源码解析Spring AOP的加载与生效

　　本次博主主要进行Spring AOP这里的解析，因为在工作中使用后，却不知道背后的实现原理并在使用的过程中发现了一些认知缺陷，所以决定写这么一篇文章以供大家参考参考，进入正题。

　　本次博主使用了@Aspect、@Around、@PointCut注解实现了一些小的需求，大家想必都用过，我就简单的举个例子吧。

  1 @Aspect
  2 @Component
  3 public class CrmCacheAspect {
  4 
  5     @Autowired
  6     StringRedisTemplate stringRedisTemplate;
  7 
  8     private ConcurrentHashMap<String, ICacheResultParser> parserMap = new ConcurrentHashMap();
  9 
 10     private ConcurrentHashMap<String, IKeyGenerator> generatorMap = new ConcurrentHashMap();
 11 
 12     private ConcurrentHashMap<String,Boolean> keyMap = new ConcurrentHashMap<>();
 13     @Pointcut("@annotation(com.bjh.hms.crm.annotation.CrmCache)")
 14     public void pointCut(){}
 15 
 16     @Around("pointCut() && @annotation(crmCache)")
 17     public Object joinPoint(ProceedingJoinPoint joinPoint, CrmCache crmCache) throws InstantiationException, IllegalAccessException {
 18         String value = "";
 19         String key = "";
 20         Object result = "";
 21         try {
 22             key = getKey(crmCache,joinPoint);
 23             value = stringRedisTemplate.opsForValue().get(key);
 24         } catch (Exception e) {
 25             XxlJobHelper.log("获取缓存{}失败：{}",crmCache.key(),e);
 26         } finally {
 27             if (StringUtils.isBlank(value)) {
 28                 value = synchronizeCache(key, joinPoint, crmCache);
 29             }
 30             result = getResult(crmCache, value, joinPoint);
 31         }
 32         return result;
 33     }
 34 
 35     private Object getResult(CrmCache crmCache,
 36                              String value,
 37                              ProceedingJoinPoint joinPoint) throws InstantiationException, IllegalAccessException {
 38         if (value == null) {
 39             return null;
 40         }
 41         String name = crmCache.parser().getName();
 42         ICacheResultParser iCacheResultParser;
 43         if (parserMap.containsKey(name)) {
 44             iCacheResultParser = parserMap.get(name);
 45         } else {
 46             iCacheResultParser = crmCache.parser().newInstance();
 47             parserMap.put(name,iCacheResultParser);
 48         }
 49         MethodSignature signature = (MethodSignature) joinPoint.getSignature();
 50         Class returnType = signature.getReturnType();
 51         Object parse = iCacheResultParser.parse(value, returnType);
 52         return parse;
 53     }
 54 
 55     /**
 56      * Title: 解决redis并发穿透
 57      * @author 2021/8/13 17:15
 58      * @return java.lang.String
 59      */
 60     private String synchronizeCache(String key,
 61                                     ProceedingJoinPoint joinPoint,
 62                                     CrmCache crmCache) {
 63         String value = "";
 64         //暂停100-200ms，线程顺序执行
 65         try {
 66             Thread.sleep((int)(Math.random()*(200 - 100 + 1) + 100));
 67         } catch (InterruptedException e) {
 68             XxlJobHelper.log("synchronizeCache error {}", ExceptionUtil.stacktraceToString(e));
 69         }
 70         while (StringUtils.isBlank(value = stringRedisTemplate.opsForValue().get(key))
 71         && (keyMap.get(key) == null || keyMap.get(key))){
 72             //防止重复调用
 73             if (keyMap.get(key) == null || !keyMap.get(key)) {
 74                 keyMap.put(key,true);
 75                 Object proceed = null;
 76                 try {
 77                     proceed = joinPoint.proceed();
 78                 } catch (Throwable e) {
 79                     XxlJobHelper.log("处理失败：{}",ExceptionUtil.stacktraceToString(e));
 80                 }
 81                 putValueByRedis(key,proceed,crmCache);
 82                 keyMap.put(key,false);
 83             }
 84         }
 85         keyMap.remove(key);
 86         return value;
 87     }
 88 
 89     private void putValueByRedis(String key, Object value, CrmCache crmCache) {
 90         if (value == null) {
 91             return;
 92         }
 93         if (value instanceof String) {
 94             stringRedisTemplate.opsForValue().set(key, value.toString());
 95         } else {
 96             String jsonString = JSONObject.toJSONString(value);
 97             stringRedisTemplate.opsForValue().set(key,jsonString);
 98         }
 99         //-1代表不过期
100         if (crmCache.expire() != -1) {
101             stringRedisTemplate.expire(key, crmCache.expire(), TimeUnit.MINUTES);
102         }
103     }
104 
105     private String getKey(CrmCache crmCache, ProceedingJoinPoint joinPoint) throws InstantiationException, IllegalAccessException {
106         MethodSignature signature = (MethodSignature) joinPoint.getSignature();
107         Method method = signature.getMethod();
108         Object[] args = joinPoint.getArgs();
109         String iKeyGeneratorName = crmCache.generator().getName();
110         String key = crmCache.key();
111         IKeyGenerator iKeyGenerator = null;
112         if (generatorMap.containsKey(iKeyGeneratorName)) {
113             iKeyGenerator = generatorMap.get(iKeyGeneratorName);
114         } else {
115             iKeyGenerator = crmCache.generator().newInstance();
116             generatorMap.put(iKeyGeneratorName,iKeyGenerator);
117         }
118         return iKeyGenerator.generate(key,method,args);
119     }
120 
121 }

