Spring异步调用原理及SpringAop拦截器链原理

一、Spring异步调用底层原理

　　开启异步调用只需一个注解@EnableAsync

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Import(AsyncConfigurationSelector.class)
public @interface EnableAsync {

    /**
     * Indicate the 'async' annotation type to be detected at either class
     * or method level.
     * <p>By default, both Spring's @{@link Async} annotation and the EJB 3.1
     * {@code @javax.ejb.Asynchronous} annotation will be detected.
     * <p>This attribute exists so that developers can provide their own
     * custom annotation type to indicate that a method (or all methods of
     * a given class) should be invoked asynchronously.
     */
    Class<? extends Annotation> annotation() default Annotation.class;

    /**
     * Indicate whether subclass-based (CGLIB) proxies are to be created as opposed
     * to standard Java interface-based proxies.
     * <p><strong>Applicable only if the {@link #mode} is set to {@link AdviceMode#PROXY}</strong>.
     * <p>The default is {@code false}.
     * <p>Note that setting this attribute to {@code true} will affect <em>all</em>
     * Spring-managed beans requiring proxying, not just those marked with {@code @Async}.
     * For example, other beans marked with Spring's {@code @Transactional} annotation
     * will be upgraded to subclass proxying at the same time. This approach has no
     * negative impact in practice unless one is explicitly expecting one type of proxy
     * vs. another &mdash; for example, in tests.
     */
    boolean proxyTargetClass() default false;

    /**
     * Indicate how async advice should be applied.
     * <p><b>The default is {@link AdviceMode#PROXY}.</b>
     * Please note that proxy mode allows for interception of calls through the proxy
     * only. Local calls within the same class cannot get intercepted that way; an
     * {@link Async} annotation on such a method within a local call will be ignored
     * since Spring's interceptor does not even kick in for such a runtime scenario.
     * For a more advanced mode of interception, consider switching this to
     * {@link AdviceMode#ASPECTJ}.
     */
    AdviceMode mode() default AdviceMode.PROXY;

    /**
     * Indicate the order in which the {@link AsyncAnnotationBeanPostProcessor}
     * should be applied.
     * <p>The default is {@link Ordered#LOWEST_PRECEDENCE} in order to run
     * after all other post-processors, so that it can add an advisor to
     * existing proxies rather than double-proxy.
     */
    int order() default Ordered.LOWEST_PRECEDENCE;

}

　　AsyncConfigurationSelector的作用是从两个异步配置类中选择一个来完成底层异步代理的工作。这个两个配置类分别是AspectJAsyncConfiguration、ProxyAsyncConfiguration。

@Override
@Nullable
public String[] selectImports(AdviceMode adviceMode) {
    switch (adviceMode) {
        case PROXY:
            return new String[] {ProxyAsyncConfiguration.class.getName()};
        case ASPECTJ:
            return new String[] {ASYNC_EXECUTION_ASPECT_CONFIGURATION_CLASS_NAME};
        default:
            return null;
    }
}

　　其中adviceMode就是@EnableAsync注解中mode()方法的值，默认是"PROXY"。接下来着重看一下ProxyAsyncConfiguration做了哪些事情。

@Configuration
@Role(BeanDefinition.ROLE_INFRASTRUCTURE)
public class ProxyAsyncConfiguration extends AbstractAsyncConfiguration {

    @Bean(name = TaskManagementConfigUtils.ASYNC_ANNOTATION_PROCESSOR_BEAN_NAME)
    @Role(BeanDefinition.ROLE_INFRASTRUCTURE)
    public AsyncAnnotationBeanPostProcessor asyncAdvisor() {
        Assert.notNull(this.enableAsync, "@EnableAsync annotation metadata was not injected");
        AsyncAnnotationBeanPostProcessor bpp = new AsyncAnnotationBeanPostProcessor();
        bpp.configure(this.executor, this.exceptionHandler);
        Class<? extends Annotation> customAsyncAnnotation = this.enableAsync.getClass("annotation");
        if (customAsyncAnnotation != AnnotationUtils.getDefaultValue(EnableAsync.class, "annotation")) {
            bpp.setAsyncAnnotationType(customAsyncAnnotation);
        }
        bpp.setProxyTargetClass(this.enableAsync.getBoolean("proxyTargetClass"));
        bpp.setOrder(this.enableAsync.<Integer>getNumber("order"));
        return bpp;
    }

