接口方法上的注解无法被 @Aspect 声明的切面拦截的原因分析

@SpringBootApplication
public class Starter {
  public static void main(String[] args) {
    SpringApplication.run(DynamicApplication.class, args);
  }
}
 
@Service
public class DemoService {
 
    @Autowired
    DemoMapper demoMapper;
 
    public List<Map<String, Object>> selectAll() {
        return demoMapper.selectAll();
    }
}
 
/**
 * mapper类
 */
@Mapper
public interface DemoMapper {
 
  @Select("SELECT * FROM demo")
  @Demo
  List<Map<String, Object>> selectAll();
 
}
 
/**
 * 切入的注解
 */
@Target({ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Demo {
  String value() default "";
}
 
/**
 * aspect切面，用于测试是否成功切入
 */
@Aspect
@Order(-10)
@Component
public class DemoAspect {
 
  @Before("@annotation(demo)")
  public void beforeDemo(JoinPoint point, Demo demo) {
    System.out.println("before demo");
  }
 
  @AfterDemo("@annotation(demo)")
  public void afterDemo(JoinPoint point, Demo demo) {
    System.out.println("after demo");
  }
}

@RunWith(SpringRunner.class) 
@SpringBootTest(classes = Starter.class)
public class BaseTest {
 
    @Autowired
    DemoService demoService;
 
    @Test
    public void testDemo() {
        demoService.selectAll();
    } 
}

public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
    if (bean != null) {
        // 缓存中尝试获取，没有则尝试包装
        Object cacheKey = getCacheKey(bean.getClass(), beanName);
        if (!this.earlyProxyReferences.contains(cacheKey)) {
            return wrapIfNecessary(bean, beanName, cacheKey);
        }
    }
    return bean;
}

protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
    // 如果是声明的需要原始Bean，则直接返回
    if (beanName != null && this.targetSourcedBeans.contains(beanName)) {
        return bean;
    }
    // 如果不需要代理，则直接返回
    if (Boolean.FALSE.equals(this.advisedBeans.get(cacheKey))) {
        return bean;
    }
    // 如果是Proxy的基础组件如Advice、Pointcut、Advisor、AopInfrastructureBean则跳过
    if (isInfrastructureClass(bean.getClass()) || shouldSkip(bean.getClass(), beanName)) {
        this.advisedBeans.put(cacheKey, Boolean.FALSE);
        return bean;
    }
 
    // Create proxy if we have advice.
    // 根据相关条件，查找interceptor，包括@Aspect生成的相关Interceptor。
    // 这里是问题的关键点，Spring Boot 1.X中这里返回为空，而Spring Boot 2.X中，则不是空
    Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
    if (specificInterceptors != DO_NOT_PROXY) {
        // 返回不是null，则需要代理
        this.advisedBeans.put(cacheKey, Boolean.TRUE);
        // 放入缓存
        Object proxy = createProxy(
                bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));
        // 自动生成代理实例
        this.proxyTypes.put(cacheKey, proxy.getClass());
        return proxy;
    }
 
    this.advisedBeans.put(cacheKey, Boolean.FALSE);
    return bean;
}

protected Object[] getAdvicesAndAdvisorsForBean(Class<?> beanClass, String beanName, TargetSource targetSource) {
    List<Advisor> advisors = findEligibleAdvisors(beanClass, beanName);
    if (advisors.isEmpty()) {
        // 如果是空，则不代理
        return DO_NOT_PROXY;
    }
    return advisors.toArray();
}
protected List<Advisor> findEligibleAdvisors(Class<?> beanClass, String beanName) {
    // 找到当前BeanFactory中的Advisor
    List<Advisor> candidateAdvisors = findCandidateAdvisors();
    // 遍历Advisor，根据Advisor中的PointCut判断，返回所有合适的Advisor
    List<Advisor> eligibleAdvisors = findAdvisorsThatCanApply(candidateAdvisors, beanClass, beanName);
    // 扩展advisor列表，这里会默认加入一个ExposeInvocationInterceptor用于暴露动态代理对象，之前文章有解释过
    extendAdvisors(eligibleAdvisors);
    if (!eligibleAdvisors.isEmpty()) {
        // 根据@Order或者接口Ordered排序
        eligibleAdvisors = sortAdvisors(eligibleAdvisors);
    }
    return eligibleAdvisors;
}
protected List<Advisor> findAdvisorsThatCanApply(
        List<Advisor> candidateAdvisors, Class<?> beanClass, String beanName) {
    ProxyCreationContext.setCurrentProxiedBeanName(beanName);
    try {
        // 真正的查找方法  
        return AopUtils.findAdvisorsThatCanApply(candidateAdvisors, beanClass);
    }
    finally {
        ProxyCreationContext.setCurrentProxiedBeanName(null);
    }
}

