Spring源码分析之AOP

Posted 2022-10-31

AOP是什么

面向切面的程序设计（Aspect-oriented programming，AOP，又译作面向方面的程序设计、剖面导向程序设计），是计算机科学中的一种程序设计思想，旨在将横切关注点与业务主体进行进一步分离，以提高程序代码的模块化程度。通过在现有代码基础上增加额外的通知（Advice）机制，能够对被声明为“切点（Pointcut）”的代码块进行统一管理与装饰。

怎么在Spring里使用AOP

在​​Spring​​里，​​AOP​​通过​​EnableAspectJAutoProxy​​注解开启。默认情况下，​​Spring​​会通过​​AopAutoConfiguration​​自动引入这个注解

@Configuration(proxyBeanMethods = false)
@ConditionalOnProperty(prefix = "spring.aop", name = "auto", havingValue = "true", matchIfMissing = true)
public class AopAutoConfiguration 

  @Configuration(proxyBeanMethods = false)
  @ConditionalOnClass(Advice.class)
  static class AspectJAutoProxyingConfiguration 

    @Configuration(proxyBeanMethods = false)
    @EnableAspectJAutoProxy(proxyTargetClass = false)
    @ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "false")
    static class JdkDynamicAutoProxyConfiguration 

    

    @Configuration(proxyBeanMethods = false)
    @EnableAspectJAutoProxy(proxyTargetClass = true)
    @ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "true",
        matchIfMissing = true)
    static class CglibAutoProxyConfiguration 

    

  

可以看到，如果我们不主动设置​​spring.aop.auto=false​​。那么​​Spring​​默认会启用AOP。接下来，我们可以通过在类上标注​​Aspect​​即可使用AOP

package org.example.aspect;

@Aspect
@Component
public class SampleAspect 

    @Pointcut("execution(* org.example.xxx.*.*(..))")
    private void executionPointcut() 

    

    @After(value = "executionPointcut()")
    public void doAfter() 

    

源码分析

1\\. AOP初始化

1.1 初始化AspectJAutoProxyRegistrar

​​EnableAspectJAutoProxy​​通过​​Import​​注解引入了​​AspectJAutoProxyRegistrar​​

@Import(AspectJAutoProxyRegistrar.class)
public @interface EnableAspectJAutoProxy 

​​AspectJAutoProxyRegistrar​​实现了​​ImportBeanDefinitionRegistrar​​，​​Spring​​在初始化​​AopAutoConfiguration​​时把所有通过​​Import​​注解引入的​​ImportBeanDefinitionRegistrar​​实现类拿出来进行初始化，并调用其​​registerBeanDefinitions​​函数

class AspectJAutoProxyRegistrar implements ImportBeanDefinitionRegistrar 

  @Override
  public void registerBeanDefinitions(
      AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) 

    AopConfigUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary(registry);

1.2 初始化AnnotationAwareAspectJAutoProxyCreator

​​AspectJAutoProxyRegistrar​​ 则在​​registerBeanDefinitions​​注册了一个​​AnnotationAwareAspectJAutoProxyCreator​​的​​BeanDefinition​​

public static BeanDefinition registerAspectJAnnotationAutoProxyCreatorIfNecessary(
      BeanDefinitionRegistry registry, @Nullable Object source) 

    return registerOrEscalateApcAsRequired(AnnotationAwareAspectJAutoProxyCreator.class, registry, source);
  

​​AnnotationAwareAspectJAutoProxyCreator​​实现了​​BeanPostProcessor​​，​​Spring​​会在初始化普通​​Bean​​之前初始化所有​​BeanPostProcessor​​。

public abstract class AbstractApplicationContext extends DefaultResourceLoader
    implements ConfigurableApplicationContext 

    public void refresh() throws BeansException, IllegalStateException 
          // 初始化BeanProcessor来拦截Bean的创建
          registerBeanPostProcessors(beanFactory);

          // 初始化所有剩下的非懒加载的Bean，比如我们写的Service
          finishBeanFactoryInitialization(beanFactory);
    

1.3 初始化切面方法跟切点

另外，​​AnnotationAwareAspectJAutoProxyCreator​​实现了​​InstantiationAwareBeanPostProcessor​​，​​Spring​​ 会在​​Bean​​创建时调用其​​postProcessBeforeInstantiation​​方法对​​Bean​​进行处理。

在第一次调用该方法时，​​AnnotationAwareAspectJAutoProxyCreator​​会初始化切面

public abstract class AbstractAutoProxyCreator extends ProxyProcessorSupport
    implements SmartInstantiationAwareBeanPostProcessor, BeanFactoryAware 

