(pca.getPointcut(), targetClass, hasIntroductions);
     8     }
     9     else {
    10         // It doesn't have a pointcut so we assume it applies.
    11         return true;
    12     }
    13 }

    第一个参数advisor的实际类型是AspectJPointcutAdvisor,它是PointcutAdvisor的子类,因此执行第7行的方法:

     1 public static boolean canApply(Pointcut pc, Class<?> targetClass, boolean hasIntroductions) {
     2     if (!pc.getClassFilter().matches(targetClass)) {
     3         return false;
     4     }
     5 
     6     MethodMatcher methodMatcher = pc.getMethodMatcher();
     7     IntroductionAwareMethodMatcher introductionAwareMethodMatcher = null;
     8     if (methodMatcher instanceof IntroductionAwareMethodMatcher) {
     9         introductionAwareMethodMatcher = (IntroductionAwareMethodMatcher) methodMatcher;
    10     }
    11 
    12     Set<Class> classes = new HashSet<Class>(ClassUtils.getAllInterfacesForClassAsSet(targetClass));
    13     classes.add(targetClass);
    14     for (Class<?> clazz : classes) {
    15         Method[] methods = clazz.getMethods();
    16         for (Method method : methods) {
    17             if ((introductionAwareMethodMatcher != null &&
    18                 introductionAwareMethodMatcher.matches(method, targetClass, hasIntroductions)) ||
    19                     methodMatcher.matches(method, targetClass)) {
    20                 return true;
    21             }
    22         }
    23     }
    24     return false;
    25 }

    这个方法其实就是拿当前Advisor对应的expression做了两层判断:

    1. 目标类必须满足expression的匹配规则
    2. 目标类中的方法必须满足expression的匹配规则,当然这里方法不是全部需要满足expression的匹配规则,有一个方法满足即可

    如果以上两条都满足,那么容器则会判断该<bean>满足条件,需要被生成代理对象,具体方式为返回一个数组对象,该数组对象中存储的是<bean>对应的Advisor。

    代理对象实例化----为<bean>生成代理代码上下文梳理

    上文分析了为<bean>生成代理的条件,现在就正式看一下Spring上下文是如何为<bean>生成代理的。回到AbstractAutoProxyCreator的wrapIfNecessary方法:

     1 protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
     2     if (this.targetSourcedBeans.contains(beanName)) {
     3         return bean;
     4     }
     5     if (this.nonAdvisedBeans.contains(cacheKey)) {
     6         return bean;
     7     }
     8     if (isInfrastructureClass(bean.getClass()) || shouldSkip(bean.getClass(), beanName)) {
     9         this.nonAdvisedBeans.add(cacheKey);
    10         return bean;
    11     }
    12 
    13     // Create proxy if we have advice.
    14     Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
    15     if (specificInterceptors != DO_NOT_PROXY) {
    16         this.advisedBeans.add(cacheKey);
    17         Object proxy = createProxy(bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));
    18         this.proxyTypes.put(cacheKey, proxy.getClass());
    19         return proxy;
    20     }
    21 
    22     this.nonAdvisedBeans.add(cacheKey);
    23     return bean;
    24 }

    第14行拿到<bean>对应的Advisor数组,第15行判断只要Advisor数组不为空,那么就会通过第17行的代码为<bean>创建代理:

     1 protected Object createProxy(
     2         Class<?> beanClass, String beanName, Object[] specificInterceptors, TargetSource targetSource) {
     3 
     4     ProxyFactory proxyFactory = new ProxyFactory();
     5     // Copy our properties (proxyTargetClass etc) inherited from ProxyConfig.
     6     proxyFactory.copyFrom(this);
     7 
     8     if (!shouldProxyTargetClass(beanClass, beanName)) {
     9         // Must allow for introductions; can't just set interfaces to
    10         // the target's interfaces only.
    11         Class<?>[] targetInterfaces = ClassUtils.getAllInterfacesForClass(beanClass, this.proxyClassLoader);
    12         for (Class<?> targetInterface : targetInterfaces) {
    13             proxyFactory.addInterface(targetInterface);
    14         }
    15     }
    16 
    17     Advisor[] advisors = buildAdvisors(beanName, specificInterceptors);
    18     for (Advisor advisor : advisors) {
    19         proxyFactory.addAdvisor(advisor);
    20     }
    21 
    22     proxyFactory.setTargetSource(targetSource);
    23     customizeProxyFactory(proxyFactory);
    24 
    25     proxyFactory.setFrozen(this.freezeProxy);
    26     if (advisorsPreFiltered()) {
    27         proxyFactory.setPreFiltered(true);
    28     }
    29 
    30     return proxyFactory.getProxy(this.proxyClassLoader);
    31 }

    第4行~第6行new出了一个ProxyFactory,Proxy,顾名思义,代理工厂的意思,提供了简单的方式使用代码获取和配置AOP代理。

    第8行的代码做了一个判断,判断的内容是<aop:config>这个节点中proxy-target-class="false"或者proxy-target-class不配置,即不使用CGLIB生成代理。如果满足条件,进判断,获取当前Bean实现的所有接口,讲这些接口Class对象都添加到ProxyFactory中。

