Spring AOP 实现

    Aop是Aspect-Oriented Programming（面向方面编程或面向切面编程）的简称。在Spring平台功能中，AOP是一个核心模块，Spring将AOP框架与IoC容器紧密集成，从而为使用AOP提供最大便利。

     AOP可以说是OOP（Object-Oriented Programing，面向对象编程）的补充和完善。OOP引入封装、继承和多态性等概念来建立一种对象层次结构，用以模拟公共行为的一个集合。当我们需要为分散的对象引入公共行为的时候，OOP则显得无能为力。也就是说，OOP允许你定义从上到下的关系，但并不适合定义从左到右的关系。例如日志功能。日志代码往往水平地散布在所有对象层次中，而与它所散布到的对象的核心功能毫无关系。对于其他类型的代码，如安全性、异常处理和透明的持续性也是如此。这种散布在各处的无关的代码被称为横切（cross-cutting）代码，在OOP设计中，它导致了大量代码的重复，而不利于各个模块的重用。

      AOP技术则恰恰相反，它利用一种称为“横切”的技术，剖解开封装的对象内部，并将那些影响了多个类的公共行为封装到一个可重用模块，并将其名为“Aspect”，即方面。所谓“方面”，简单地说，就是将那些与业务无关，却为业务模块所共同调用的逻辑或责任封装起来，便于减少系统的重复代码，降低模块间的耦合度，并有利于未来的可操作性和可维护性。

   Spring AOP实现中，使用Java语言来实现增强对象与切面增强应用，并为这两者的结合提供了配置环境。

1、常用Spring AOP 的两种方式实现

     通常使用Spring AOP主要有两种方式，一种是基于xml配置的纯POJO切面方式，另一种是通过@AspectJ注解驱动的切面方式。如果是纯POJO切面的配置AOP，我们需要在Spring容器xml资源定义文件中声明切面bean，并将切面、切点和通知配置信息通过<aop:config>标签加以说明。如果是@AspectJ注解驱动的切面，我们知道为了支持注解，我们需要在配置文件中加入<aop:aspectj-autoproxy />标签。不论何种形式，Spring容器在处理切面时的思路是一致的，在Spring容器的初始化过程中，当载入、解析配置文件中的这些标签，都需要动态的通过自定义标签机制解析这些标签，从而生成相应的BeanDefine。

 1）Spring Aop 自定义标签处理

    在类AopNamespaceHandler中，源码如下：

public class AopNamespaceHandler extends NamespaceHandlerSupport {public AopNamespaceHandler() {    }public void init() {this.registerBeanDefinitionParser("config", new ConfigBeanDefinitionParser());        this.registerBeanDefinitionParser("aspectj-autoproxy", new AspectJAutoProxyBeanDefinitionParser());        this.registerBeanDefinitionDecorator("scoped-proxy", new ScopedProxyBeanDefinitionDecorator());        this.registerBeanDefinitionParser("spring-configured", new SpringConfiguredBeanDefinitionParser());}}

   在init()函数中，注册了ConfigBeanDefinitionParser()用于解析处理纯POJO切面方式<aop:config>标签，然后注册了AspectJAutoProxyBeanDefinitionParser()用于解析处理<aop:aspectj-autoproxy />标签。

2）注册Aop自动代理创建器

   不管是  ConfigBeanDefinitionParser还是AspectJAutoProxyBeanDefinitionParser，他们都是实现BeanDefinitionParse接口，主要的方法是BeanDefinition parse(Element element, ParserContext parserContext)。

ConfigBeanDefinitionParser的parse()方法：

public BeanDefinition parse(Element element, ParserContext parserContext) {    CompositeComponentDefinition compositeDef = new CompositeComponentDefinition(element.getTagName(), parserContext.extractSource(element));    parserContext.pushContainingComponent(compositeDef); this.configureAutoProxyCreator(parserContext, element);    List childElts = DomUtils.getChildElements(element);    Iterator var5 = childElts.iterator();    while(var5.hasNext()) {        Element elt = (Element)var5.next();        String localName = parserContext.getDelegate().getLocalName(elt);        if("pointcut".equals(localName)) {            this.parsePointcut(elt, parserContext);        } else if("advisor".equals(localName)) {            this.parseAdvisor(elt, parserContext);        } else if("aspect".equals(localName)) {            this.parseAspect(elt, parserContext);        }    }    parserContext.popAndRegisterContainingComponent();    return null;}