  @Override
  public Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) 
    Object cacheKey = getCacheKey(beanClass, beanName);

    if (!StringUtils.hasLength(beanName) || !this.targetSourcedBeans.contains(beanName)) 
      //判断当前BeanName对应的Bean是否应该被代理
      //并将判断结果保存下来，避免后续的后处理方法重复计算
      //在第一次判断时，会在shouldSkip里扫描所有Bean进行切面初始化
      if (isInfrastructureClass(beanClass) || shouldSkip(beanClass, beanName)) 
        this.advisedBeans.put(cacheKey, Boolean.FALSE);
        return null;
      
    

    //如果为AbstractAutoProxyCreator注入了自定义的TargetSourceCreator
    //则通过TargetSourceCreator创建的Bean都被被AOP代理
    //TargetSourceCreator默认为空
    TargetSource targetSource = getCustomTargetSource(beanClass, beanName);
    if (targetSource != null) 
      if (StringUtils.hasLength(beanName)) 
        this.targetSourcedBeans.add(beanName);
      
      Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(beanClass, beanName, targetSource);
      Object proxy = createProxy(beanClass, beanName, specificInterceptors, targetSource);
      this.proxyTypes.put(cacheKey, proxy.getClass());
      return proxy;
    

    return null;
  

切面初始化

public class BeanFactoryAspectJAdvisorsBuilder 

    public List<Advisor> buildAspectJAdvisors() 
        List<String> aspectNames = this.aspectBeanNames;

        //如果还未进行初始化
        if (aspectNames == null) 
          synchronized (this) 
            aspectNames = this.aspectBeanNames;
            if (aspectNames == null) 
              List<Advisor> advisors = new ArrayList<>();
              aspectNames = new ArrayList<>();

              //拿到容器里所有的beanName
              String[] beanNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(
                  this.beanFactory, Object.class, true, false);
              for (String beanName : beanNames) 

                Class<?> beanType = this.beanFactory.getType(beanName, false);

                //判断类上是否标注Aspect，以及判断该class是否已经被代码式的Aspectj处理过
                if (this.advisorFactory.isAspect(beanType)) 
                  aspectNames.add(beanName);
                  AspectMetadata amd = new AspectMetadata(beanType, beanName);
                  MetadataAwareAspectInstanceFactory factory =
                        new BeanFactoryAspectInstanceFactory(this.beanFactory, beanName);

                    //从类中拿到所有带有Before、Around等注解的方法，
                    //将这些方法包装成MethodInterceptor放入Advisor，MethodInterceptor#invoke为增强方法的调用入口
                    //将Advisor排好顺序组成List返回
                    List<Advisor> classAdvisors = this.advisorFactory.getAdvisors(factory);

                    this.advisorsCache.put(beanName, classAdvisors);
                    advisors.addAll(classAdvisors);
                
              
              this.aspectBeanNames = aspectNames;
              return advisors;
            
          
        
      

Advisor排序

public class ReflectiveAspectJAdvisorFactory extends AbstractAspectJAdvisorFactory implements Serializable 
    private static final Comparator<Method> adviceMethodComparator;

    static 

      Comparator<Method> adviceKindComparator = new ConvertingComparator<>(
          //按照注解顺序设置方法对应的advisor的顺序
          //在AspectJAfterAdvice里，会先将请求继续向拦截器链后传播，
          //对增强方法的调用是在后面的finnaly块里。所以这里的After顺序即使在AfterReturning前面也没关系
          //另外，因为在finnly块里触发，所以即使后续的调用抛出了未捕获的异常，After指定的增强方法也会被执行
          new InstanceComparator<>(
              Around.class, Before.class, After.class, AfterReturning.class, AfterThrowing.class),

          (Converter<Method, Annotation>) method -> 
              //如果方法上没有标注上面的几个注解，则返回null，null会排在最后
            AspectJAnnotation<?> ann = AbstractAspectJAdvisorFactory.findAspectJAnnotationOnMethod(method);
            return (ann != null ? ann.getAnnotation() : null);
          );
      Comparator<Method> methodNameComparator = new ConvertingComparator<>(Method::getName);
      adviceMethodComparator = adviceKindComparator.thenComparing(methodNameComparator);
    

判断method是否属于切面方法

public class ReflectiveAspectJAdvisorFactory extends AbstractAspectJAdvisorFactory implements Serializable 
    public Advisor getAdvisor(Method candidateAdviceMethod, MetadataAwareAspectInstanceFactory aspectInstanceFactory,
        int declarationOrderInAspect, String aspectName) 

        //获取切点信息，如果candidateAdviceMethod不是切面方法，则返回null
        AspectJExpressionPointcut expressionPointcut = getPointcut(
            candidateAdviceMethod, aspectInstanceFactory.getAspectMetadata().getAspectClass());
        if (expressionPointcut == null) 
          return null;
        

        return new InstantiationModelAwarePointcutAdvisorImpl(expressionPointcut, candidateAdviceMethod,
            this, aspectInstanceFactory, declarationOrderInAspect, aspectName);
     

    private AspectJExpressionPointcut getPointcut(Method candidateAdviceMethod, Class<?> candidateAspectClass) 
        //在方法上查找Aspectj的相关注解（Around、After等）
        AspectJAnnotation<?> aspectJAnnotation =
            AbstractAspectJAdvisorFactory.findAspectJAnnotationOnMethod(candidateAdviceMethod);
        if (aspectJAnnotation == null) 
          return null;
        

        ...
    