　　本例子主要是对结果与请求进行解析缓存，spring其实有自带的，但是不可以使用缓存时间，有缓存时间又需要引入其他依赖包，公司内部私服又是内网访问的，所以就自写了一个简单的注解实现了缓存有限时间功能。这不是重点，我们来分析一下注解是如何加载进来的，又是如何被spring走进来解析的吧。

　　讲解之前，博主还是一如既往的为大家画了几张草图，以便大家防止看代码看晕，先来第一张：aspect注解源码分析加载与生效

　　https://www.processon.com/view/link/6134aae163768906a2203894

　　我们开始走代码，我们直接走bean的创建开始，如果有小伙伴不知道整个bean创建流程的话，可以看一下博主以前的画的草图脑补一下：

　　https://www.processon.com/view/link/5f704050f346fb166d0f3e3c

　　代码走起，任意的bean创建都可以，如果看不了静态代码，自行debug就可以了。

 1     protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) {
 2         Object bean = null;
 3         if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) {
 4             // 确定是否有aspect注解
 5             if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
 6                 Class<?> targetType = determineTargetType(beanName, mbd);
 7                 if (targetType != null) {
 8                     //解析注解
 9                     bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);
10                     if (bean != null) {
11                         //此处会给有自定义注解的bean创建代理类返回
12                         bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);
13                     }
14                 }
15             }
16             mbd.beforeInstantiationResolved = (bean != null);
17         }
18         return bean;
19     }

　　我们分析一下hasInstantiationAwareBeanPostProcessors方法，看看是如何走进来的，从方法名字可以看出，是否有InstantiationAwareBeanPostProcessors后置处理器，那我们本身并没有去填加这个类，那怎么就有了呢，原因就在我们引入aop包依赖后，有一个默认的自动配置AopAutoConfiguration，EnableAspectJAutoProxy注解中间引入了一个AspectJAutoProxyRegistrar类，实现这个registerBeanDefinitions方法后，引入了一个AnnotationAwareAspectJAutoProxyCreator类，这个类就是AspectJAutoProxyRegistrar的实现类，所以hasInstantiationAwareBeanPostProcessors方法走通了。