 在该函数中首先注册AutoProxyCreator，然后根据不同切面元素解析成为响应的BeanDefinition。this.configureAutoProxyCreator(parserContext, element);最终会执行

public static BeanDefinition registerAspectJAutoProxyCreatorIfNecessary(BeanDefinitionRegistry registry, Object source) {return registerOrEscalateApcAsRequired(AspectJAwareAdvisorAutoProxyCreator.class, registry, source);}

注册AspectJAwareAdvisorAutoProxyCreator。

AspectJAutoProxyBeanDefinitionParser的parse()方法：

public BeanDefinition parse(Element element, ParserContext parserContext) { AopNamespaceUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary(parserContext, element);    this.extendBeanDefinition(element, parserContext);    return null;}public static BeanDefinition registerAspectJAnnotationAutoProxyCreatorIfNecessary(BeanDefinitionRegistry registry, Object source) {return registerOrEscalateApcAsRequired(AnnotationAwareAspectJAutoProxyCreator.class, registry, source);}

可以看到，同样的注册AspectJAnnotationAutoProxyCreator，不同于之前的是，AspectJAnnotationAutoProxyCreator继承自AspectJAwareAdvisorAutoProxyCreator，添加了对于注解的处理。

3）AutoProxyCreator类继承体系

     通过类的继承体系，可以看到AutoProxyCreator类实现了BeanPostProcessor接口，其实际是一

个InstantiationAwareBeanPostProcessor。在bean的生命周期中，可以知道，当容器注册了BeanPostProcessor类型的bean后，从容器中获取任何的bean，都将经过BeanPostProcessor相关方法的处理。那么，当一个bean，被切入切面，进行功能增强时，也是通过InstantiationAwareBeanPostProcessor相关方法对这个bean进行处理，具体就是通过代理方式，将切面植入方法调用流程。而这个处理方法就是 Object postProcessAfterInitialization(Object bean, String beanName)。

public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {if(bean != null) {        Object cacheKey = this.getCacheKey(bean.getClass(), beanName);        if(!this.earlyProxyReferences.contains(cacheKey)) {return this.wrapIfNecessary(bean, beanName, cacheKey);}    }return bean;}

在this.wrapIfNecessary(bean, beanName, cacheKey)中封装bean生成代理对象返回。

4）生成代理相关类继承体系

   在生成代理对象时，通过ProxyFactory类Object getProxy(TargetSource targetSource)方法生成，targetSource是被代理的对象bean封装。

public static Object getProxy(TargetSource targetSource) {if(targetSource.getTargetClass() == null) {throw new IllegalArgumentException("Cannot create class proxy for TargetSource with null target class");} else {        ProxyFactory proxyFactory = new ProxyFactory();proxyFactory.setTargetSource(targetSource);proxyFactory.setProxyTargetClass(true);        return proxyFactory.getProxy();}}

最终调用DefaultAopProxyFactory类的AopProxy createAopProxy(AdvisedSupport config)方法：

public AopProxy createAopProxy(AdvisedSupport config) throws AopConfigException {if(!config.isOptimize() && !config.isProxyTargetClass() && !this.hasNoUserSuppliedProxyInterfaces(config)) {return new JdkDynamicAopProxy(config);} else {        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.");} else {return (AopProxy)(targetClass.isInterface()?new JdkDynamicAopProxy(config):new ObjenesisCglibAopProxy(config));}    }}

Spring提供了两种方式来生成代理对象: JDKProxy和Cglib，具体使用哪种方式生成由AopProxyFactory根据AdvisedSupport对象的配置来决定。默认的策略是如果目标类是接口，则使用JDK动态代理技术，否则使用Cglib来生成代理。

  而JdkDynamicAopProxy是标准的java动态代理使用方式，可以看到其实现了InvocationHandler接口。在JdkDynamicAopProxy中，通过Object getProxy(ClassLoader classLoader)函数，获取代理：