2\\. 生成代理对象

​​AbstractAutoProxyCreator​​实现了​​BeanPostProcessor​​，在创建​​Bean​​时，​​Spring​​会调用​​AbstractAutoProxyCreator​​#​​postProcessAfterInitialization​​对​​Bean​​进行处理

public abstract class AbstractAutoProxyCreator extends ProxyProcessorSupport
    implements SmartInstantiationAwareBeanPostProcessor, BeanFactoryAware 

    @Override
    public Object postProcessAfterInitialization(@Nullable Object bean, String beanName) 
      if (bean != null) 
        Object cacheKey = getCacheKey(bean.getClass(), beanName);
        if (this.earlyProxyReferences.remove(cacheKey) != bean) 
          //对bean进行包装，返回代理bean
          return wrapIfNecessary(bean, beanName, cacheKey);
        
      
      return bean;
    

    protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) 
        //如果bean有TargetSourceCreator创建，说明已经被代理过了，直接返回
        if (StringUtils.hasLength(beanName) && this.targetSourcedBeans.contains(beanName)) 
          return bean;
        
        //拿出缓存的检测的结果进行判断
        if (Boolean.FALSE.equals(this.advisedBeans.get(cacheKey))) 
          return bean;
        
        //初步判断bean是否可以被代理
        if (isInfrastructureClass(bean.getClass()) || shouldSkip(bean.getClass(), beanName)) 
          this.advisedBeans.put(cacheKey, Boolean.FALSE);
          return bean;
        

        //根据切点Point的表达式获得符合当前bean的所有advisor
        //如果当前bean不在切点的指向中，则返回DO_NOT_PROXY
        Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
        if (specificInterceptors != DO_NOT_PROXY) 
          this.advisedBeans.put(cacheKey, Boolean.TRUE);
          //创建代理对象，将所有advisor包装成DynamicAdvisedInterceptor，
          //其intercept方法为所有增强方法的统一入口，这个类来自Spring
          Object proxy = createProxy(
              bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));
          this.proxyTypes.put(cacheKey, proxy.getClass());
          return proxy;
        

        //缓存判断结果
        this.advisedBeans.put(cacheKey, Boolean.FALSE);
        return bean;
  

选择代理对象的创建方式

public class DefaultAopProxyFactory implements AopProxyFactory, Serializable   
    @Override
    public AopProxy createAopProxy(AdvisedSupport config) throws AopConfigException 
      if (!NativeDetector.inNativeImage() &&
          (config.isOptimize() || config.isProxyTargetClass() || hasNoUserSuppliedProxyInterfaces(config))) 
        Class<?> targetClass = config.getTargetClass();

        //如果targetClass是接口类型或者是通过Proxy.getProxyClass生成的或者类名里包含$$Lambda
        //则使用JDK动态代理
        if (targetClass.isInterface() || Proxy.isProxyClass(targetClass) || ClassUtils.isLambdaClass(targetClass)) 
          return new JdkDynamicAopProxy(config);
        
        //使用cglib，ASM修改字节码的方式生产代理类
        return new ObjenesisCglibAopProxy(config);
      
      else 
        return new JdkDynamicAopProxy(config);
      
    

3\\. 访问代理对象

​​DispatcherServlet​​ 找到对应的实例跟方法后通过反射进行调用（前面​​Spring Mvc​​博文里已分析过）,此时会遍历代理对象上的所有​​MethodInterceptor​​,

public class ReflectiveMethodInvocation implements ProxyMethodInvocation, Cloneable 

    public Object proceed() throws Throwable 
      //如果拦截器遍历完了，则调用目标方法
      if (this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size() - 1) 
        return invokeJoinpoint();
      

      //遍历所有增强器MethodInterceptor，
      //遍历方式是在MethodInterceptor里调用MethodInvocation#proceed
      //每次进入该方法，都会使currentInterceptorIndex增加1，从而达成遍历
      Object interceptorOrInterceptionAdvice =
          this.interceptorsAndDynamicMethodMatchers.get(++this.currentInterceptorIndex);

      //调用MethodInterceptor#invoke
      return ((MethodInterceptor) interceptorOrInterceptionAdvice).invoke(this);
    