}

　　ProxyAsyncConfiguration主要是创建了一个基于异步调用的后置处理器（AsyncAnnotationBeanPostProcessor），改BPP中设置了executor（异步线程池）、exceptionHandler（异常处理器）、AsyncAnnotationType（异步注解类型）、proxyTargetClass（代理创建模式）、order（后置处理器执行顺序）。那么executor和exceptionHandler是哪里来的呢、默认值是什么？接着继续向父级探索。

@Configuration
public abstract class AbstractAsyncConfiguration implements ImportAware {

    .....

    /**
     * Collect any {@link AsyncConfigurer} beans through autowiring.
     */
    @Autowired(required = false)
    void setConfigurers(Collection<AsyncConfigurer> configurers) {
        if (CollectionUtils.isEmpty(configurers)) {
            return;
        }
        if (configurers.size() > 1) {
            throw new IllegalStateException("Only one AsyncConfigurer may exist");
        }
        AsyncConfigurer configurer = configurers.iterator().next();
        this.executor = configurer::getAsyncExecutor;
        this.exceptionHandler = configurer::getAsyncUncaughtExceptionHandler;
    }

}

　　由此可见，executor和exceptionHandler可以通过AsyncConfigurer自定义配置。需要注意的是，spring容器中只能有一个AsyncConfigurer类型的实例呦。

　　进入异步实现的正题了，当然是好好研究一下AsyncAnnotationBeanPostProcessor这个后置处理器做了哪些事情了。

public class AsyncAnnotationBeanPostProcessor extends AbstractBeanFactoryAwareAdvisingPostProcessor {

    ....

    @Override
    public void setBeanFactory(BeanFactory beanFactory) {
        super.setBeanFactory(beanFactory);

        AsyncAnnotationAdvisor advisor = new AsyncAnnotationAdvisor(this.executor, this.exceptionHandler);
        if (this.asyncAnnotationType != null) {
            advisor.setAsyncAnnotationType(this.asyncAnnotationType);
        }
        advisor.setBeanFactory(beanFactory);
        this.advisor = advisor;
    }

}

　　1、添加一个AOP advisor（AsyncAnnotationAdvisor），识别带有@Async注解或者指定类型注解的方法，创建代理类。

　　2、找一个合适的TaskExecutor来异步调用带有@Async注解或者指定类型注解的方法。

　　3、如果方法在异步调用过程中抛出异常，将使用合适的ExceptionHandler进行处理。

　　看到这里，已经有两个疑问了。AsyncAnnotationAdvisor做了什么？如何创建的异步调用代理类？

　　AsyncAnnotationAdvisor实现原理

　　大家都知道，Spring Aop中，一个advisor包含一个advice（通知）、pointcut（切点）。

　　创建advice

protected Advice buildAdvice(
        @Nullable Supplier<Executor> executor, @Nullable Supplier<AsyncUncaughtExceptionHandler> exceptionHandler) {

    AnnotationAsyncExecutionInterceptor interceptor = new AnnotationAsyncExecutionInterceptor(null);
    interceptor.configure(executor, exceptionHandler);
    return interceptor;
}

　　AnnotationAsyncExecutionInterceptor是一个方法拦截器，父级接口是我们最熟悉的org.aopalliance.intercept.MethodInterceptor。这个拦截器有个优秀的功能，可以根据不同的方法选择不同的taskexecutor来异步执行，即Async#value()方法的值。

public Object invoke(final MethodInvocation invocation) throws Throwable {
    Class<?> targetClass = (invocation.getThis() != null ? AopUtils.getTargetClass(invocation.getThis()) : null);
    Method specificMethod = ClassUtils.getMostSpecificMethod(invocation.getMethod(), targetClass);
    final Method userDeclaredMethod = BridgeMethodResolver.findBridgedMethod(specificMethod);

    AsyncTaskExecutor executor = determineAsyncExecutor(userDeclaredMethod);
    if (executor == null) {
        throw new IllegalStateException(
                "No executor specified and no default executor set on AsyncExecutionInterceptor either");
    }