// AopProxyUtils.java
public static List<Advisor> findAdvisorsThatCanApply(List<Advisor> candidateAdvisors, Class<?> clazz) {
    // ... 省略
    for (Advisor candidate : candidateAdvisors) {
        if (canApply(candidate, clazz, hasIntroductions)) {
            eligibleAdvisors.add(candidate);
        }
    }
    // ... 省略
}
public static boolean canApply(Advisor advisor, Class<?> targetClass, boolean hasIntroductions) {
    if (advisor instanceof IntroductionAdvisor) {
        return ((IntroductionAdvisor) advisor).getClassFilter().matches(targetClass);
    }
    else if (advisor instanceof PointcutAdvisor) {
        // 对于@Aspect的切面，是这段代码在生效
        PointcutAdvisor pca = (PointcutAdvisor) advisor;
        return canApply(pca.getPointcut(), targetClass, hasIntroductions);
    }
    else {
        // It doesn't have a pointcut so we assume it applies.
        return true;
    }
}

/**
 * targetClass是com.sun.proxy.$Proxy??即JDK动态代理生成的类
 * hasIntroductions是false，先不管
 */
public static boolean canApply(Pointcut pc, Class<?> targetClass, boolean hasIntroductions) {
    Assert.notNull(pc, "Pointcut must not be null");
    // 先判断class，这里两个版本都为true
    if (!pc.getClassFilter().matches(targetClass)) {
        return false;
    }
 
    MethodMatcher methodMatcher = pc.getMethodMatcher();
    // 如果method是固定true，即拦截所有method，则返回true。这里当然为false
    if (methodMatcher == MethodMatcher.TRUE) {
        // No need to iterate the methods if we're matching any method anyway...
        return true;
    }
 
    // 特殊类型，做下转换，Aspect生成的属于这个类型
    IntroductionAwareMethodMatcher introductionAwareMethodMatcher = null;
    if (methodMatcher instanceof IntroductionAwareMethodMatcher) {
        introductionAwareMethodMatcher = (IntroductionAwareMethodMatcher) methodMatcher;
    }
 
    // 取到目标class的所有接口
    Set<Class<?>> classes = new LinkedHashSet<Class<?>>(ClassUtils.getAllInterfacesForClassAsSet(targetClass));
    // 再把目标calss加入遍历列表
    classes.add(targetClass);
    for (Class<?> clazz : classes) {
        Method[] methods = ReflectionUtils.getAllDeclaredMethods(clazz);
        // 遍历每个类的每个方法，尝试判断是否match
        for (Method method : methods) {
            if ((introductionAwareMethodMatcher != null &&
                    introductionAwareMethodMatcher.matches(method, targetClass, hasIntroductions)) ||
                    methodMatcher.matches(method, targetClass)) {
                return true;
            }
        }
    }
 
    return false;
}

public static boolean canApply(Pointcut pc, Class<?> targetClass, boolean hasIntroductions) {
    Assert.notNull(pc, "Pointcut must not be null");
    if (!pc.getClassFilter().matches(targetClass)) {
        return false;
    }
 
    MethodMatcher methodMatcher = pc.getMethodMatcher();
    if (methodMatcher == MethodMatcher.TRUE) {
        // No need to iterate the methods if we're matching any method anyway...
        return true;
    }
 
    IntroductionAwareMethodMatcher introductionAwareMethodMatcher = null;
    if (methodMatcher instanceof IntroductionAwareMethodMatcher) {
        introductionAwareMethodMatcher = (IntroductionAwareMethodMatcher) methodMatcher;
    }
 
    Set<Class<?>> classes = new LinkedHashSet<>();
    // 这里与1.X版本不同，使用Jdk动态代理Proxy，先判断是否是Proxy，如果不是则加入用户Class，即被动态代理的class，以便查找真正的Class中是否符合判断条件
    // 因为动态代理可能只把被代理类的方法实现了，被代理类的注解之类的没有复制到生成的子类中，故要使用原始的类进行判断
    // JDK动态代理一样不会为动态代理生成类上加入接口的注解
    // 如果是JDK动态代理，不需要把动态代理生成的类方法遍历列表中，因为实现的接口中真实的被代理接口。
    if (!Proxy.isProxyClass(targetClass)) {
        classes.add(ClassUtils.getUserClass(targetClass));
    }
    classes.addAll(ClassUtils.getAllInterfacesForClassAsSet(targetClass));
 
    for (Class<?> clazz : classes) {
        Method[] methods = ReflectionUtils.getAllDeclaredMethods(clazz);
        for (Method method : methods) {
            // 比1.X版本少遍历了Proxy生成的动态代理类，但是遍历内容都包含了真实的接口，其实是相同的，为什么结果不一样呢？
            if ((introductionAwareMethodMatcher != null &&
                    introductionAwareMethodMatcher.matches(method, targetClass, hasIntroductions)) ||
                    methodMatcher.matches(method, targetClass)) {
                return true;
            }
        }
    }
 
    return false;
}

/** AspectJExpressionPointcut.java
 * method是上面接口中遍历的方法，targetClass是目标class，即生成的动态代理class
 */
public boolean matches(Method method, @Nullable Class<?> targetClass, boolean beanHasIntroductions) {
    obtainPointcutExpression();
    Method targetMethod = AopUtils.getMostSpecificMethod(method, targetClass);
    ShadowMatch shadowMatch = getShadowMatch(targetMethod, method);
 
