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Spring如何实现AOP，请不要再说cglib了！

最近工作中我都是基于注解实现AOP功能，常用的开启AOP的注解是@EnableAspectJAutoProxy，我们就从它入手。

上面的动图的流程的步骤就是：

@EnableAspectJAutoProxy

--> AspectJAutoProxyRegistrar

-->AopConfigUtils .registerAspectJAnnotationAutoProxyCreatorIfNecessary

-->AnnotationAwareAspectJAutoProxyCreator.class

AnnotationAwareAspectJAutoProxyCreator查看其中文注释（如下），确定它就是AOP的核心类！--温安适 20191020

/**
1.AspectJAwareAdvisorAutoProxyCreator的子类
，用于处理当前应用上下文中的注解切面
2.任何被AspectJ注解的类将自动被识别。
3.若SpringAOP代理模式可以识别，优先使用Spring代理模式。
4.它覆盖了方法执行连接点
5.如果使用<aop:include>元素，
  则只有名称与include模式匹配的@aspectj bean才被视为切面
  ，并由spring自动代理。
6. Spring Advisors的处理请查阅，
org.springframework.aop
.framework.autoproxy.AbstractAdvisorAutoProxyCreator
 */
@SuppressWarnings("serial")
public class AnnotationAwareAspectJAutoProxyCreator 
extends AspectJAwareAdvisorAutoProxyCreator {
    //...省略实现
    }注解切面
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虽然找到了核心类，但是并没有找到核心方法！下面我们尝试画类图确定核心方法。

2.画核心类类图，猜测核心方法

AnnotationAwareAspectJAutoProxyCreator的部分类图。

从类图看到了AnnotationAwareAspectJAutoProxyCreator实现了BeanPostProcessor，而AOP功能应该在创建完Bean之后执行，猜测AnnotationAwareAspectJAutoProxyCreator实现BeanPostProcessor的postProcessAfterInitialization（实例化bean后处理）是核心方法。查看AnnotationAwareAspectJAutoProxyCreator实现的postProcessAfterInitialization方法，实际该方法在其父类AbstractAutoProxyCreator中。

//AbstractAutoProxyCreator中的postProcessAfterInitialization实现
@Override
public Object postProcessAfterInitialization(Object bean, String beanName)
 throws BeansException {
   if (bean != null) {
      Object cacheKey = getCacheKey(bean.getClass(), beanName);
      if (!this.earlyProxyReferences.contains(cacheKey)) {
         return wrapIfNecessary(bean, beanName, cacheKey);
      }
   }
   return bean;
}
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发现发现疑似方法wrapIfNecessary，查看其源码如下，发现createProxy方法。确定找对了地方。

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;
   }
   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;
}
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即AnnotationAwareAspectJAutoProxyCreator实现BeanPostProcessor的postProcessAfterInitialization方法，在该方法中由wrapIfNecessary实现了AOP的功能。 wrapIfNecessary中有2个和核心方法

getAdvicesAndAdvisorsForBean获取当前bean匹配的增强器
createProxy为当前bean创建代理
要想明白核心流程还需要分析这2个方法。

3.读重点方法，理核心流程

3.1 getAdvicesAndAdvisorsForBean获取当前bean匹配的增强器

查看源码如下，默认实现在AbstractAdvisorAutoProxyCreator中。

@Override
@Nullable
protected Object[] getAdvicesAndAdvisorsForBean(
      Class<?> beanClass, String beanName, 
      @Nullable TargetSource targetSource) {
   List<Advisor> advisors = findEligibleAdvisors(beanClass, beanName);
   if (advisors.isEmpty()) {
      return DO_NOT_PROXY;
   }
   return advisors.toArray();
}
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查阅findEligibleAdvisors方法，就干了3件事