    Callable<Object> task = () -> {
        try {
            Object result = invocation.proceed();
            if (result instanceof Future) {
                return ((Future<?>) result).get();
            }
        }
        catch (ExecutionException ex) {
            handleError(ex.getCause(), userDeclaredMethod, invocation.getArguments());
        }
        catch (Throwable ex) {
            handleError(ex, userDeclaredMethod, invocation.getArguments());
        }
        return null;
    };

    return doSubmit(task, executor, invocation.getMethod().getReturnType());
}

　　拦截器的invoke方法看一下瞬间豁然开朗，寻找方法对应的桥接方法、选择一个合适的异步执行的executor、创建Callback实例（异常的处理）、提交异步调用任务到executor中。

　　创建pointcut

public interface Pointcut {

    /**
     * Return the ClassFilter for this pointcut.
     * @return the ClassFilter (never {@code null})
     */
    ClassFilter getClassFilter();

    /**
     * Return the MethodMatcher for this pointcut.
     * @return the MethodMatcher (never {@code null})
     */
    MethodMatcher getMethodMatcher();

    /**
     * Canonical Pointcut instance that always matches.
     */
    Pointcut TRUE = TruePointcut.INSTANCE;

}

public interface Pointcut {

    /**
     * Return the ClassFilter for this pointcut.
     * @return the ClassFilter (never {@code null})
     */
    ClassFilter getClassFilter();

    /**
     * Return the MethodMatcher for this pointcut.
     * @return the MethodMatcher (never {@code null})
     */
    MethodMatcher getMethodMatcher();

    /**
     * Canonical Pointcut instance that always matches.
     */
    Pointcut TRUE = TruePointcut.INSTANCE;

}

　　一个切点主要包含两个对象ClassFilter（class过滤器）、MethodMatcher（方法匹配器）。AnnotationMatchingPointcut主要匹配注有@Async或者指定类型注解的class或者方法。

　异步代理类调用创建过程

　　继续回到AsyncAnnotationBeanPostProcessor这个后置处理器，父类AbstractBeanFactoryAwareAdvisingPostProcessor是和AbstractAutoProxyCreator（Spring Aop中最常见的创建Aop Proxy的BPP）同一级别的，主要是曝光代理对象的class、强制设置target-class mode。

@Override
protected ProxyFactory prepareProxyFactory(Object bean, String beanName) {
    if (this.beanFactory != null) {
        AutoProxyUtils.exposeTargetClass(this.beanFactory, beanName, bean.getClass());
    }

    ProxyFactory proxyFactory = super.prepareProxyFactory(bean, beanName);
    if (!proxyFactory.isProxyTargetClass() && this.beanFactory != null &&
            AutoProxyUtils.shouldProxyTargetClass(this.beanFactory, beanName)) {
        proxyFactory.setProxyTargetClass(true);
    }
    return proxyFactory;
}

　　如何判断bean是否需要创建proxy呢？

@Override
protected boolean isEligible(Object bean, String beanName) {
    return (!AutoProxyUtils.isOriginalInstance(beanName, bean.getClass()) &&
            super.isEligible(bean, beanName));
}

AbstractAdvisingBeanPostProcessor.java

protected boolean isEligible(Class<?> targetClass) {
    Boolean eligible = this.eligibleBeans.get(targetClass);
    if (eligible != null) {
        return eligible;
    }
    if (this.advisor == null) {
        return false;
    }
    eligible = AopUtils.canApply(this.advisor, targetClass);
    this.eligibleBeans.put(targetClass, eligible);
    return eligible;
}

AopUtils.java

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) {
        PointcutAdvisor pca = (PointcutAdvisor) advisor;
        return canApply(pca.getPointcut(), targetClass, hasIntroductions);
    }
    else {
        // It doesn't have a pointcut so we assume it applies.
        return true;
    }
}