    // Special handling for this, target, @this, @target, @annotation
    // in Spring - we can optimize since we know we have exactly this class,
    // and there will never be matching subclass at runtime.
    if (shadowMatch.alwaysMatches()) {
        return true;
    }
    else if (shadowMatch.neverMatches()) {
        return false;
    }
    else {
        // the maybe case
        if (beanHasIntroductions) {
            return true;
        }
        // A match test returned maybe - if there are any subtype sensitive variables
        // involved in the test (this, target, at_this, at_target, at_annotation) then
        // we say this is not a match as in Spring there will never be a different
        // runtime subtype.
        RuntimeTestWalker walker = getRuntimeTestWalker(shadowMatch);
        return (!walker.testsSubtypeSensitiveVars() ||
                (targetClass != null && walker.testTargetInstanceOfResidue(targetClass)));
    }
}

// 1.X
public static Method getMostSpecificMethod(Method method, Class<?> targetClass) {
    // 这里返回了targetClass上的重写的method方法。
    Method resolvedMethod = ClassUtils.getMostSpecificMethod(method, targetClass);
    // If we are dealing with method with generic parameters, find the original method.
    return BridgeMethodResolver.findBridgedMethod(resolvedMethod);
}
 
// 2.X
public static Method getMostSpecificMethod(Method method, @Nullable Class<?> targetClass) {
    // 比1.X多了个逻辑判断，如果是JDK的Proxy，则specificTargetClass为null，否则取被代理的Class。
    Class<?> specificTargetClass = (targetClass != null && !Proxy.isProxyClass(targetClass) ?
            ClassUtils.getUserClass(targetClass) : null);
    // 如果specificTargetClass为空，直接返回原始method。
    // 如果不为空，返回被代理的Class上的方法
    Method resolvedMethod = ClassUtils.getMostSpecificMethod(method, specificTargetClass);
    // If we are dealing with method with generic parameters, find the original method.
    // 获取真实桥接的方法，泛型支持
    return BridgeMethodResolver.findBridgedMethod(resolvedMethod);
}

// AbstractFallbackCacheOperationSource.computeCacheOperations
if (specificMethod != method) {
    //  Fallback is to look at the original method
    opDef = findCacheOperations(method);
    if (opDef != null) {
        return opDef;
    }
    // Last fallback is the class of the original method.
    opDef = findCacheOperations(method.getDeclaringClass());
    if (opDef != null && ClassUtils.isUserLevelMethod(method)) {
        return opDef;
    }
}

/**
 * Indicates that an annotation type is automatically inherited.  If
 * an Inherited meta-annotation is present on an annotation type
 * declaration, and the user queries the annotation type on a class
 * declaration, and the class declaration has no annotation for this type,
 * then the class's superclass will automatically be queried for the
 * annotation type.  This process will be repeated until an annotation for this
 * type is found, or the top of the class hierarchy (Object)
 * is reached.  If no superclass has an annotation for this type, then
 * the query will indicate that the class in question has no such annotation.
 *
 * <p>Note that this meta-annotation type has no effect if the annotated
 * type is used to annotate anything other than a class.  Note also
 * that this meta-annotation only causes annotations to be inherited
 * from superclasses; annotations on implemented interfaces have no
 * effect.
 * 上面这句话说明了只在父类上的注解可被继承，接口上的都是无效的
 *
 * @author  Joshua Bloch
 * @since 1.5
 */
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Inherited {
}

https://github.com/spring-projects/spring-framework/blob/master/spring-aop/src/main/java/org/springframework/aop/support/AopUtils.java

Consistent treatment of proxy classes and interfaces for introspection
 
Issue: SPR-16675
Issue: SPR-16677

https://jira.spring.io/browse/SPR-16677

public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
    if (mapperProxyFactory == null) {
        throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
    }
    try {
        // 调用下面的方法生成代理实例
        return mapperProxyFactory.newInstance(sqlSession);
    } catch (Exception e) {
        throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
}
public T newInstance(SqlSession sqlSession) {
    // 创建MapperProxy这个InvocationHandler实例
    final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
    return newInstance(mapperProxy);
}
protected T newInstance(MapperProxy<T> mapperProxy) {
    // 调用jdk动态代理生成实例，代理的InvocationHandler是MapperProxy
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
}

@Configuration
public class DemoConfiguraion {
 
    @Bean
    public FactoryBean<DemoMapper> getDemoMapper() {
        return new FactoryBean<DemoMapper>() {
            @Override
            public DemoMapper getObject() throws Exception {
                InvocationHandler invocationHandler = (proxy, method, args) -> {
                    System.out.println("调用动态代理方法" + method.getName());
                    return Collections.singletonList(new HashMap<String, Object>());
                };
                return (DemoMapper) Proxy.newProxyInstance(this.getClass().getClassLoader(), new Class[] {DemoMapper.class}, invocationHandler);
            }
            @Override
            public Class<?> getObjectType() {
                return DemoMapper.class;
            }
            @Override
            public boolean isSingleton() {
                return true;
            }
        };
    }
}