Spring创建及使用切面过程

快来打我* 2022-09-11 01:26 178阅读 0赞

### 文章目录 ###

*  AspectJ和Spring aop
 *  Spring创建切面
 *   *  创建入口
     *   *  1.普通场景代理
         *   *  1-c.为AspectJ创建对应的advisor
             *   *  c-I.先找到使用了@Aspect注解的类或者使用ajc编译的类
                 *  c-II.创建普通Advisor及IntroductionAdvisor
             *  2.再找出匹配上的切面
             *  3.看看是不是需要其他辅助切面【AspectJ】
         *  2.根据名字代理
 *  切面执行代理
 *   *  执行代理的入口
     *  执行切面
     *  代理顺序
     *   *  拼接原理
         *   *  AspectJAroundAdvice
             *  MethodBeforeAdviceInterceptor
             *  AspectJAfterThrowingAdvice
             *  AfterReturningAdviceInterceptor
             *  AspectJAfterAdvice
             *  通知时机顺序总结
 *  相关知识点
 *   *  ExposeInvocationInterceptor
     *  PointcutAdvisor和IntroductionAdvisor
     *  Pointcut表达式
     *   *  args和@args
         *  execution和@annotation
         *  within和@within
         *  this
         *  target和@target
     *  Spring aop
     *   *  jdk代理示例
         *  cglib代理示例

# AspectJ和Spring aop #

AspectJ是一个强大的aop框架，可以单独实现静态字节码代理，故而可以用于代理final，abstract方法；而Spring aop是基于动态代理，并且也涵盖了AspectJ实现代理。普通情况下，为了实现方便，都是直接使用spring aop实现动态代理。

# Spring创建切面 #

Spring是在创建bean的时候整理其对应的切面，主要过程如下。

## 创建入口 ##

org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory\#doCreateBean

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)
          throws BeanCreationException { 
    
       // 创建bean的实例
       BeanWrapper instanceWrapper = null;
       if (mbd.isSingleton()) { 
          instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
       }
       if (instanceWrapper == null) { 
          instanceWrapper = createBeanInstance(beanName, mbd, args);
       }
       final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
       Class<?> beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);
       mbd.resolvedTargetType = beanType;
    
       // 前置处理器修改beandefinition
       synchronized (mbd.postProcessingLock) { 
          if (!mbd.postProcessed) { 
             try { 
                applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
             }
             catch (Throwable ex) { 
                throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                      "Post-processing of merged bean definition failed", ex);
             }
             mbd.postProcessed = true;
          }
       }
    
       // 更早的暴露防止循环引用
       boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
             isSingletonCurrentlyInCreation(beanName));
       if (earlySingletonExposure) { 
          if (logger.isDebugEnabled()) { 
             logger.debug("Eagerly caching bean '" + beanName +
                   "' to allow for resolving potential circular references");
          }
          addSingletonFactory(beanName, new ObjectFactory<Object>() { 
             @Override
             public Object getObject() throws BeansException { 
                return getEarlyBeanReference(beanName, mbd, bean);
             }
          });
       }
    
       Object exposedObject = bean;
       try { 
          // 填充bean的属性
          populateBean(beanName, mbd, instanceWrapper);
          if (exposedObject != null) { 
              // ！！！初始化bean，这里就是织入切面
             exposedObject = initializeBean(beanName, exposedObject, mbd);
          }
       }
       catch (Throwable ex) { 
          if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) { 
             throw (BeanCreationException) ex;
          }
          else { 
             throw new BeanCreationException(
                   mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
          }
       }
    
       if (earlySingletonExposure) { 
           // 获取单例
          Object earlySingletonReference = getSingleton(beanName, false);
          if (earlySingletonReference != null) { 
             if (exposedObject == bean) { 
                exposedObject = earlySingletonReference;
             }
             else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) { 
                 //获取相关依赖的bean
                String[] dependentBeans = getDependentBeans(beanName);
                Set<String> actualDependentBeans = new LinkedHashSet<String>(dependentBeans.length);
                for (String dependentBean : dependentBeans) { 
                   if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) { 
                      actualDependentBeans.add(dependentBean);
                   }
                }
                if (!actualDependentBeans.isEmpty()) { 
                   throw new BeanCurrentlyInCreationException(beanName,
                         "Bean with name '" + beanName + "' has been injected into other beans [" +
                         StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
                         "] in its raw version as part of a circular reference, but has eventually been " +
                         "wrapped. This means that said other beans do not use the final version of the " +
                         "bean. This is often the result of over-eager type matching - consider using " +
                         "'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
                }
             }
          }
       }
    