　　首先当前处理的bean是最原始的实例，然后通过advisor的pointcut去判断。

　　继续追踪父级AbstractAdvisingBeanPostProcessor。

public Object postProcessAfterInitialization(Object bean, String beanName) {
    if (this.advisor == null || bean instanceof AopInfrastructureBean) {
        // Ignore AOP infrastructure such as scoped proxies.
        return bean;
    }

    if (bean instanceof Advised) {
        Advised advised = (Advised) bean;
        if (!advised.isFrozen() && isEligible(AopUtils.getTargetClass(bean))) {
            // Add our local Advisor to the existing proxy's Advisor chain...
            if (this.beforeExistingAdvisors) {
                advised.addAdvisor(0, this.advisor);
            }
            else {
                advised.addAdvisor(this.advisor);
            }
            return bean;
        }
    }

    if (isEligible(bean, beanName)) {
        ProxyFactory proxyFactory = prepareProxyFactory(bean, beanName);
        if (!proxyFactory.isProxyTargetClass()) {
            evaluateProxyInterfaces(bean.getClass(), proxyFactory);
        }
        proxyFactory.addAdvisor(this.advisor);
        customizeProxyFactory(proxyFactory);
        return proxyFactory.getProxy(getProxyClassLoader());
    }

    // No proxy needed.
    return bean;
}

　　这个抽象类中实现了BPP的postProcessAfterInitialization方法。如果bean是Advised，则将AsyncAnnotationAdvisor添加到Advised实例中去；如果是一个可以创建异步调用代理的bean，通过ProxyFactory创建代理对象。

二、正确实现异步调用

　　1、启动类新增注解@EnableAsync

　　2、通过AsyncConfigurerSupport创建异步调用线程池，合理设置相关配置参数，如下。

@Configuration
public class MyAsyncConfigurer extends AsyncConfigurerSupport {
    private static Logger LOGGER = LoggerFactory.getLogger(MyAsyncConfigurer.class);

    @Override
    public Executor getAsyncExecutor() {
        ThreadPoolTaskExecutor taskExecutor = new ThreadPoolTaskExecutor();
        taskExecutor.setCorePoolSize(2);
        taskExecutor.setMaxPoolSize(4);
        taskExecutor.setQueueCapacity(10);
        taskExecutor.setRejectedExecutionHandler((runnable, executor) -> LOGGER.error("异步线程池拒绝任务..." + runnable));
        taskExecutor.setThreadFactory(new MyAsyncThreadFactory());
        taskExecutor.initialize();
        return taskExecutor;
    }

    static class MyAsyncThreadFactory implements ThreadFactory {
        private static final AtomicInteger poolNumber = new AtomicInteger(1);
        private final ThreadGroup group;
        private final AtomicInteger threadNumber = new AtomicInteger(1);
        private final String namePrefix;

        MyAsyncThreadFactory() {
            SecurityManager s = System.getSecurityManager();
            group = (s != null) ? s.getThreadGroup() :
                    Thread.currentThread().getThreadGroup();
            namePrefix = "myasync-pool-" +
                    poolNumber.getAndIncrement() +
                    "-thread-";
        }

        @Override
        public Thread newThread(Runnable r) {
            Thread t = new Thread(group, r,
                    namePrefix + threadNumber.getAndIncrement(),
                    0);
            if (t.isDaemon())
                t.setDaemon(false);
            if (t.getPriority() != Thread.NORM_PRIORITY)
                t.setPriority(Thread.NORM_PRIORITY);
            return t;
        }
    }
}

　　3、成员方法异步调用、内部类方法异步调用、spring retry功能整合到异步调用

@Component
public class MyAsyncTask {

    private static Logger LOGGER = LoggerFactory.getLogger(MyAsyncConfigurer.class);

    /**
     * Lazy 功能
     *
     * @see DefaultListableBeanFactory#resolveDependency(DependencyDescriptor, String, Set, TypeConverter)
     * <p>
     * Spring Bean创建-解决依赖 参考链接：https://blog.csdn.net/finalcola/article/details/81537380
     */
    @Lazy
    @Autowired
    private MyInnerAsyncTask myInnerAsyncTask;

    @Autowired
    private AsyncWrapped asyncWrapped;

    @Async
    public void async() {
        LOGGER.error("async");
    }

    public void asyncInner() {
        myInnerAsyncTask.async();
    }

    public void asyncWrapped() {
        asyncWrapped.asyncProcess(() -> LOGGER.error("async wrapped"), null, null);
    }

    public void asyncWrappedWithRetry() {
        Retry retry = new Retry(2, 1000);
        asyncWrapped.asyncProcess(() -> {
            throw new RuntimeException("async wrapped with retry");
        }, null, retry);
    }

    public void asyncWrappedWithRetry2() {
        try {
            asyncWrapped.asyncProcess(() -> {
                throw new RuntimeException("async wrapped with retry2");
            });
        } catch (Exception e) {
            LOGGER.error("异步调用异常...", e);
        }
    }

    private class MyInnerAsyncTask {
        @Async
        public void async() {
            LOGGER.error("async inner");
        }
    }