　　再看一下applyBeanPostProcessorsBeforeInstantiation方法解析注解流程。

 1 public Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) throws BeansException {
 2         Object cacheKey = getCacheKey(beanClass, beanName);
 3 
 4         if (!StringUtils.hasLength(beanName) || !this.targetSourcedBeans.contains(beanName)) {
 5             if (this.advisedBeans.containsKey(cacheKey)) {
 6                 return null;
 7             }//我们主要分析一下shouldSkip方法
 8             if (isInfrastructureClass(beanClass) || shouldSkip(beanClass, beanName)) {
 9                 this.advisedBeans.put(cacheKey, Boolean.FALSE);
10                 return null;
11             }
12         }
13 
14         // Create proxy here if we have a custom TargetSource.
15         // Suppresses unnecessary default instantiation of the target bean:
16         // The TargetSource will handle target instances in a custom fashion.
17         TargetSource targetSource = getCustomTargetSource(beanClass, beanName);
18         if (targetSource != null) {
19             if (StringUtils.hasLength(beanName)) {
20                 this.targetSourcedBeans.add(beanName);
21             }
22             Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(beanClass, beanName, targetSource);
23             Object proxy = createProxy(beanClass, beanName, specificInterceptors, targetSource);
24             this.proxyTypes.put(cacheKey, proxy.getClass());
25             return proxy;
26         }
27 
28         return null;
29     }

 1     protected boolean shouldSkip(Class<?> beanClass, String beanName) {
 2         // TODO: Consider optimization by caching the list of the aspect names
 3         //主要这里获取了注解
 4         List<Advisor> candidateAdvisors = findCandidateAdvisors();
 5         for (Advisor advisor : candidateAdvisors) {
 6             if (advisor instanceof AspectJPointcutAdvisor &&
 7                     ((AspectJPointcutAdvisor) advisor).getAspectName().equals(beanName)) {
 8                 return true;
 9             }
10         }
11         return super.shouldSkip(beanClass, beanName);
12     }

 1     //此方法分为两步
 2     protected List<Advisor> findCandidateAdvisors() {
 3         // Add all the Spring advisors found according to superclass rules.
 4         //第一步从bean工厂中找到所有Advisor的实现类
 5         List<Advisor> advisors = super.findCandidateAdvisors();
 6         // Build Advisors for all AspectJ aspects in the bean factory.
 7         if (this.aspectJAdvisorsBuilder != null) {
 8         //主要是第二步：从bean工厂中找到所有带有@aspect注解的类
 9             advisors.addAll(this.aspectJAdvisorsBuilder.buildAspectJAdvisors());
10         }
11         return advisors;
12     }

　　我们直接看第二步即可

 1     public List<Advisor> buildAspectJAdvisors() {
 2         List<String> aspectNames = this.aspectBeanNames;
 3 
 4         if (aspectNames == null) {
 5             synchronized (this) {
 6                 aspectNames = this.aspectBeanNames;
 7                 if (aspectNames == null) {
 8                     List<Advisor> advisors = new ArrayList<>();
 9                     aspectNames = new ArrayList<>();
10                     //获取所有类
11                     String[] beanNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(
12                             this.beanFactory, Object.class, true, false);
13                     for (String beanName : beanNames) {
14                         if (!isEligibleBean(beanName)) {
15                             continue;
16                         }
17                         // We must be careful not to instantiate beans eagerly as in this case they
18                         // would be cached by the Spring container but would not have been weaved.
19                         Class<?> beanType = this.beanFactory.getType(beanName);
20                         if (beanType == null) {
21                             continue;
22                         }
23                         //改类是否是我们写的aspect注解类
24                         if (this.advisorFactory.isAspect(beanType)) {
25                             aspectNames.add(beanName);
26                             AspectMetadata amd = new AspectMetadata(beanType, beanName);
27                             if (amd.getAjType().getPerClause().getKind() == PerClauseKind.SINGLETON) {
28                                 MetadataAwareAspectInstanceFactory factory =
29                                         new BeanFactoryAspectInstanceFactory(this.beanFactory, beanName);
30                                 //开始在这里解析
31                                 List<Advisor> classAdvisors = this.advisorFactory.getAdvisors(factory);
32                                 if (this.beanFactory.isSingleton(beanName)) {
33                                     this.advisorsCache.put(beanName, classAdvisors);
34                                 }
35                                 else {
36                                     this.aspectFactoryCache.put(beanName, factory);
37                                 }
38                                 advisors.addAll(classAdvisors);
39         .......
40         return advisors;
41     }