增强方法实际执行的顺序图

接下来看看每个增强注解对应的​​MethodInterceptor​​处理类是怎么进行请求处理与传递的

1\\. AspectJAroundAdvice

public class AspectJAroundAdvice extends AbstractAspectJAdvice implements MethodInterceptor, Serializable 

  @Override
  @Nullable
  public Object invoke(MethodInvocation mi) throws Throwable 
    ...
    ProxyMethodInvocation pmi = (ProxyMethodInvocation) mi;
    ProceedingJoinPoint pjp = lazyGetProceedingJoinPoint(pmi);
    JoinPointMatch jpm = getJoinPointMatch(pmi);
    //调用Around对应的增强方法，并将mi传如增强方法
    //mi持有了所有的拦截/增强器信息，通过Joinpoint#proceed实现请求的传递
    //所以，around增强方法里需要注意接收Joinpont的实例并调用其proceed方法
    return invokeAdviceMethod(pjp, jpm, null, null);
  

2\\. MethodBeforeAdviceInterceptor

public class MethodBeforeAdviceInterceptor implements MethodInterceptor, BeforeAdvice, Serializable 

  private final MethodBeforeAdvice advice;

  public MethodBeforeAdviceInterceptor(MethodBeforeAdvice advice) 
    Assert.notNull(advice, "Advice must not be null");
    this.advice = advice;
  

  @Override
  @Nullable
  public Object invoke(MethodInvocation mi) throws Throwable 
    //调用before增强方法
    this.advice.before(mi.getMethod(), mi.getArguments(), mi.getThis());
    //向后传递
    return mi.proceed();
  

3\\. AspectJAfterAdvice

public class AspectJAfterAdvice extends AbstractAspectJAdvice
    implements MethodInterceptor, AfterAdvice, Serializable 

    public AspectJAfterAdvice(
        Method aspectJBeforeAdviceMethod, AspectJExpressionPointcut pointcut, AspectInstanceFactory aif) 

      super(aspectJBeforeAdviceMethod, pointcut, aif);
    

    @Override
    @Nullable
    public Object invoke(MethodInvocation mi) throws Throwable 
      try 
        //先向后传递
        return mi.proceed();
      
      finally 
        //执行After方法
        invokeAdviceMethod(getJoinPointMatch(), null, null);
      
    

4\\. AfterReturningAdviceInterceptor

public class AfterReturningAdviceInterceptor implements MethodInterceptor, AfterAdvice, Serializable 

  private final AfterReturningAdvice advice;

  public AfterReturningAdviceInterceptor(AfterReturningAdvice advice) 
    Assert.notNull(advice, "Advice must not be null");
    this.advice = advice;
  

  @Override
  @Nullable
  public Object invoke(MethodInvocation mi) throws Throwable 

    //向后传递
    Object retVal = mi.proceed();
    //调用AfterReturning方法
    this.advice.afterReturning(retVal, mi.getMethod(), mi.getArguments(), mi.getThis());
    return retVal;
  

5\\. AspectJAfterThrowingAdvice

public class AspectJAfterThrowingAdvice extends AbstractAspectJAdvice
    implements MethodInterceptor, AfterAdvice, Serializable 

    public AspectJAfterThrowingAdvice(
        Method aspectJBeforeAdviceMethod, AspectJExpressionPointcut pointcut, AspectInstanceFactory aif) 

      super(aspectJBeforeAdviceMethod, pointcut, aif);
    

    @Override
    @Nullable
    public Object invoke(MethodInvocation mi) throws Throwable 
      try 
        //向后传递
        return mi.proceed();
      
      catch (Throwable ex) 
        //判断增强器上定义的异常类型是否匹配
        if (shouldInvokeOnThrowing(ex)) 
          //调用AfterThrowing方法
          invokeAdviceMethod(getJoinPointMatch(), null, ex);
        
        throw ex;
      
    

    /**
     * In AspectJ semantics, after throwing advice that specifies a throwing clause
     * is only invoked if the thrown exception is a subtype of the given throwing type.
     */
    private boolean shouldInvokeOnThrowing(Throwable ex) 
      return getDiscoveredThrowingType().isAssignableFrom(ex.getClass());
    

6\\. ExposeInvocationInterceptor

public final class ExposeInvocationInterceptor implements MethodInterceptor, PriorityOrdered, Serializable 

    private static final ThreadLocal<MethodInvocation> invocation =
          new NamedThreadLocal<>("Current AOP method invocation");

    @Override
    @Nullable
    public Object invoke(MethodInvocation mi) throws Throwable 
      MethodInvocation oldInvocation = invocation.get();
      //将MethodInvocation绑定到当前线程的本地变量里，
      //从而实现在其他地方访问MethodInvocation
      //但不建议这样做，因为AOP对于被代理对象来说应该是无感知的，不应该产生这种依赖
      invocation.set(mi);
      try 
        return mi.proceed();
      
      finally 
        invocation.set(oldInvocation);