    @Configuration
    public static class MyAsyncTaskConfiguration {
        @Bean
        public MyInnerAsyncTask myInnerAsyncTask(MyAsyncTask myAsyncTask) {
            return myAsyncTask.new MyInnerAsyncTask();
        }
    }
}

@Component
public class AsyncWrapped {
    protected static Logger LOGGER = LoggerFactory.getLogger(AsyncWrapped.class);

    @Async
    public void asyncProcess(Runnable runnable, Callback callback, Retry retry) {
        try {
            if (retry == null) {
                retry = new Retry(1);
            }
            retry.execute(ctx -> {
                runnable.run();
                return null;
            }, ctx -> {
                if (callback != null) {
                    callback.call();
                }
                return null;
            });
        } catch (Exception e) {
            LOGGER.error("异步调用异常...", e);
        }
    }

    @Async
    @Retryable(value = Exception.class, maxAttempts = 3, backoff = @Backoff(delay = 2000, multiplier = 1.5))
    public void asyncProcess(Runnable runnable) throws Exception {
        System.out.println("重试中...");
        runnable.run();
    }

    @FunctionalInterface
    public interface Runnable {
        void run() throws Exception;
    }

    @FunctionalInterface
    public interface Callback {
        void call();
    }
}

三、Spring Aop拦截器链

　　本来没有写这块的东西，Spring异步调用整合了Spring Retry功能之后，就像看一下二者是如何协调工作的。

　　开启异步和重试功能，仅需要加上这两个注解@EnableAsync、@EnableRetry。

　　大家可以看一下RetryConfiguration这个类，直接告诉大家了，它是一个advisor，直接注册到spring容器当中的。AbstractAutoProxyCreator会拿到这个advisor，对具有@Retryable注解的bean创建代理类。分析流程和AsyncAnnotationAdvisor一致，大家可以按照上面讲过的流程分析一下Spring Retry的底层实现原理，这里就不详细说明了。

　　如下是AbstractAutowireCapableBeanFactory#applyBeanPostProcessorsAfterInitialization（）方法。

@Override
public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
        throws BeansException {

    Object result = existingBean;
    for (BeanPostProcessor processor : getBeanPostProcessors()) {
        Object current = processor.postProcessAfterInitialization(result, beanName);
        if (current == null) {
            return result;
        }
        result = current;
    }
    return result;
}

　　

　　上图中AnnotationAwareAspectJAutoProxyCreator是AbstractAdvisorAutoProxyCreator的实例。也就是说AbstractAdvisorAutoProxyCreator类型的后置处理器优先于AsyncAnnotationBeanPostProcessor类型的后置处理器执行。AbstractAdvisorAutoProxyCreator BPP通过BeanFactoryAdvisorRetrievalHelper从当前的BeanFactory中拿到所有的advisor。

　　

　　然后针对当前的bean（beanName = asyncWrapped ）筛选出合适的advisor集合（包含RetryConfiguration实例）。最后是通过ProxyFactory创建的代理类，具体如下。

　　

　　ProxyFactory通过默认AopProxyFactory即DefaultAopProxyFactory来创建Aop Proxy。

　　

　　到这里，beanName = asyncWrapped 关于Retryable的代理对象已经创建完毕，并返回代理对象替代当前的bean。然后继续到AsyncAnnotationBeanPostProcessor#postProcessAfterInitialization（）方法，处理关于带有@Async注解的bean。

//如果是advised
if (bean instanceof Advised) {
    Advised advised = (Advised) bean;
    if (!advised.isFrozen() && isEligible(AopUtils.getTargetClass(bean))) {
        // Add our local Advisor to the existing proxy's Advisor chain...
        if (this.beforeExistingAdvisors) {
            advised.addAdvisor(0, this.advisor);
        }
        else {
            advised.addAdvisor(this.advisor);
        }
        return bean;
    }
}

