@ComponentScan(value = "com.wang")
@Configuration
@EnableAsync
public class AppConfig {
}

@Service
public class CycleService2 {

	@Autowired
	private CycleService1 cycleService1;

	@Async
	public void alsoDo() {
		System.out.println("create cycleService2");
	}

}

@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.
	 * 此處說明的是方法執(zhí)行變成異步，掃描的是哪個注解，目前默認的是Async和Asynchronous，開發(fā)者也可以自定義
	 */
	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.
	 * 如何proxyTargetClass被設(shè)置成true，那么spring的所有proxy都會通過CGLIB方式實現(xiàn)，不再使用Java默認的基于接口的代理實現(xiàn)方式；而且此處如果設(shè)置，不僅僅是會影響添加了@Async注解的類的proxy方式，加了@Transactional的類也會變成CGLIB代理，不推薦修改；這個注解只有mode是默認的PROXY，才有意義
	 */
	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}.
	 * 代理方式的不同，默認的是使用Spring的proxy方式，也可以換成原生的AspectJ的proxy方式。
	 * 這兩個的區(qū)別作用還是很明顯的
	 */
	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.
	 * 因為在beanPostProcessor執(zhí)行的時候，會根據(jù)order值進行排序，此處設(shè)置為最低值，就是想讓其最后執(zhí)行
	 * 其實即使不設(shè)置這個值，因為AsyncAnnotationBeanPostProcessor繼承了ProxyProcessorSupport，ProxyProcessorSupport中的order默認也是最小優(yōu)先級
	 * 
	 */
	int order() default Ordered.LOWEST_PRECEDENCE;

}

public class AsyncConfigurationSelector extends AdviceModeImportSelector<EnableAsync> {

	private static final String ASYNC_EXECUTION_ASPECT_CONFIGURATION_CLASS_NAME =
			"org.springframework.scheduling.aspectj.AspectJAsyncConfiguration";


	/**
	 * Returns {@link ProxyAsyncConfiguration} or {@code AspectJAsyncConfiguration}
	 * for {@code PROXY} and {@code ASPECTJ} values of {@link EnableAsync#mode()},
	 * respectively.
	 */
	@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;
		}
	}

}

@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;
	}

}

public class AsyncAnnotationBeanPostProcessor extends AbstractBeanFactoryAwareAdvisingPostProcessor {

    // 刪除了一些無關(guān)緊要，或者默認不會設(shè)置的屬性
	public AsyncAnnotationBeanPostProcessor() {
		setBeforeExistingAdvisors(true);
	}

	/**
	 * 因為AsyncAnnotationBeanPostProcessor實現(xiàn)了BeanFactoryAware接口
	 * 所以在實例化的過程中執(zhí)行到initializeBean步驟的時候，里面第一步就是執(zhí)行各種實現(xiàn)了Aware接口的接口方法
	 * 在此處new了一個advisor。advisor簡單理解就是：advice+pointcut
	 * @param beanFactory
	 */
	@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;
	}

}

public AsyncAnnotationAdvisor(
			@Nullable Supplier<Executor> executor, @Nullable Supplier<AsyncUncaughtExceptionHandler> exceptionHandler) {

		Set<Class<? extends Annotation>> asyncAnnotationTypes = new LinkedHashSet<>(2);
		/**
		 * 這兒設(shè)置符合pointCut需要的注解
		 * 此處的executor就是一個擴展點，如果不想用spring的默認單線程線程池，可以自定義一個線程池
		 * exceptionHandler，顧名思義，就是我們的方法在線程池中執(zhí)行時拋出exception該如何handle使用的
		 * advice也就是咱們的interceptor
		 * pointCut就不多解釋了，就是把設(shè)置符合什么條件會進行interceptor的invoke方法
		 */
		asyncAnnotationTypes.add(Async.class);
		try {
			asyncAnnotationTypes.add((Class<? extends Annotation>)
					ClassUtils.forName("javax.ejb.Asynchronous", AsyncAnnotationAdvisor.class.getClassLoader()));
		}
		catch (ClassNotFoundException ex) {
			// If EJB 3.1 API not present, simply ignore.
		}
		this.advice = buildAdvice(executor, exceptionHandler);
		this.pointcut = buildPointcut(asyncAnnotationTypes);
	}

protected Advice buildAdvice(
			@Nullable Supplier<Executor> executor, @Nullable Supplier<AsyncUncaughtExceptionHandler> exceptionHandler) {
		// new了一個interceptor
		AnnotationAsyncExecutionInterceptor interceptor = new AnnotationAsyncExecutionInterceptor(null);
		interceptor.configure(executor, exceptionHandler);
		return interceptor;
	}

