一、SpringAOP应用
先搭建一个SpringBoot项目

<?xml version="1.0" encoding="UTF-8"?>

4.0.0

org.springframework.boot
spring-boot-starter-parent
2.1.7.RELEASE


com.mmc
springboot-study
0.0.1-SNAPSHOT
springboot-study
Demo project for Spring Boot

<properties><java.version>1.8</java.version>
</properties><dependencies><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-web</artifactId></dependency></dependencies><build><plugins><plugin><groupId>org.springframework.boot</groupId><artifactId>spring-boot-maven-plugin</artifactId></plugin></plugins>
</build>

定义一个业务逻辑类，作为切面 public interface CalculationService {

/*** 加法运算* @param x* @param y* @return*/
public Integer add(Integer x,Integer y);

}

/**

• @description:

• @author: mmc

• @create: 2020-06-01 14:22
  **/
  @Service
  public class CalculationServiceImpl implements CalculationService {

  @Override
  public Integer add(Integer x, Integer y) {
  if(xnull||ynull){
  throw new NullPointerException(“参数不能为空”);
  }
  return x+y;
  }
  }
  定义一个切面类，添加通知方法
  前置通知(@Before)：logStart：在目标方法(div)运行之前运行
  后置通知(@After)：logEnd：在目标方法(add)运行结束之后运行（无论方法正常结束还是异常结束）
  返回通知(@AfterReturning)：logReturn：在目标方法(add)正常返回之后运行
  异常通知(@AfterThrowing)：logException：在目标方法(add)出现异常以后运行
  环绕通知(@Around)：动态代理，手动推进目标方法运行（joinPoint.procced()）

/**

• @description: 切面类

• @author: mmc

• @create: 2020-06-01 14:24
  **/
  @Aspect
  @Component
  public class LogAspects {

  //抽取公共的切入点表达式
  //1、本类引用
  //2、其他的切面引用
  @Pointcut(“execution(public Integer com.mmc.springbootstudy.service.CalculationService.*(…))”)
  public void pointCut(){};

  @Before(“pointCut()”)
  public void logStart(JoinPoint joinPoint){
  Object[] args = joinPoint.getArgs();
  System.out.println(""+joinPoint.getSignature().getName()+“运行。。。@Before:参数列表是：{”+Arrays.asList(args)+"}");
  }

  @After(“pointCut()”)
  public void logEnd(JoinPoint joinPoint){
  System.out.println(""+joinPoint.getSignature().getName()+“结束。。。@After”);
  }

  //JoinPoint一定要出现在参数表的第一位
  @AfterReturning(value=“pointCut()”,returning=“result”)
  public void logReturn(JoinPoint joinPoint,Object result){
  System.out.println(""+joinPoint.getSignature().getName()+“正常返回。。。@AfterReturning:运行结果：{”+result+"}");
  }

  @AfterThrowing(value=“pointCut()”,throwing=“exception”)
  public void logException(JoinPoint joinPoint,Exception exception){
  System.out.println(""+joinPoint.getSignature().getName()+“异常。。。异常信息：{”+exception+"}");
  }
  }
  写一个controller测试
  @RequestMapping("/testaop")
  @ResponseBody
  public Integer testaop(Integer x,Integer y){
  Integer result = calculationService.add(x, y);
  return result;
  }
  测试
  add运行。。。@Before:参数列表是：{[2, 3]}
  add结束。。。@After
  add正常返回。。。@AfterReturning:运行结果：{5}

二、源码分析
主线流程图：

spring.factories文件里引入了AopAutoConfiguration类
@Configuration
@ConditionalOnClass({ EnableAspectJAutoProxy.class, Aspect.class, Advice.class, AnnotatedElement.class })
@ConditionalOnProperty(prefix = “spring.aop”, name = “auto”, havingValue = “true”, matchIfMissing = true)
public class AopAutoConfiguration {

@Configuration
@EnableAspectJAutoProxy(proxyTargetClass = false)
//看配置文件，如果配置的spring.aop.proxy-target-class为false则引入JdkDynamicAutoProxyConfiguration
@ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "false",matchIfMissing = false)
public static class JdkDynamicAutoProxyConfiguration {}@Configuration
//开启AspectJAutoProxy
@EnableAspectJAutoProxy(proxyTargetClass = true)
//看配置文件，如果配置的spring.aop.proxy-target-class为true则引入CglibAutoProxyConfiguration 
@ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "true",matchIfMissing = true)
public static class CglibAutoProxyConfiguration {}

}
在包目录下找到配置文件，并且发现他的值为true

在上面的方法上有EnableAspectJAutoProxy注解，并传入了proxyTargetClass=true

进入@EnableAspectJAutoProxy注解
@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
//引入了AspectJAutoProxyRegistrar
@Import({AspectJAutoProxyRegistrar.class})
public @interface EnableAspectJAutoProxy {
boolean proxyTargetClass() default false;

boolean exposeProxy() default false;

}
进入AspectJAutoProxyRegistrar类
class AspectJAutoProxyRegistrar implements ImportBeanDefinitionRegistrar {
AspectJAutoProxyRegistrar() {
}

public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {//注册了自动自动代理类AopConfigUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary(registry);AnnotationAttributes enableAspectJAutoProxy = AnnotationConfigUtils.attributesFor(importingClassMetadata, EnableAspectJAutoProxy.class);if (enableAspectJAutoProxy != null) {if (enableAspectJAutoProxy.getBoolean("proxyTargetClass")) {AopConfigUtils.forceAutoProxyCreatorToUseClassProxying(registry);}if (enableAspectJAutoProxy.getBoolean("exposeProxy")) {AopConfigUtils.forceAutoProxyCreatorToExposeProxy(registry);}}}