       // Register bean as disposable.
       try { 
          registerDisposableBeanIfNecessary(beanName, bean, mbd);
       }
       catch (BeanDefinitionValidationException ex) { 
          throw new BeanCreationException(
                mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
       }
    
       return exposedObject;
    }

org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory\#initializeBean(java.lang.String, java.lang.Object, org.springframework.beans.factory.support.RootBeanDefinition)

protected Object initializeBean(final String beanName, final Object bean, RootBeanDefinition mbd) { 
       if (System.getSecurityManager() != null) { 
          AccessController.doPrivileged(new PrivilegedAction<Object>() { 
             @Override
             public Object run() { 
                invokeAwareMethods(beanName, bean);
                return null;
             }
          }, getAccessControlContext());
       }
       else { 
           //如果实现了BeanNameAware，BeanClassLoaderAware或者BeanFactoryAware，则调用对应的特殊方法
           //setBeanName/setBeanClassLoader/setBeanFactory
          invokeAwareMethods(beanName, bean);
       }
    
       Object wrappedBean = bean;
       if (mbd == null || !mbd.isSynthetic()) { 
          wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
       }
    
       try { 
           //调用init方法，比如实现了InitializingBean则调用afterPropertiesSet方法；如果自定义了init方法，则反射调用
          invokeInitMethods(beanName, wrappedBean, mbd);
       }
       catch (Throwable ex) { 
          throw new BeanCreationException(
                (mbd != null ? mbd.getResourceDescription() : null),
                beanName, "Invocation of init method failed", ex);
       }
       if (mbd == null || !mbd.isSynthetic()) { 
           //这里执行各类bean前置处理器，此处就含有创建代理
          wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
       }
       return wrappedBean;
    }

org.springframework.aop.framework.autoproxy.AbstractAutoProxyCreator\#postProcessAfterInitialization->  
org.springframework.aop.framework.autoproxy.AbstractAutoProxyCreator\#wrapIfNecessary

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;
       }
        //如果是基础类（Advice、Pointcut、Advisor、AopInfrastructureBean）则不需要代理
        //如果是Aspect类则不需要代理直接跳过
       if (isInfrastructureClass(bean.getClass()) || shouldSkip(bean.getClass(), beanName)) { 
          this.advisedBeans.put(cacheKey, Boolean.FALSE);
          return bean;
       }
    
       //！！！创建代理
       Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
       if (specificInterceptors != DO_NOT_PROXY) { 
          this.advisedBeans.put(cacheKey, Boolean.TRUE);
          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;
    }

### 1.普通场景代理 ###

org.springframework.aop.framework.autoproxy.AbstractAdvisorAutoProxyCreator\#getAdvicesAndAdvisorsForBean->  
org.springframework.aop.framework.autoproxy.AbstractAdvisorAutoProxyCreator\#findEligibleAdvisors

protected List<Advisor> findEligibleAdvisors(Class<?> beanClass, String beanName) { 
             //1.先找到所有的切面，主要是a.找到所有advisor定义b.创建advisorc.创建AspectJ的advisor
    		List<Advisor> candidateAdvisors = findCandidateAdvisors();
             //2.再找出匹配上的切面
    		List<Advisor> eligibleAdvisors = findAdvisorsThatCanApply(candidateAdvisors, beanClass, beanName);
        	//3.看看是不是需要其他辅助切面【AspectJ】
    		extendAdvisors(eligibleAdvisors);
        	//4.切面排序(@Order/@Priority)
    		if (!eligibleAdvisors.isEmpty()) { 
    			eligibleAdvisors = sortAdvisors(eligibleAdvisors);
    		}
    		return eligibleAdvisors;
    	}

#### 1-c.为AspectJ创建对应的advisor ####

org.springframework.aop.aspectj.annotation.AnnotationAwareAspectJAutoProxyCreator\#findCandidateAdvisors

protected List<Advisor> findCandidateAdvisors() { 
       // a.找到所有advisor定义b.创建advisor
       List<Advisor> advisors = super.findCandidateAdvisors();
       // c.创建AspectJ的advisor
       advisors.addAll(this.aspectJAdvisorsBuilder.buildAspectJAdvisors());
       return advisors;
    }