 1     public List<Advisor> getAdvisors(MetadataAwareAspectInstanceFactory aspectInstanceFactory) {
 2     //获取我们的注解类
 3         Class<?> aspectClass = aspectInstanceFactory.getAspectMetadata().getAspectClass();
 4         //获取名称
 5         String aspectName = aspectInstanceFactory.getAspectMetadata().getAspectName();
 6         validate(aspectClass);
 7 
 8         // We need to wrap the MetadataAwareAspectInstanceFactory with a decorator
 9         // so that it will only instantiate once.
10         MetadataAwareAspectInstanceFactory lazySingletonAspectInstanceFactory =
11                 new LazySingletonAspectInstanceFactoryDecorator(aspectInstanceFactory);
12 
13         List<Advisor> advisors = new ArrayList<>();
14         //这里循环获取我们类的方法，找到除Pointcut注解外的注解方法
15         for (Method method : getAdvisorMethods(aspectClass)) {
16         //解析方法，如果找到Around.class, Before.class, After.class, AfterReturning.class, AfterThrowing.class注解，则返回InstantiationModelAwarePointcutAdvisorImpl生成类
17             Advisor advisor = getAdvisor(method, lazySingletonAspectInstanceFactory, advisors.size(), aspectName);
18             if (advisor != null) {
19                 advisors.add(advisor);
20             }
21         }
22 
23         .......
24 
25         return advisors;
26     }

　　自此，我们的注解就解析完成了，不过心细的同学发现了，直解析了除Pointcut注解外的注解，Pointcut直接没有解析啊，这个注解一般我们都配置在了Around等注解里面，会有解析类去解析这个方法的。我们看看实例化后的后置处理器逻辑再

 1     @Override
 2     public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
 3             throws BeansException {
 4 
 5         Object result = existingBean;
 6         //遍历所有后置处理器，但是我们只看AbstractAutoProxyCreator类的
 7         for (BeanPostProcessor processor : getBeanPostProcessors()) {
 8             Object current = processor.postProcessAfterInitialization(result, beanName);
 9             if (current == null) {
10                 return result;
11             }
12             result = current;
13         }
14         return result;
15     }

 1     //调用此方法
 2     protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
 3         if (StringUtils.hasLength(beanName) && this.targetSourcedBeans.contains(beanName)) {
 4             return bean;
 5         }
 6         if (Boolean.FALSE.equals(this.advisedBeans.get(cacheKey))) {
 7             return bean;
 8         }
 9         if (isInfrastructureClass(bean.getClass()) || shouldSkip(bean.getClass(), beanName)) {
10             this.advisedBeans.put(cacheKey, Boolean.FALSE);
11             return bean;
12         }
13 
14         // Create proxy if we have advice.
15         //是否有注解
16         Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
17         if (specificInterceptors != DO_NOT_PROXY) {
18             this.advisedBeans.put(cacheKey, Boolean.TRUE);
19             //有则创建代理类返回
20             Object proxy = createProxy(
21                     bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));
22             this.proxyTypes.put(cacheKey, proxy.getClass());
23             return proxy;
24         }
25 
26         this.advisedBeans.put(cacheKey, Boolean.FALSE);
27         return bean;
28     }

　　为了清晰逻辑，中间的环节代码就不看了，直接看一下返回的是啥。

 1     public AopProxy createAopProxy(AdvisedSupport config) throws AopConfigException {
 2     //config.isProxyTargetClass()这个默认时true，为什么走cglib代理呢？
 3         if (config.isOptimize() || config.isProxyTargetClass() || hasNoUserSuppliedProxyInterfaces(config)) {
 4             Class<?> targetClass = config.getTargetClass();
 5             if (targetClass == null) {
 6                 throw new AopConfigException("TargetSource cannot determine target class: " +
 7                         "Either an interface or a target is required for proxy creation.");
 8             }
 9             if (targetClass.isInterface() || Proxy.isProxyClass(targetClass)) {
10                 return new JdkDynamicAopProxy(config);
11             }
12             return new ObjenesisCglibAopProxy(config);
13         }
14         else {
15             return new JdkDynamicAopProxy(config);
16         }
17     }