/**
	 * Intercept the given method invocation, submit the actual calling of the method to
	 * the correct task executor and return immediately to the caller.
	 * @param invocation the method to intercept and make asynchronous
	 * @return {@link Future} if the original method returns {@code Future}; {@code null}
	 * otherwise.
	 */
	@Override
	@Nullable
	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);
		/**獲取一個任務(wù)執(zhí)行器
		 * 1. 從@Async注解里面獲取配置的任務(wù)執(zhí)行器
		 * 2. 從Spring容器中找TaskExecutor類的bean
		 * 3. 從spring容器中獲取名為"taskExecutor"的bean，
		 * 4. 如果還沒有，new SimpleAsyncTaskExecutor()）
		 */
		AsyncTaskExecutor executor = determineAsyncExecutor(userDeclaredMethod);
		if (executor == null) {
			throw new IllegalStateException(
					"No executor specified and no default executor set on AsyncExecutionInterceptor either");
		}

		//將當(dāng)前方法執(zhí)行封裝成一個callable對象，然后放入到線程池里
		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;
		};

		//任務(wù)提交
		return doSubmit(task, executor, invocation.getMethod().getReturnType());
	}

	protected void doExecute(Runnable task) {
		Thread thread = (this.threadFactory != null ? this.threadFactory.newThread(task) : createThread(task));
		thread.start();
	}

/**
	 * Calculate a pointcut for the given async annotation types, if any.
	 * @param asyncAnnotationTypes the async annotation types to introspect
	 * @return the applicable Pointcut object, or {@code null} if none
	 */
	protected Pointcut buildPointcut(Set<Class<? extends Annotation>> asyncAnnotationTypes) {
		ComposablePointcut result = null;
		for (Class<? extends Annotation> asyncAnnotationType : asyncAnnotationTypes) {
			// 就是根據(jù)這兩個匹配器進行匹配的
			// 檢查類上是否有@Async注解
			Pointcut cpc = new AnnotationMatchingPointcut(asyncAnnotationType, true);
			//檢查方法上是否有@Async注解
			Pointcut mpc = new AnnotationMatchingPointcut(null, asyncAnnotationType, true);
			if (result == null) {
				result = new ComposablePointcut(cpc);
			}
			else {
			// 取并集：類上加了@Async或者類的方法上加了@Async
				result.union(cpc);
			}
			result = result.union(mpc);
		}
		return (result != null ? result : Pointcut.TRUE);
	}

@Override
	public Object postProcessAfterInitialization(Object bean, String beanName) {
		// 沒有通知，或者是AOP的基礎(chǔ)設(shè)施類，那么不進行代理
		if (this.advisor == null || bean instanceof AopInfrastructureBean) {
			// Ignore AOP infrastructure such as scoped proxies.
			return bean;
		}

		// 對已經(jīng)被代理的類，不再生成代理，只是將通知添加到代理類的邏輯中
		// 這里通過beforeExistingAdvisors決定是將通知添加到所有通知之前還是添加到所有通知之后
		// 在使用@Async注解的時候，beforeExistingAdvisors被設(shè)置成了true，
		// @Async注解之所以把beforeExistingAdvisors設(shè)置為true，是因為該advisor和其他的advisor差別太大了，從情理上講，也應(yīng)該第一個執(zhí)行
		// 意味著整個方法及其攔截邏輯都會異步執(zhí)行
		if (bean instanceof Advised) {
			Advised advised = (Advised) bean;
			// 判斷bean是否符合該advisor的使用范圍，通過pointcut來判斷
			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;
			}
		}

		// 如果還不是一個代理類，也需要通過eligible來判斷是否符合使用該advisor的條件
		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;
	}

protected boolean isEligible(Class<?> targetClass) {
		Boolean eligible = this.eligibleBeans.get(targetClass);
		if (eligible != null) {
			return eligible;
		}
		if (this.advisor == null) {
			return false;
		}
		// 其實就是判斷類是否可以進行添加該advisor，也就是判斷是否符合該advisor的使用條件
		// 就是把advisor的pointCut拿出來，pointCut里的classMatcher和methodMatcher拿出來對類及其方法進行判斷
		eligible = AopUtils.canApply(this.advisor, targetClass);
		this.eligibleBeans.put(targetClass, eligible);
		return eligible;
	}