##### c-I.先找到使用了@Aspect注解的类或者使用ajc编译的类 #####

org.springframework.aop.aspectj.annotation.BeanFactoryAspectJAdvisorsBuilder\#buildAspectJAdvisors

##### c-II.创建普通Advisor及IntroductionAdvisor #####

org.springframework.aop.aspectj.annotation.ReflectiveAspectJAdvisorFactory\#getAdvisors

public List<Advisor> getAdvisors(MetadataAwareAspectInstanceFactory aspectInstanceFactory) { 
       Class<?> aspectClass = aspectInstanceFactory.getAspectMetadata().getAspectClass();
       String aspectName = aspectInstanceFactory.getAspectMetadata().getAspectName();
       validate(aspectClass);
    
       MetadataAwareAspectInstanceFactory lazySingletonAspectInstanceFactory =
             new LazySingletonAspectInstanceFactoryDecorator(aspectInstanceFactory);
    
       List<Advisor> advisors = new ArrayList<>();
        //找出该类的所有方法，包含其父类或接口的方法
       for (Method method : getAdvisorMethods(aspectClass)) { 
          //为通知方法创建一个InstantiationModelAwarePointcutAdvisorImpl，如果是普通方法则返回null
          Advisor advisor = getAdvisor(method, lazySingletonAspectInstanceFactory, 0, aspectName);
          if (advisor != null) { 
             advisors.add(advisor);
          }
       }
    
       // 如果切面需要懒加载，则在首位加入一个SyntheticInstantiationAdvisor
       if (!advisors.isEmpty() && lazySingletonAspectInstanceFactory.getAspectMetadata().isLazilyInstantiated()) { 
          Advisor instantiationAdvisor = new SyntheticInstantiationAdvisor(lazySingletonAspectInstanceFactory);
          advisors.add(0, instantiationAdvisor);
       }
    
       // 寻找类中使用@DeclareParents的字段，为之创建DeclareParentsAdvisor
       // @DeclareParents可用于AspectJ类的字段上，表示哪些方法需要整合其他方法代理【此处就是将两个毫无关系的类的方法建立关系】
       for (Field field : aspectClass.getDeclaredFields()) { 
          Advisor advisor = getDeclareParentsAdvisor(field);
          if (advisor != null) { 
             advisors.add(advisor);
          }
       }
    
       return advisors;
    }

#### 2.再找出匹配上的切面 ####

org.springframework.aop.framework.autoproxy.AbstractAdvisorAutoProxyCreator\#findAdvisorsThatCanApply->

org.springframework.aop.support.AopUtils\#findAdvisorsThatCanApply->

org.springframework.aop.support.AopUtils\#canApply(org.springframework.aop.Advisor, java.lang.Class<?>, boolean)

public static boolean canApply(Advisor advisor, Class<?> targetClass, boolean hasIntroductions) { 
       if (advisor instanceof IntroductionAdvisor) { 
           // a.IntroductionAdvisor则根据类过滤器匹配
          return ((IntroductionAdvisor) advisor).getClassFilter().matches(targetClass);
       }
       else if (advisor instanceof PointcutAdvisor) { 
          PointcutAdvisor pca = (PointcutAdvisor) advisor;
           // b.PointcutAdvisor匹配切点
          return canApply(pca.getPointcut(), targetClass, hasIntroductions);
       }
       else { 
          // c.其他则直接匹配成功
          return true;
       }
    }

#### 3.看看是不是需要其他辅助切面【AspectJ】 ####

org.springframework.aop.aspectj.autoproxy.AspectJAwareAdvisorAutoProxyCreator\#extendAdvisors->

org.springframework.aop.aspectj.AspectJProxyUtils\#makeAdvisorChainAspectJCapableIfNecessary

public static boolean makeAdvisorChainAspectJCapableIfNecessary(List<Advisor> advisors) { 
       // Don't add advisors to an empty list; may indicate that proxying is just not required
       if (!advisors.isEmpty()) { 
          boolean foundAspectJAdvice = false;
          for (Advisor advisor : advisors) { 
             //看是否包含AspectJ通知
             if (isAspectJAdvice(advisor)) { 
                foundAspectJAdvice = true;
             }
          }
          if (foundAspectJAdvice && !advisors.contains(ExposeInvocationInterceptor.ADVISOR)) { 
              //添加ExposeInvocationInterceptor，暴露当前的methodInvocation
             advisors.add(0, ExposeInvocationInterceptor.ADVISOR);
             return true;
          }
       }
       return false;
    }