找所有增强器，也就是所有@Aspect注解的Bean
找匹配的增强器，也就是根据 @Before ，@After等注解上的表达式，与当前bean进行匹配，暴露匹配上的。
对匹配的增强器进行扩展和排序，就是按照@Order或者PriorityOrdered的getOrder的数据值进行排序，越小的越靠前。

protected List<Advisor> findEligibleAdvisors(Class<?> beanClass, String beanName) {
   //找所有增强器
   List<Advisor> candidateAdvisors = findCandidateAdvisors();
   //找所有匹配的增强器
   List<Advisor> eligibleAdvisors = findAdvisorsThatCanApply(candidateAdvisors, beanClass, beanName);
   extendAdvisors(eligibleAdvisors);
   if (!eligibleAdvisors.isEmpty()) {
       //排序
      eligibleAdvisors = sortAdvisors(eligibleAdvisors);
   }
   return eligibleAdvisors;
}
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AnnotationAwareAspectJAutoProxyCreator 重写了findCandidateAdvisors，下面我们看看具体实现了什么

3.1.1findCandidateAdvisors找所有增强器，也就是所有@Aspect注解的Bean

@Override
protected List<Advisor> findCandidateAdvisors() {
   // Add all the Spring advisors found according to superclass rules.
   List<Advisor> advisors = super.findCandidateAdvisors();
   // Build Advisors for all AspectJ aspects in the bean factory.
   if (this.aspectJAdvisorsBuilder != null) {
      //@Aspect注解的类在这里除了
      advisors.addAll(this.aspectJAdvisorsBuilder.buildAspectJAdvisors());
   }
   return advisors;
}
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从该方法我们可以看到处理@Aspect注解的bean的方法是：this.aspectJAdvisorsBuilder.buildAspectJAdvisors() 。这个方法如下：

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<>();
            //找到所有BeanName
            String[] beanNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(
                  this.beanFactory, Object.class, true, false);
            for (String beanName : beanNames) {
               if (!isEligibleBean(beanName)) {
                  continue;
               }
               // 必须注意，bean会提前暴露，并被Spring容器缓存，但是这时还不能织入。
               Class<?> beanType = this.beanFactory.getType(beanName);
               if (beanType == null) {
                  continue;
               }
               if (this.advisorFactory.isAspect(beanType)) {
                  //找到所有被@Aspect注解的类
                  aspectNames.add(beanName);
                  AspectMetadata amd = new AspectMetadata(beanType, beanName);
                  if (amd.getAjType().getPerClause().getKind() == PerClauseKind.SINGLETON) {
                     MetadataAwareAspectInstanceFactory factory =
                           new BeanFactoryAspectInstanceFactory(this.beanFactory, beanName);
                     //解析封装为Advisor返回
                     List<Advisor> classAdvisors = this.advisorFactory.getAdvisors(factory);
                     if (this.beanFactory.isSingleton(beanName)) {
                        this.advisorsCache.put(beanName, classAdvisors);
                     }
                     else {
                        this.aspectFactoryCache.put(beanName, factory);
                     }
                     advisors.addAll(classAdvisors);
                  }
                  else {
                     // Per target or per this.
                     if (this.beanFactory.isSingleton(beanName)) {
                        throw new IllegalArgumentException("Bean with name '" + beanName +
                              "' is a singleton, but aspect instantiation model is not singleton");
                     }
                     MetadataAwareAspectInstanceFactory factory =
                           new PrototypeAspectInstanceFactory(this.beanFactory, beanName);
                     this.aspectFactoryCache.put(beanName, factory);
                     advisors.addAll(this.advisorFactory.getAdvisors(factory));
                  }
               }
            }
            this.aspectBeanNames = aspectNames;
            return advisors;
         }
      }
   }

   if (aspectNames.isEmpty()) {
      return Collections.emptyList();
   }
   List<Advisor> advisors = new ArrayList<>();
   for (String aspectName : aspectNames) {
      List<Advisor> cachedAdvisors = this.advisorsCache.get(aspectName);
      if (cachedAdvisors != null) {
         advisors.addAll(cachedAdvisors);
      }
      else {
         MetadataAwareAspectInstanceFactory factory = this.aspectFactoryCache.get(aspectName);
         advisors.addAll(this.advisorFactory.getAdvisors(factory));
      }
   }
   return advisors;
}
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这个方法可以概括为：