    第17行~第28行的代码没什么看的必要,向ProxyFactory中添加一些参数而已。重点看第30行proxyFactory.getProxy(this.proxyClassLoader)这句:

     1 public Object getProxy(ClassLoader classLoader) {
     2     return createAopProxy().getProxy(classLoader);
     3 }

    实现代码就一行,但是却明确告诉我们做了两件事情:

    1. 创建AopProxy接口实现类
    2. 通过AopProxy接口的实现类的getProxy方法获取<bean>对应的代理

    就从这两个点出发,分两部分分析一下。

    代理对象实例化----创建AopProxy接口实现类

    看一下createAopProxy()方法的实现,它位于DefaultAopProxyFactory类中:

    1 protected final synchronized AopProxy createAopProxy() {
    2     if (!this.active) {
    3         activate();
    4     }
    5     return getAopProxyFactory().createAopProxy(this);
    6 }

    前面的部分没什么必要看,直接进入重点即createAopProxy方法:

     1 public AopProxy createAopProxy(AdvisedSupport config) throws AopConfigException {
     2     if (config.isOptimize() || config.isProxyTargetClass() || hasNoUserSuppliedProxyInterfaces(config)) {
     3         Class targetClass = config.getTargetClass();
     4         if (targetClass == null) {
     5             throw new AopConfigException("TargetSource cannot determine target class: " +
     6                     "Either an interface or a target is required for proxy creation.");
     7         }
     8         if (targetClass.isInterface()) {
     9             return new JdkDynamicAopProxy(config);
    10         }
    11         if (!cglibAvailable) {
    12             throw new AopConfigException(
    13                     "Cannot proxy target class because CGLIB2 is not available. " +
    14                     "Add CGLIB to the class path or specify proxy interfaces.");
    15         }
    16         return CglibProxyFactory.createCglibProxy(config);
    17     }
    18     else {
    19         return new JdkDynamicAopProxy(config);
    20     }
    21 }

    平时我们说AOP原理三句话就能概括:

    1. 对类生成代理使用CGLIB
    2. 对接口生成代理使用JDK原生的Proxy
    3. 可以通过配置文件指定对接口使用CGLIB生成代理

    这三句话的出处就是createAopProxy方法。看到默认是第19行的代码使用JDK自带的Proxy生成代理,碰到以下三种情况例外:

    1. ProxyConfig的isOptimize方法为true,这表示让Spring自己去优化而不是用户指定
    2. ProxyConfig的isProxyTargetClass方法为true,这表示配置了proxy-target-class="true"
    3. ProxyConfig满足hasNoUserSuppliedProxyInterfaces方法执行结果为true,这表示<bean>对象没有实现任何接口或者实现的接口是SpringProxy接口

    在进入第2行的if判断之后再根据目标<bean>的类型决定返回哪种AopProxy。简单总结起来就是:

    1. proxy-target-class没有配置或者proxy-target-class="false",返回JdkDynamicAopProxy
    2. proxy-target-class="true"或者<bean>对象没有实现任何接口或者只实现了SpringProxy接口,返回Cglib2AopProxy

    当然,不管是JdkDynamicAopProxy还是Cglib2AopProxy,AdvisedSupport都是作为构造函数参数传入的,里面存储了具体的Advisor。

    代理对象实例化----通过getProxy方法获取<bean>对应的代理

    其实代码已经分析到了JdkDynamicAopProxy和Cglib2AopProxy,剩下的就没什么好讲的了,无非就是看对这两种方式生成代理的熟悉程度而已。

    Cglib2AopProxy生成代理的代码就不看了,对Cglib不熟悉的朋友可以看Cglib及其基本使用一文。

    JdkDynamicAopProxy生成代理的方式稍微看一下:

    1 public Object getProxy(ClassLoader classLoader) {
    2     if (logger.isDebugEnabled()) {
    3         logger.debug("Creating JDK dynamic proxy: target source is " + this.advised.getTargetSource());
    4     }
    5     Class[] proxiedInterfaces = AopProxyUtils.completeProxiedInterfaces(this.advised);
    6     findDefinedEqualsAndHashCodeMethods(proxiedInterfaces);
    7     return Proxy.newProxyInstance(classLoader, proxiedInterfaces, this);
    8 }

    这边解释一下第5行和第6行的代码,第5行代码的作用是拿到所有要代理的接口,第6行代码的作用是尝试寻找这些接口方法里面有没有equals方法和hashCode方法,同时都有的话打个标记,寻找结束,equals方法和hashCode方法有特殊处理。

    最终通过第7行的Proxy.newProxyInstance方法获取接口/类对应的代理对象,Proxy是JDK原生支持的生成代理的方式。

    代理方法调用原理

    前面已经详细分析了为接口/类生成代理的原理,生成代理之后就要调用方法了,这里看一下使用JdkDynamicAopProxy调用方法的原理。

    由于JdkDynamicAopProxy本身实现了InvocationHandler接口,因此具体代理前后处理的逻辑在invoke方法中:

     1 public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
     2     MethodInvocation invocation;
     3     Object oldProxy = null;
     4     boolean setProxyContext = false;
     5 
     6     TargetSource targetSource = this.advised.targetSource;
     7     Class targetClass = null;
     8     Object target = null;
     9 
    10     try {
    11         if (!this.equalsDefined && AopUtils.isEqualsMethod(method)) {
    12             // The target does not implement the equals(Object) method itself.
    13             return equals(args[0]);
    14         }
    15         if (!this.hashCodeDefined && AopUtils.isHashCodeMethod(method)) {
    16             // The target does not implement the hashCode() method itself.
    17             return hashCode();
    18         }
    19         if (!this.advised.opaque && method.getDeclaringClass().isInterface() &&
    20                 method.getDeclaringClass().isAssignableFrom(Advised.class)) {
    21             // Service invocations on ProxyConfig with the proxy config...
    22             return AopUtils.invokeJoinpointUsingReflection(this.advised, method, args);
    23         }
    24 
    25         Object retVal;
    26 
    27         if (this.advised.exposeProxy) {
    28             // Make invocation available if necessary.
    29             oldProxy = AopContext.setCurrentProxy(proxy);
    30             setProxyContext = true;
    31         }
    32 
    33         // May be null. Get as late as possible to minimize the time we "own" the target,
    34         // in case it comes from a pool.
    35         target = targetSource.getTarget();
    36         if (target != null) {
    37             targetClass = target.getClass();
    38         }
    39 
    40         // Get the interception chain for this method.
    41         List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);
    42 
    43         // Check whether we have any advice. If we don't, we can fallback on direct
    44         // reflective invocation of the target, and avoid creating a MethodInvocation.
    45         if (chain.isEmpty()) {
    46             // We can skip creating a MethodInvocation: just invoke the target directly
    47             // Note that the final invoker must be an InvokerInterceptor so we know it does
    48             // nothing but a reflective operation on the target, and no hot swapping or fancy proxying.
    49             retVal = AopUtils.invokeJoinpointUsingReflection(target, method, args);
    50         }
    51         else {
    52             // We need to create a method invocation...
    53             invocation = new ReflectiveMethodInvocation(proxy, target, method, args, targetClass, chain);
    54             // Proceed to the joinpoint through the interceptor chain.
    55             retVal = invocation.proceed();
    56         }
    57 
    58         // Massage return value if necessary.
    59         if (retVal != null && retVal == target && method.getReturnType().isInstance(proxy) &&
    60                 !RawTargetAccess.class.isAssignableFrom(method.getDeclaringClass())) {
    61             // Special case: it returned "this" and the return type of the method
    62             // is type-compatible. Note that we can't help if the target sets
    63             // a reference to itself in another returned object.
    64             retVal = proxy;
    65         }
    66         return retVal;
    67     }
    68     finally {
    69         if (target != null && !targetSource.isStatic()) {
    70             // Must have come from TargetSource.
    71             targetSource.releaseTarget(target);
    72         }
    73         if (setProxyContext) {
    74             // Restore old proxy.
    75             AopContext.setCurrentProxy(oldProxy);
    76         }
    77     }
    78 }

    第11行~第18行的代码,表示equals方法与hashCode方法即使满足expression规则,也不会为之产生代理内容,调用的是JdkDynamicAopProxy的equals方法与hashCode方法。至于这两个方法是什么作用,可以自己查看一下源代码。

    第19行~第23行的代码,表示方法所属的Class是一个接口并且方法所属的Class是AdvisedSupport的父类或者父接口,直接通过反射调用该方法。

    第27行~第30行的代码,是用于判断是否将代理暴露出去的,由<aop:config>标签中的expose-proxy="true/false"配置。

    第41行的代码,获取AdvisedSupport中的所有拦截器和动态拦截器列表,用于拦截方法,具体到我们的实际代码,列表中有三个Object,分别是:

    • chain.get(0):ExposeInvocationInterceptor,这是一个默认的拦截器,对应的原Advisor为DefaultPointcutAdvisor
    • chain.get(1):MethodBeforeAdviceInterceptor,用于在实际方法调用之前的拦截,对应的原Advisor为AspectJMethodBeforeAdvice
    • chain.get(2):AspectJAfterAdvice,用于在实际方法调用之后的处理

    第45行~第50行的代码,如果拦截器列表为空,很正常,因为某个类/接口下的某个方法可能不满足expression的匹配规则,因此此时通过反射直接调用该方法。

    第51行~第56行的代码,如果拦截器列表不为空,按照注释的意思,需要一个ReflectiveMethodInvocation,并通过proceed方法对原方法进行拦截,proceed方法感兴趣的朋友可以去看一下,里面使用到了递归的思想对chain中的Object进行了层层的调用。

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  • 原文地址:https://www.cnblogs.com/xrq730/p/6757608.html
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