　　为什么spring默认走cglib代理呢？我们大家可能还知道一个注解是@EnableAspectJAutoProxy，其实这个才是控制的开关。如果我们写成false的话是走jdk代理的，但是为什么我们自己的配置类配置EnableAspectJAutoProxy注解了也是无效的呢？这时候就要看一下AopAutoConfiguration自动配置类了，为了防止大家看晕，博主也画了一张草图：

　　https://www.processon.com/view/link/6134bef3e401fd1fb6a91dc6

 1 public class AopAutoConfiguration {
 2     //jdk和cglib都有注解，但是默认只有一个生效了，就是CglibAutoProxyConfiguration，因为ConditionalOnProperty注解说明了一起
 3     @Configuration
 4     @EnableAspectJAutoProxy(proxyTargetClass = false)
 5     @ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "false", matchIfMissing = false)
 6     public static class JdkDynamicAutoProxyConfiguration {
 7 
 8     }
 9 
10     @Configuration
11     @EnableAspectJAutoProxy(proxyTargetClass = true)
12     @ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "true", matchIfMissing = true)
13     public static class CglibAutoProxyConfiguration {
14 
15     }
16 
17 }

　　当我们不去在配置文件中明确标明spring.aop.proxy-target-class属性时，只有就是CglibAutoProxyConfiguration是生效的，怕有些小伙伴不知道ConditionalOnProperty注解的作用，博主就简单带带大家看一下，熟悉同学可以自行略过，在spring解析配置类时，就会解析该注解

 1     //这是校验配置类的时候解析的，路径-》org.springframework.context.annotation.ConditionEvaluator#shouldSkip
 2     public boolean shouldSkip(@Nullable AnnotatedTypeMetadata metadata, @Nullable ConfigurationPhase phase) {
 3     //由于jdk和cglib类都有Conditional的子注解，所以都通过了
 4         if (metadata == null || !metadata.isAnnotated(Conditional.class.getName())) {
 5             return false;
 6         }
 7 
 8         if (phase == null) {
 9             if (metadata instanceof AnnotationMetadata &&
10                     ConfigurationClassUtils.isConfigurationCandidate((AnnotationMetadata) metadata)) {
11                 return shouldSkip(metadata, ConfigurationPhase.PARSE_CONFIGURATION);
12             }
13             return shouldSkip(metadata, ConfigurationPhase.REGISTER_BEAN);
14         }
15 
16         List<Condition> conditions = new ArrayList<>();
17         //找到ConditionalOnProperty注解
18         for (String[] conditionClasses : getConditionClasses(metadata)) {
19             for (String conditionClass : conditionClasses) {
20                 Condition condition = getCondition(conditionClass, this.context.getClassLoader());
21                 conditions.add(condition);
22             }
23         }
24 
25         AnnotationAwareOrderComparator.sort(conditions);
26 
27         for (Condition condition : conditions) {
28             ConfigurationPhase requiredPhase = null;
29             if (condition instanceof ConfigurationCondition) {
30                 requiredPhase = ((ConfigurationCondition) condition).getConfigurationPhase();
31             }
32             //开始检验是否匹配
33             if ((requiredPhase == null || requiredPhase == phase) && !condition.matches(this.context, metadata)) {
34                 return true;
35             }
36         }
37 
38         return false;
39     }

 1     public final boolean matches(ConditionContext context,
 2             AnnotatedTypeMetadata metadata) {
 3         String classOrMethodName = getClassOrMethodName(metadata);
 4         try {
 5         //走这里查看OnPropertyCondition匹配即可
 6             ConditionOutcome outcome = getMatchOutcome(context, metadata);
 7             logOutcome(classOrMethodName, outcome);
 8             recordEvaluation(context, classOrMethodName, outcome);
 9             return outcome.isMatch();
10         }
11         ......
12     }

 1         private void collectProperties(PropertyResolver resolver, List<String> missing,
 2                 List<String> nonMatching) {
 3             for (String name : this.names) {
 4                 String key = this.prefix + name;
 5                 if (resolver.containsProperty(key)) {
 6                     if (!isMatch(resolver.getProperty(key), this.havingValue)) {
 7                         nonMatching.add(name);
 8                     }
 9                 }
10                 else {
11                 //直接查看关键代码，如果配置文件中没有该属性，查看是否注解中写了matchIfMissing属性，而我们的cglib是true，所以，不会missing，而是装配起来了，所以默认走cglib代理
12                     if (!this.matchIfMissing) {
13                         missing.add(name);
14                     }
15                 }
16             }
17         }