}
进入registerAspectJAnnotationAutoProxyCreatorIfNecessary方法里面
public static BeanDefinition registerAspectJAnnotationAutoProxyCreatorIfNecessary(BeanDefinitionRegistry registry, @Nullable Object source) {
return registerOrEscalateApcAsRequired(AnnotationAwareAspectJAutoProxyCreator.class, registry, source);
}
可以看到返回了一个BeanDefinition，里面的BeanClass类型是AnnotationAwareAspectJAutoProxyCreator，这个类看名字是一个AOP的动态代理创建类，里面没有啥可疑的方法。在IDEA里按Ctrl+H看他的继承结构。有一个父类AbstractAutoProxyCreator，这个类实现了BeanPostProcessor接口。这个接口是Bean的扩展接口，在bean初始化完成后会调用到他的postProcessAfterInitialization(Object bean, String beanName)方法。

方法内容如下
public Object postProcessAfterInitialization(@Nullable Object bean, String beanName) {
if (bean != null) {
Object cacheKey = this.getCacheKey(bean.getClass(), beanName);
if (this.earlyProxyReferences.remove(cacheKey) != bean) {
//如果有必要，进行包装
return this.wrapIfNecessary(bean, beanName, cacheKey);
}
}

    return bean;
}protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {if (StringUtils.hasLength(beanName) && this.targetSourcedBeans.contains(beanName)) {return bean;} else if (Boolean.FALSE.equals(this.advisedBeans.get(cacheKey))) {return bean;} else if (!this.isInfrastructureClass(bean.getClass()) && !this.shouldSkip(bean.getClass(), beanName)) {//获取切面的方法，第9点那里展开讨论Object[] specificInterceptors = this.getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, (TargetSource)null);if (specificInterceptors != DO_NOT_PROXY) {this.advisedBeans.put(cacheKey, Boolean.TRUE);//创建动态代理Object proxy = this.createProxy(bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));this.proxyTypes.put(cacheKey, proxy.getClass());return proxy;} else {this.advisedBeans.put(cacheKey, Boolean.FALSE);return bean;}} else {this.advisedBeans.put(cacheKey, Boolean.FALSE);return bean;}
}

可以看出这里已经在开始创建动态代理了
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);
}
//动态代理工厂
ProxyFactory proxyFactory = new ProxyFactory();
proxyFactory.copyFrom(this);
if (!proxyFactory.isProxyTargetClass()) {
if (this.shouldProxyTargetClass(beanClass, beanName)) {
proxyFactory.setProxyTargetClass(true);
} else {
this.evaluateProxyInterfaces(beanClass, proxyFactory);
}
}

    Advisor[] advisors = this.buildAdvisors(beanName, specificInterceptors);//切面那里的方法proxyFactory.addAdvisors(advisors);proxyFactory.setTargetSource(targetSource);this.customizeProxyFactory(proxyFactory);proxyFactory.setFrozen(this.freezeProxy);if (this.advisorsPreFiltered()) {proxyFactory.setPreFiltered(true);}//获取动态代理类return proxyFactory.getProxy(this.getProxyClassLoader());
}

学过AOP的人都知道动态代理的方式有两种，一种JDK代理，一种CGLIB动态代理。那么Spring里面是怎么选择的呢？答案就在这里：
public AopProxy createAopProxy(AdvisedSupport config) throws AopConfigException {
// 1.config.isOptimize()是否使用优化的代理策略，目前使用与CGLIB
// config.isProxyTargetClass() 是否目标类本身被代理而不是目标类的接口
// hasNoUserSuppliedProxyInterfaces()是否存在代理接口

    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 {//目标类不是接口或不是代理类就使用cglib代理return (AopProxy)(!targetClass.isInterface() && !Proxy.isProxyClass(targetClass) ? new ObjenesisCglibAopProxy(config) : new JdkDynamicAopProxy(config));}}
}

Cglib的代理类是CglibAopProxy、ObjenesisCglibAopProxy，JDK的代理类是JdkDynamicAopProxy。在这些类里面对目标类进行了代理，在执行方法的时候就是执行的代理类的方法，而实现了切面编程的效果。
主线流程就是这些了，还有一个没说的就是我们如何获取的切面方法，@Before(“pointCut()”)这些注解又是如何生效的？再回到AbstractAutoProxyCreator的wrapIfNecessary()方法
里面有这句代码：
Object[] specificInterceptors = this.getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, (TargetSource)null);

@Nullable
protected Object[] getAdvicesAndAdvisorsForBean(Class<?> beanClass, String beanName, @Nullable TargetSource targetSource) {
List advisors = this.findEligibleAdvisors(beanClass, beanName);
return advisors.isEmpty() ? DO_NOT_PROXY : advisors.toArray();
}

protected List<Advisor> findEligibleAdvisors(Class<?> beanClass, String beanName) {//查找候选的要切面附加的方法，这里加进去的List<Advisor> candidateAdvisors = this.findCandidateAdvisors();List<Advisor> eligibleAdvisors = this.findAdvisorsThatCanApply(candidateAdvisors, beanClass, beanName);this.extendAdvisors(eligibleAdvisors);if (!eligibleAdvisors.isEmpty()) {eligibleAdvisors = this.sortAdvisors(eligibleAdvisors);}return eligibleAdvisors;
}
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