//这里的逻辑不会执行了
if (isEligible(bean, beanName)) {
    ProxyFactory proxyFactory = prepareProxyFactory(bean, beanName);
    if (!proxyFactory.isProxyTargetClass()) {
        evaluateProxyInterfaces(bean.getClass(), proxyFactory);
    }
    proxyFactory.addAdvisor(this.advisor);
    customizeProxyFactory(proxyFactory);
    return proxyFactory.getProxy(getProxyClassLoader());
}

　　以前总以为多个注解，就会多次创建代理，一层一层嵌套。现在明白了，是通过拦截器链来完成的。此时beanName = asyncWrapped对应的bean已经是Advised类型的实例了，然后将AsyncAnnotationAdvisor实例添加到Advised实例的advisors集合中。

　　为啥beanName = asyncWrapped对应的bean是Advised类型的实例？那还要从对beanName = asyncWrapped的bean创建代理类说起。那么接着回到通过DefaultAopProxyFactory来创建Aop Proxy。这里看一下CglibAopProxy，JdkDynamicAopProxy请自行查看。以下代码来自CglibAopProxy#getProxy（）方法。

......
//设置需要代理的接口
enhancer.setInterfaces(AopProxyUtils.completeProxiedInterfaces(this.advised));
......
//获取callbacks
Callback[] callbacks = getCallbacks(rootClass);
......
//设置callback filter
enhancer.setCallbackFilter(new ProxyCallbackFilter(
        this.advised.getConfigurationOnlyCopy(), this.fixedInterceptorMap, this.fixedInterceptorOffset));
.....

　　设置需要代理的接口，除了目标类包含的接口，还需要添加一些额外的接口。如下是AopProxyUtils#completeProxiedInterfaces（）方法中的内容。

......
if (addSpringProxy) {
    proxiedInterfaces[index] = SpringProxy.class;
    index++;
}
if (addAdvised) {
    proxiedInterfaces[index] = Advised.class;
    index++;
}
if (addDecoratingProxy) {
    proxiedInterfaces[index] = DecoratingProxy.class;
}
......

　　看到了Advised.class哈，这就是为啥最终的代理对象是Advised类型的实例了。

　　获取callbacks集合，注意this.advisedDispatcher在数组中的索引是4，下面会用到。

Callback[] mainCallbacks = new Callback[] {
            aopInterceptor,  // for normal advice
            targetInterceptor,  // invoke target without considering advice, if optimized
            new SerializableNoOp(),  // no override for methods mapped to this
            targetDispatcher, this.advisedDispatcher,
            new EqualsInterceptor(this.advised),
            new HashCodeInterceptor(this.advised)
    };

　　设置callback filters，如下是ProxyCallbackFilter#accept(Method method)部分源码。

......
if (!this.advised.isOpaque() && method.getDeclaringClass().isInterface() &&
            method.getDeclaringClass().isAssignableFrom(Advised.class)) {
        if (logger.isTraceEnabled()) {
            logger.trace("Method is declared on Advised interface: " + method);
        }
        return DISPATCH_ADVISED;
    }
......

　　ProxyCallbackFilter的作用主要是根据不同类型的method，返回callbacks数组的索引。上面的DISPATCH_ADVISED变量的值是4。

　　这个AdvisedDispatcher是干什么的呢？

//Dispatcher for any methods declared on the Advised class.
private static class AdvisedDispatcher implements Dispatcher, Serializable {

    private final AdvisedSupport advised;

    public AdvisedDispatcher(AdvisedSupport advised) {
        this.advised = advised;
    }

    @Override
    public Object loadObject() throws Exception {
        return this.advised;
    }
}

　　也就是如果method是Advised.class声明的，则使用AdvisedDispatcher进行分发。

AsyncAnnotationBeanPostProcessor#postProcessAfterInitialization（）

//如果是advised
if (bean instanceof Advised) {
    Advised advised = (Advised) bean;
    if (!advised.isFrozen() && isEligible(AopUtils.getTargetClass(bean))) {
        // Add our local Advisor to the existing proxy's Advisor chain...
        if (this.beforeExistingAdvisors) {
            advised.addAdvisor(0, this.advisor);
        }
        else {
            advised.addAdvisor(this.advisor);
        }
        return bean;
    }
}

　　上面的advised.addAdvisor(0, this.advisor); 相当于如下代码。

//spring aop cglib代理对象
public class XXXX$$EnhancerBySpringCGLIB$$8f47b115 implements Advised {
    private org.springframework.cglib.proxy.Dispatcher advisedDispatcher;//AdvisedDispatcher实例
    ......