　　现在我们的注解不仅加载完了，而且被注解表明的也生成了代理类，我们看看切面注解是如何生效的，我们就以cglib举例了，jdk类似

 1 //CglibAopProxy
 2 public Object intercept(Object proxy, Method method, Object[] args, MethodProxy methodProxy) throws Throwable {
 3             Object oldProxy = null;
 4             boolean setProxyContext = false;
 5             Object target = null;
 6             TargetSource targetSource = this.advised.getTargetSource();
 7             try {
 8                 .....
 9                 //获取是否有拦截链，并不是我们的请求拦截器，这里把切面认为是一种拦截器了
10                 List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);
11                 Object retVal;
12                 // Check whether we only have one InvokerInterceptor: that is,
13                 // no real advice, but just reflective invocation of the target.
14                 if (chain.isEmpty() && Modifier.isPublic(method.getModifiers())) {
15                     // We can skip creating a MethodInvocation: just invoke the target directly.
16                     // Note that the final invoker must be an InvokerInterceptor, so we know
17                     // it does nothing but a reflective operation on the target, and no hot
18                     // swapping or fancy proxying.
19                     Object[] argsToUse = AopProxyUtils.adaptArgumentsIfNecessary(method, args);
20                     retVal = methodProxy.invoke(target, argsToUse);
21                 }
22                 else {
23                     // We need to create a method invocation...
24                     //主要就是走后面的.proceed()方法
25                     retVal = new CglibMethodInvocation(proxy, target, method, args, targetClass, chain, methodProxy).proceed();
26                 }
27                 retVal = processReturnType(proxy, target, method, retVal);
28                 return retVal;
29             .....
30                 }
31             }
32         }

 1     //这里就像走我们的请求过滤器一样，每个拦截器都走一遍，最后都调用proceed()再回到这个方法，直到++this.currentInterceptorIndex到头终止
 2     public Object proceed() throws Throwable {
 3         //    We start with an index of -1 and increment early.
 4         if (this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size() - 1) {
 5             return invokeJoinpoint();
 6         }
 7 
 8         Object interceptorOrInterceptionAdvice =
 9                 this.interceptorsAndDynamicMethodMatchers.get(++this.currentInterceptorIndex);
10         if (interceptorOrInterceptionAdvice instanceof InterceptorAndDynamicMethodMatcher) {
11             // Evaluate dynamic method matcher here: static part will already have
12             // been evaluated and found to match.
13             InterceptorAndDynamicMethodMatcher dm =
14                     (InterceptorAndDynamicMethodMatcher) interceptorOrInterceptionAdvice;
15             if (dm.methodMatcher.matches(this.method, this.targetClass, this.arguments)) {
16             //别的我们不看，就看我们自己定义的@around
17                 return dm.interceptor.invoke(this);
18             }
19             else {
20                 // Dynamic matching failed.
21                 // Skip this interceptor and invoke the next in the chain.
22                 return proceed();
23             }
24         }
25         else {
26             // It's an interceptor, so we just invoke it: The pointcut will have
27             // been evaluated statically before this object was constructed.
28             return ((MethodInterceptor) interceptorOrInterceptionAdvice).invoke(this);
29         }
30     }

　　调用反射的时候，就会发现我们的around中才会去解析pointcut方法，因为我们在around注解里面写了，具体设这个类PointcutParser#parsePointcutExpression去进行解析的，将pointcut的表达式放入到around中作为参数传递。

　　对此，Spring AOP就全部讲解完毕了，里面为了减少文章篇幅，去掉了一些中间的跳转代码，具体可以看一下，博主发的草图，草图中所以的逻辑都很清晰，也贴了一些关键性的逻辑代码。希望大家可以在深入了解了解。

ps：以上内容，纯属个人见解，有任何问题下方评论！关注博主公众号，你想要的都有，每周不停更哦！原创编写不易，转载请说明出处！

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原文地址：https://www.cnblogs.com/guoxiaoyu/p/15230623.html