### 2.根据名字代理 ###

org.springframework.aop.framework.autoproxy.BeanNameAutoProxyCreator\#getAdvicesAndAdvisorsForBean  
通过bean的名字来进行匹配，来创建特定代理。

# 切面执行代理 #

## 执行代理的入口 ##

org.springframework.aop.framework.CglibAopProxy.DynamicAdvisedInterceptor\#intercept

因为spring在高版本后，统一使用的Cglib代理，所以在调用方法时，会先为这个方法找到代理链路。然后每个链路依次嵌套调用，实现代理。

public Object intercept(Object proxy, Method method, Object[] args, MethodProxy methodProxy) throws Throwable { 
       Object oldProxy = null;
       boolean setProxyContext = false;
       Object target = null;
       TargetSource targetSource = this.advised.getTargetSource();
       try { 
          if (this.advised.exposeProxy) { 
             // Make invocation available if necessary.
             oldProxy = AopContext.setCurrentProxy(proxy);
             setProxyContext = true;
          }
          // Get as late as possible to minimize the time we "own" the target, in case it comes from a pool...
          target = targetSource.getTarget();
          Class<?> targetClass = (target != null ? target.getClass() : null);
           //1. 获取切面链路
          List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);
          Object retVal;
          if (chain.isEmpty() && Modifier.isPublic(method.getModifiers())) { 
              //没有切面则直接执行方法
             Object[] argsToUse = AopProxyUtils.adaptArgumentsIfNecessary(method, args);
             retVal = methodProxy.invoke(target, argsToUse);
          }
          else { 
             //2. 创建MethodInvocation，并执行切面
             retVal = new CglibMethodInvocation(proxy, target, method, args, targetClass, chain, methodProxy).proceed();
          }
           //处理返回值
          retVal = processReturnType(proxy, target, method, retVal);
          return retVal;
       }
       finally { 
          if (target != null && !targetSource.isStatic()) { 
             targetSource.releaseTarget(target);
          }
          if (setProxyContext) { 
             // Restore old proxy.
             AopContext.setCurrentProxy(oldProxy);
          }
       }
    }

## 执行切面 ##

@Override
    @Nullable
    public Object proceed() throws Throwable { 
       // 最后一个切面的时候，如果是public方法（记录的methodProxy），则直接执行方法，否则通过反射的方式执行方法
       if (this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size() - 1) { 
          return invokeJoinpoint();
       }
       //每次递增使用的切面下标
       Object interceptorOrInterceptionAdvice =
             this.interceptorsAndDynamicMethodMatchers.get(++this.currentInterceptorIndex);
       if (interceptorOrInterceptionAdvice instanceof InterceptorAndDynamicMethodMatcher) { 
          // 动态匹配【比如参数切点】
          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 { 
             // 动态匹配失败，则略过本切面，执行下一个切面
             return proceed();
          }
       }
       else { 
          // 依次调用切面
          return ((MethodInterceptor) interceptorOrInterceptionAdvice).invoke(this);
       }
    }

## 代理顺序 ##

切面的顺序是在创建切面的时候就已经给每个切面排好序了，但是每个Aspect的不同时期的通知是运行时拼接的。

### 拼接原理 ###

不同的通知，invoke方法不一样，通过调整执行proceed的顺序，保证了各个通知最终能按照一定顺序执行。

#### AspectJAroundAdvice ####

直接执行通知方法，所以around在before之前

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 ####

在MethodInvocation执行proceed之前执行before通知

public Object invoke(MethodInvocation mi) throws Throwable { 
       this.advice.before(mi.getMethod(), mi.getArguments(), mi.getThis());
       return mi.proceed();
    }

#### AspectJAfterThrowingAdvice ####

在catch块执行afterThrowing通知

public Object invoke(MethodInvocation mi) throws Throwable { 
       try { 
          return mi.proceed();
       }
       catch (Throwable ex) { 
          if (shouldInvokeOnThrowing(ex)) { 
             invokeAdviceMethod(getJoinPointMatch(), null, ex);
          }
          throw ex;
       }
    }

#### AfterReturningAdviceInterceptor ####

在执行MethodInvocation之后，返回结果前，执行afterReturning通知

public Object invoke(MethodInvocation mi) throws Throwable { 
       Object retVal = mi.proceed();
       this.advice.afterReturning(retVal, mi.getMethod(), mi.getArguments(), mi.getThis());
       return retVal;
    }