找到所有BeanName
根据BeanName筛选出被@Aspect注解的类
针对类中被Around.class, Before.class, After.class, AfterReturning.class, AfterThrowing.class注解的方法，先按上边的注解顺序排序后按方法名称排序，每一个方法对应一个Advisor。

3.2 createProxy为当前bean创建代理。

3.2.1 创建代理的2种方式

众所周知，创建代理的常用的2种方式是：JDK创建和CGLIB，下面我们就看看这2中创建代理的例子。

3.2.1 .1 jdk创建代理的例子

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;

public class JDKProxyMain {

    public static void main(String[] args) {
        JDKProxyTestInterface target = new JDKProxyTestInterfaceImpl();
        // 根据目标对象创建代理对象
        JDKProxyTestInterface proxy =
         (JDKProxyTestInterface) Proxy
         .newProxyInstance(target.getClass().getClassLoader(),
                target.getClass().getInterfaces(), 
                new JDKProxyTestInvocationHandler(target));
        // 调用代理对象方法
        proxy.testProxy();
    }

    interface JDKProxyTestInterface {
        void testProxy();
    }
    static class JDKProxyTestInterfaceImpl 
                    implements JDKProxyTestInterface {
        @Override
        public void testProxy() {
            System.out.println("testProxy");
        }
    }
   static class JDKProxyTestInvocationHandler 
                           implements InvocationHandler {
        private  Object target;
        public JDKProxyTestInvocationHandler(Object target){
            this.target=target;
        }
        @Override
        public Object invoke(Object proxy, Method method,
                             Object[] args) throws Throwable {
            System.out.println("执行前");
            Object result=  method.invoke(this.target,args);
            System.out.println("执行后");
            return result;
        }
    }
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3.2.1 .2 cglib创建代理的例子

import org.springframework.cglib.proxy.Enhancer;
import org.springframework.cglib.proxy.MethodInterceptor;
import org.springframework.cglib.proxy.MethodProxy;
import java.lang.reflect.Method;
public class CglibProxyTest {

   static class CglibProxyService {
        public  CglibProxyService(){
        }
        void sayHello(){
            System.out.println(" hello !");
        }
    }

    static class CglibProxyInterceptor implements MethodInterceptor{
        @Override
        public Object intercept(Object sub, Method method,
             Object[] objects, MethodProxy methodProxy)
                                          throws Throwable {
            System.out.println("before hello");
            Object object = methodProxy.invokeSuper(sub, objects);
            System.out.println("after hello");
            return object;
        }
    }

    public static void main(String[] args) {
        // 通过CGLIB动态代理获取代理对象的过程
        Enhancer enhancer = new Enhancer();
        // 设置enhancer对象的父类
        enhancer.setSuperclass(CglibProxyService.class);
        // 设置enhancer的回调对象
        enhancer.setCallback(new CglibProxyInterceptor());
        // 创建代理对象
        CglibProxyService proxy= (CglibProxyService)enhancer.create();
        System.out.println(CglibProxyService.class);
        System.out.println(proxy.getClass());
        // 通过代理对象调用目标方法
        proxy.sayHello();
    }
}
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3.2.1 .3 jdk创建代理与cglib创建代理的区别

类型	jdk创建动态代理	cglib创建动态代理
原理	java动态代理是利用反射机制生成一个实现代理接口的匿名类，在调用具体方法前调用InvokeHandler来处理	cglib动态代理是利用asm开源包，对代理对象类的class文件加载进来，通过修改其字节码生成子类来处理
核心类	Proxy 创建代理利用反射机制生成一个实现代理接口的匿名类InvocationHandler 方法拦截器接口,需要实现invoke方法	net.sf.cglib.proxy.Enhancer：主要增强类，通过字节码技术动态创建委托类的子类实例net.sf.cglib.proxy.MethodInterceptor：方法拦截器接口，需要实现intercept方法
局限性	只能代理实现了接口的类	不能对final修饰的类进行代理，也不能处理final修饰的方法

3.2.2 Spring如何选择的使用哪种方式

Spring的选择选择如何代理时在DefaultAopProxyFactory 中。

public class DefaultAopProxyFactory implements AopProxyFactory, 
                                                        Serializable {
   @Override
   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);
      }
   }
   //...
   }
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