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

我们知道InvocationHandler是JDK动态代理的核心，生成的代理对象的方法调用都会委托到InvocationHandler.invoke()方法。

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {    Object oldProxy = null;    boolean setProxyContext = false;TargetSource targetSource = this.advised.targetSource;Class targetClass = null;Object target = null;Object retVal;    try {      //eqauls()方法，具目标对象未实现此方法 if(!this.equalsDefined && AopUtils.isEqualsMethod(method)) {            Boolean retVal2 = Boolean.valueOf(this.equals(args[0]));            return retVal2;} //hashCode()方法，具目标对象未实现此方法  if(!this.hashCodeDefined && AopUtils.isHashCodeMethod(method)) {            Integer retVal1 = Integer.valueOf(this.hashCode());            return retVal1;} //Advised接口或者其父接口中定义的方法,直接反射调用,不应用通知  if(this.advised.opaque || !method.getDeclaringClass().isInterface() || !method.getDeclaringClass().isAssignableFrom(Advised.class)) {if(this.advised.exposeProxy) {                oldProxy = AopContext.setCurrentProxy(proxy);setProxyContext = true;} //获得目标对象的类              target = targetSource.getTarget();            if(target != null) {                targetClass = target.getClass();} //获取可以应用到此方法上的Interceptor列表   <strong> List chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);</strong>            if(chain.isEmpty()) {   //如果没有可以应用到此方法的通知(Interceptor)，此直接反射调用 method.invoke(target, args)                 retVal = AopUtils.invokeJoinpointUsingReflection(target, method, args);} else {           //创建MethodInvocation ，封装 Interceptor调用链 ReflectiveMethodInvocation invocation = new ReflectiveMethodInvocation(proxy, target, method, args, targetClass, chain);         //调用proceed()方法， 依次执行 Interceptor调用链<strong>retVal = invocation.proceed();</strong>}            Class returnType = method.getReturnType();            if(retVal != null && retVal == target && returnType.isInstance(proxy) && !RawTargetAccess.class.isAssignableFrom(method.getDeclaringClass())) {                retVal = proxy;} else if(retVal == null && returnType != Void.TYPE && returnType.isPrimitive()) {throw new AopInvocationException("Null return value from advice does not match primitive return type for: " + method);}            Object var13 = retVal;            return var13;}        retVal = AopUtils.invokeJoinpointUsingReflection(this.advised, method, args);} finally {if(target != null && !targetSource.isStatic()) {            targetSource.releaseTarget(target);}if(setProxyContext) {            AopContext.setCurrentProxy(oldProxy);}    }return retVal;}