#### AspectJAfterAdvice ####

在finally块执行after通知

public Object invoke(MethodInvocation mi) throws Throwable { 
       try { 
          return mi.proceed();
       }
       finally { 
          invokeAdviceMethod(getJoinPointMatch(), null, null);
       }
    }

#### 通知时机顺序总结 ####

按照上述5个通知的执行原理，可以得到以下顺序图。  
![在这里插入图片描述][watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5Y-k6Imv6YGT_size_20_color_FFFFFF_t_70_g_se_x_16_pic_center]

# 相关知识点 #

## ExposeInvocationInterceptor ##

在spring寻找切面的时候，会经过org.springframework.aop.aspectj.AspectJProxyUtils\#makeAdvisorChainAspectJCapableIfNecessary，看是否需要为AspectJ添加一个辅助类——ExposeInvocationInterceptor。

这个特殊切面是用于存储当前的invocation，供AspectJ使用，比如在使用before方法切入，则此时不会直接传入一个invocation，需要通过ExposeInvocationInterceptor来获取当前的invocation。当切面优先级为最高级，则ExposeInvocationInterceptor的会报错，因为没有过这个切面，没有往invocation放入过当前线程的值；由于invocation是threadlocal变量，故而不允许跨线程调用，一旦跨线程也会报错。

那为什么需要ExposeInvocationInterceptor？

因为AspectJ有很多切的时机，除了around的入参是ProceedingJoinPoint以外，其他方法入参都是JointPoint，ProceedingJoinPoint和JoinPoint的区别是前者暴露了MethodInvocation的proceed方法，可以执行方法。所以对于其他时机的切面来说，无法直接获取当前的MethodInvocation，需要通过一个辅助类来帮忙。

## PointcutAdvisor和IntroductionAdvisor ##

Advisor有许多实现，其中PointcutAdvisor和IntroductionAdvisor是主要的两个实现

*  PointcutAdvisor是基于切点的，适用范围很广，并且切点很精确
 *  IntroductionAdvisor是使用于类的，目的是在不修改java文件的情况下，使得某个类具有特殊接口

## Pointcut表达式 ##

pointcut表达式十分丰富，从对参数、方法、类、包到注解等都有对应的表达式。多个pointcut之间可以用|| , &&, !进行组合。

### args和@args ###

args：匹配参数；@args匹配含有某些注解的参数

案例：

<table> 
 <thead> 
  <tr> 
   <th>场景</th> 
   <th>表达式</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>没有参数</td> 
   <td>args()</td> 
  </tr> 
  <tr> 
   <td>第一个参数是User类型</td> 
   <td>args(test.User,…)</td> 
  </tr> 
  <tr> 
   <td>任意参数</td> 
   <td>args(…)</td> 
  </tr> 
  <tr> 
   <td>含有具有@RequestBody的参数</td> 
   <td>@args(org.springframework.web.bind.annotation.RequestBody)</td> 
  </tr> 
 </tbody> 
</table>

### execution和@annotation ###

execution：匹配方法，格式：访问标识符 返回值类型 包路径.类名.方法名(参数类型)

@annotation：匹配具有某些注解的方法

案例：

<table> 
 <thead> 
  <tr> 
   <th>场景</th> 
   <th>表达式</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>所有方法</td> 
   <td>execution(* *(…))</td> 
  </tr> 
  <tr> 
   <td>User类的方法</td> 
   <td>execution(public * test.User.*(…))</td> 
  </tr> 
  <tr> 
   <td>User类最后一个参数是int类型的方法</td> 
   <td>execution(public * test.User.*(…,int))</td> 
  </tr> 
 </tbody> 
</table>

### within和@within ###

within：指定包含的类范围；@within：指定具有某些注解的类

案例：

<table> 
 <thead> 
  <tr> 
   <th>场景</th> 
   <th>表达式</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>某个包下所有类方法</td> 
   <td>within(test.*)</td> 
  </tr> 
  <tr> 
   <td>具体类的所有方法</td> 
   <td>within(test.User)</td> 
  </tr> 
  <tr> 
   <td>使用@Aspect的类的所有方法</td> 
   <td>@within(org.aspectj.lang.annotation.Aspect)</td> 
  </tr> 
 </tbody> 
</table>