    @Override
    public void addAdvisor(int pos, Advisor advisor) throws AopConfigException() {
        advisedDispatcher.loadObject().addAdvisor(pos, advisor);
    }

    ......
}

　　还需要补充的一个地方就是callbacks数组中有个aopInterceptor，对应的类型是DynamicAdvisedInterceptor（General purpose AOP callback. Used when the target is dynamic or when the proxy is not frozen.）。

　　　　

　　如上图所示，intercept方法中会通过advised(AdvisedSupport type, The configuration used to configure this proxy.)实例获取一个拦截器链，如果不为空，则返回一个CglibMethodInvocation实例。

　　简单总结一下获取拦截器链的过程， 如下。

　　1、从缓存中获取当前方法的拦截器链
　　2、若缓存未命中，则调用 getInterceptorsAndDynamicInterceptionAdvice 获取拦截器链
　　3、遍历通知器列表
　　4、对于 PointcutAdvisor 类型的通知器，这里要调用通知器所持有的切点（Pointcut）对类和方法进行匹配，匹配成功说明应向当前方法织入通知逻辑
　　5、调用 getInterceptors 方法对非 MethodInterceptor 类型的通知进行转换
　　6、返回拦截器数组，并在随后存入缓存中

　　CglibMethodInvocation的父类是ReflectiveMethodInvocation，ReflectiveMethodInvocation 贯穿于拦截器链执行的始终。

public class ReflectiveMethodInvocation implements ProxyMethodInvocation {

    private int currentInterceptorIndex = -1;

    public Object proceed() throws Throwable {
        // 拦截器链中的最后一个拦截器执行完后，即可执行目标方法
        if (this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size() - 1) {
            // 执行目标方法
            return invokeJoinpoint();
        }

        Object interceptorOrInterceptionAdvice =
                this.interceptorsAndDynamicMethodMatchers.get(++this.currentInterceptorIndex);
        if (interceptorOrInterceptionAdvice instanceof InterceptorAndDynamicMethodMatcher) {
            InterceptorAndDynamicMethodMatcher dm =
                    (InterceptorAndDynamicMethodMatcher) interceptorOrInterceptionAdvice;
            /*
             * 调用具有三个参数（3-args）的 matches 方法动态匹配目标方法，
             * 两个参数（2-args）的 matches 方法用于静态匹配
             */
            if (dm.methodMatcher.matches(this.method, this.targetClass, this.arguments)) {
                // 调用拦截器逻辑
                return dm.interceptor.invoke(this);
            }
            else {
                // 如果匹配失败，则忽略当前的拦截器
                return proceed();
            }
        }
        else {
            // 调用拦截器逻辑，并传递 ReflectiveMethodInvocation 对象
            return ((MethodInterceptor) interceptorOrInterceptionAdvice).invoke(this);
        }
    }
}

　　所以整个拦截器链的调用流程大约长这样（盗图一张）。

　　

　　　　大家在写MethodInterceptor 的时候注意了，一定要调用MethodInvocation 的 proceed（）方法，否则不能执行拦截器链。

public SelfMethodInterceptor implements MethodInterceptor {
    public Object invoke(MethodInvocation invocation) throws Throwable {
        //前置逻辑
        Object ret=invocation.proceed();

        //错误的写法，无法执行拦截器链
        //Object ret = invocation.getMethod().invoke(invocation.getThis(), invocation.getArguments());

        //后置逻辑
        return ret;
    }
}

四、参考

　　 Spring AOP 源码分析 - 拦截器链的执行过程

五、总结

　　至此，Spring异步调用原理及SpringAop拦截器链都已经分析完毕，希望对大家使用spring异步调用有所帮助。另外我自己也重新温习了spring aop相关的知识，也希望大家对spring aop有一个新的认识。如果有需要源码的同学，请f访问我的github：Spring异步调用原理及实现方案demo。