通过this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass)方法，获取通知拦截器链 

public List<Object> getInterceptorsAndDynamicInterceptionAdvice(Method method, Class<?> targetClass) {//这里使用了cache，利用cache获取已经有了的intercepterAdvisedSupport.MethodCacheKey cacheKey = new AdvisedSupport.MethodCacheKey(method);List cached = (List)this.methodCache.get(cacheKey);if(cached == null) {//这里是利用DefaultAdvisorChainFactory，获取intercepter链cached = this.advisorChainFactory.getInterceptorsAndDynamicInterceptionAdvice(this, method, targetClass);this.methodCache.put(cacheKey, cached);}    return cached;}
在DefaultAdvisorChainFactory类（生成连接器的工厂）中：
public List<Object> getInterceptorsAndDynamicInterceptionAdvice(Advised config, Method method, Class<?> targetClass) {    ArrayList interceptorList = new ArrayList(config.getAdvisors().length);    Class actualClass = targetClass != null?targetClass:method.getDeclaringClass();    boolean hasIntroductions = hasMatchingIntroductions(config, actualClass);    AdvisorAdapterRegistry registry = GlobalAdvisorAdapterRegistry.getInstance();    Advisor[] var8 = config.getAdvisors();    int var9 = var8.length;    for(int var10 = 0; var10 < var9; ++var10) {        Advisor advisor = var8[var10];        MethodInterceptor[] interceptors1;        if(advisor instanceof PointcutAdvisor) {            PointcutAdvisor interceptors = (PointcutAdvisor)advisor;            if(config.isPreFiltered() || interceptors.getPointcut().getClassFilter().matches(actualClass)) {                interceptors1 = registry.getInterceptors(advisor);                MethodMatcher mm = interceptors.getPointcut().getMethodMatcher();                if(MethodMatchers.matches(mm, method, actualClass, hasIntroductions)) {                    if(mm.isRuntime()) {                        MethodInterceptor[] var15 = interceptors1;                        int var16 = interceptors1.length;                        for(int var17 = 0; var17 < var16; ++var17) {                            MethodInterceptor interceptor = var15[var17];                            interceptorList.add(new InterceptorAndDynamicMethodMatcher(interceptor, mm));                        }                    } else {                        interceptorList.addAll(Arrays.asList(interceptors1));                    }                }            }        } else if(advisor instanceof IntroductionAdvisor) {            IntroductionAdvisor var19 = (IntroductionAdvisor)advisor;            if(config.isPreFiltered() || var19.getClassFilter().matches(actualClass)) {                interceptors1 = registry.getInterceptors(advisor);                interceptorList.addAll(Arrays.asList(interceptors1));            }        } else {            MethodInterceptor[] var20 = registry.getInterceptors(advisor);            interceptorList.addAll(Arrays.asList(var20));        }    }    return interceptorList;}
在这个函数中，首先设置一个List，用于保存最后返回的interceptorList， 然后DefaultAdvisorChainFactory会通过一个AdvisorAdapterRegistry 来实现拦截器的注册，将配置中Advice适配包装为interceptor，最终返回。
通过 GlobalAdvisorAdapterRegistry.getInstance()获取
AdvisorAdapterRegistry registry。
GlobalAdvisorAdapterRegistry运用单例模式：其实例是DefaultAdvisorAdapterRegistry类对象：
public abstract class GlobalAdvisorAdapterRegistry {    private static AdvisorAdapterRegistry instance = new DefaultAdvisorAdapterRegistry();    public GlobalAdvisorAdapterRegistry() {    }    public static AdvisorAdapterRegistry getInstance() {        return instance;    }    static void reset() {        instance = new DefaultAdvisorAdapterRegistry();    }}

public class DefaultAdvisorAdapterRegistry implements AdvisorAdapterRegistry, Serializable {    private final List<AdvisorAdapter> adapters = new ArrayList(3);    public DefaultAdvisorAdapterRegistry() {        this.registerAdvisorAdapter(new MethodBeforeAdviceAdapter());        this.registerAdvisorAdapter(new AfterReturningAdviceAdapter());        this.registerAdvisorAdapter(new ThrowsAdviceAdapter());    }    public Advisor wrap(Object adviceObject) throws UnknownAdviceTypeException {        if(adviceObject instanceof Advisor) {            return (Advisor)adviceObject;        } else if(!(adviceObject instanceof Advice)) {            throw new UnknownAdviceTypeException(adviceObject);        } else {            Advice advice = (Advice)adviceObject;            if(advice instanceof MethodInterceptor) {                return new DefaultPointcutAdvisor(advice);            } else {                Iterator var3 = this.adapters.iterator();                AdvisorAdapter adapter;                do {                    if(!var3.hasNext()) {                        throw new UnknownAdviceTypeException(advice);                    }                    adapter = (AdvisorAdapter)var3.next();                } while(!adapter.supportsAdvice(advice));                return new DefaultPointcutAdvisor(advice);            }        }    }    public MethodInterceptor[] getInterceptors(Advisor advisor) throws UnknownAdviceTypeException {        ArrayList interceptors = new ArrayList(3);        Advice advice = advisor.getAdvice();        if(advice instanceof MethodInterceptor) {            interceptors.add((MethodInterceptor)advice);        }        Iterator var4 = this.adapters.iterator();        while(var4.hasNext()) {            AdvisorAdapter adapter = (AdvisorAdapter)var4.next();            if(adapter.supportsAdvice(advice)) {                interceptors.add(adapter.getInterceptor(advisor));            }        }        if(interceptors.isEmpty()) {            throw new UnknownAdviceTypeException(advisor.getAdvice());        } else {            return (MethodInterceptor[])interceptors.toArray(new MethodInterceptor[interceptors.size()]);        }    }    public void registerAdvisorAdapter(AdvisorAdapter adapter) {        this.adapters.add(adapter);    }}