### this ###

匹配代理后的对象的所有方法

案例：

<table> 
 <thead> 
  <tr> 
   <th>场景</th> 
   <th>表达式</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>代理后的对象是User类型</td> 
   <td>this(test.User)</td> 
  </tr> 
 </tbody> 
</table>

### target和@target ###

target：匹配代理前的对象的所有方法；@target：匹配代理前的对象的类得使用某些注解

案例：

<table> 
 <thead> 
  <tr> 
   <th>场景</th> 
   <th>表达式</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>代理前的对象是User类型</td> 
   <td>target(test.User)</td> 
  </tr> 
  <tr> 
   <td>代理前的对象的类使用了@Aspect注解</td> 
   <td>@target(org.aspectj.lang.annotation.Aspect)</td> 
  </tr> 
 </tbody> 
</table>

## Spring aop ##

Spring的动态代理最终是通过 **Cglib和jdk**动态代理实现的，分别是为类、接口产生代理类。两者均是在运行时产生代理类，\*\*所以如果是有大量类需要代理时，要考虑下生成的代理类所占空间大小。\*\*因为Cglib是通过字节码技术实现，所以在调用方法时，效率会更高。

### jdk代理示例 ###

**代理类实现InvocationHandler**  
总接口：Person

public interface Person { 
        void printName();
    }

实现类：User

@Data
    public class User implements Person{ 
        String city;
        String name;
    
        @Override
        public void printName() { 
            System.out.println(name);;
        }
    }

代理类：UserProxy

@Slf4j
    public class UserProxy implements InvocationHandler { 
        User user;
    
        public UserProxy(User user) { 
            this.user = user;
        }
    
        public static Object newProxy(User user) { 
            UserProxy proxy = new UserProxy(user);
            return Proxy.newProxyInstance(UserProxy.class.getClassLoader(), User.class.getInterfaces(), proxy);
        }
    
        @Override
        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { 
            log.info("准备执行{}方法", method.getName());
            return method.invoke(this.user, args);
        }
    }

测试代码：

public static void main(String[] args) { 
            User user = new User();
            user.setName("jdk代理");
            Person person = (Person) UserProxy.newProxy(user);
            person.printName();
        }

测试结果

18:56:18.166 [main] INFO cn.dotfashion.soa.CacheDemo.jdk.UserProxy - 准备执行printName方法
    jdk代理

### cglib代理示例 ###

代理类实现  
实体类：User。注意，此类可以不实现接口，因为Cglib是用继承该类User的方式实现。

@Data
    public class User implements Person { 
        String city;
        String name;
    
        @Override
        public void printName() { 
            System.out.println(name);;
        }
    }

代理类：UserProxy

@Slf4j
    public class UserProxy implements MethodInterceptor { 
        User user;
    
        public UserProxy(User user) { 
            this.user = user;
        }
    
        public static Object newProxy(User user) { 
            UserProxy userProxy = new UserProxy(user);
            Enhancer enhancer = new Enhancer();
            enhancer.setSuperclass(user.getClass());
            enhancer.setCallback(userProxy);
            return enhancer.create();
        }
    
        @Override
        public Object intercept(Object obj, Method method, Object[] args, MethodProxy proxy) throws Throwable { 
            log.info("准备执行{}方法", method.getName());
            return method.invoke(this.user, args);
        }
    }

测试代码：

public static void main(String[] args) { 
            User user = new User();
            user.setName("cglib代理");
            User pp = (User) UserProxy.newProxy(user);
            pp.printName();
        }

测试结果：

19:00:30.452 [main] INFO cn.dotfashion.soa.CacheDemo.cglib.UserProxy - 准备执行printName方法
    cglib代理

<table> 
 <thead> 
  <tr> 
   <th></th> 
   <th>JDK代理</th> 
   <th>Cglib代理</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>代理类必须实现接口</td> 
   <td>是</td> 
   <td>无要求</td> 
  </tr> 
  <tr> 
   <td>实现方式</td> 
   <td>通过实现接口，<strong>只会对接口的方法进行代理(public/default)</strong>，生成代理类，然后生成代理对象（Proxy.newProxyInstance），多接口场景更倾向使用jdk代理</td> 
   <td>通过继承代理类，生成子类，<strong>覆盖被代理类的方法(非private)</strong>，通过子类代理对象实现(Enhancer.create)</td> 
  </tr> 
  <tr> 
   <td>代理类实现的接口</td> 
   <td>InvocationHandler</td> 
   <td>MethodInterceptor</td> 
  </tr> 
 </tbody> 
</table>

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