SpringFramework 之EnableAsync

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.
	 * 
By default, both Spring's @{@link Async} annotation and the EJB 3.1
	 * {@code @javax.ejb.Asynchronous} annotation will be detected.
	 * 
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.
	 * 
Applicable only if the {@link #mode} is set to {@link AdviceMode#PROXY}.
	 * 
The default is {@code false}.
	 * 
Note that setting this attribute to {@code true} will affect all
	 * 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 — for example, in tests.
	 */
	boolean proxyTargetClass() default false;

	/**
	 * Indicate how async advice should be applied.
	 * 
The default is {@link AdviceMode#PROXY}.
	 * 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.
	 * 
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;

}
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1, either a unique org.springframework.core.task.TaskExecutor bean in the context,
2, java.util.concurrent.Executor bean named “taskExecutor” otherwise
3, a org.springframework.core.task.SimpleAsyncTaskExecutor will be used to process async method invocations

注意，如果什么都不配置， 那么SimpleAsyncTaskExecutor 就会被创建
NOTE: This implementation does not reuse threads! Consider a thread-pooling TaskExecutor implementation instead, in particular for executing a large number of short-lived tasks.

spring对pool做了很多抽象， 参见org.springframework.core.task.TaskExecutor

4，使用AsyncConfigurer或者 AsyncConfigurerSupport自定义配置

public interface AsyncConfigurer {

	/**
	 * The {@link Executor} instance to be used when processing async
	 * method invocations.
	 */
	@Nullable
	default Executor getAsyncExecutor() {
		return null;
	}

	/**
	 * The {@link AsyncUncaughtExceptionHandler} instance to be used
	 * when an exception is thrown during an asynchronous method execution
	 * with {@code void} return type.
	 */
	@Nullable
	default AsyncUncaughtExceptionHandler getAsyncUncaughtExceptionHandler() {
		return null;
	}

}
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AsyncConfigurationSelector

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;
		}
	}

}
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这个AdviceModeImportSelector很值得研究

public abstract class AdviceModeImportSelector<A extends Annotation> implements ImportSelector {

	/**
	 * The default advice mode attribute name.
	 */
	public static final String DEFAULT_ADVICE_MODE_ATTRIBUTE_NAME = "mode";


	/**
	 * The name of the {@link AdviceMode} attribute for the annotation specified by the
	 * generic type {@code A}. The default is {@value #DEFAULT_ADVICE_MODE_ATTRIBUTE_NAME},
	 * but subclasses may override in order to customize.
	 */
	protected String getAdviceModeAttributeName() {
		return DEFAULT_ADVICE_MODE_ATTRIBUTE_NAME;
	}

	/**
	 * This implementation resolves the type of annotation from generic metadata and
	 * validates that (a) the annotation is in fact present on the importing
	 * {@code @Configuration} class and (b) that the given annotation has an
	 * {@linkplain #getAdviceModeAttributeName() advice mode attribute} of type
	 * {@link AdviceMode}.
	 * 
The {@link #selectImports(AdviceMode)} method is then invoked, allowing the
	 * concrete implementation to choose imports in a safe and convenient fashion.
	 * @throws IllegalArgumentException if expected annotation {@code A} is not present
	 * on the importing {@code @Configuration} class or if {@link #selectImports(AdviceMode)}
	 * returns {@code null}
	 */
	@Override
	public final String[] selectImports(AnnotationMetadata importingClassMetadata) {
		Class<?> annType = GenericTypeResolver.resolveTypeArgument(getClass(), AdviceModeImportSelector.class);
		Assert.state(annType != null, "Unresolvable type argument for AdviceModeImportSelector");

		AnnotationAttributes attributes = AnnotationConfigUtils.attributesFor(importingClassMetadata, annType);
		if (attributes == null) {
			throw new IllegalArgumentException(String.format(
					"@%s is not present on importing class '%s' as expected",
					annType.getSimpleName(), importingClassMetadata.getClassName()));
		}

		AdviceMode adviceMode = attributes.getEnum(getAdviceModeAttributeName());
		String[] imports = selectImports(adviceMode);
		if (imports == null) {
			throw new IllegalArgumentException("Unknown AdviceMode: " + adviceMode);
		}
		return imports;
	}

	/**
	 * Determine which classes should be imported based on the given {@code AdviceMode}.
	 * 
Returning {@code null} from this method indicates that the {@code AdviceMode}
	 * could not be handled or was unknown and that an {@code IllegalArgumentException}
	 * should be thrown.
	 * @param adviceMode the value of the {@linkplain #getAdviceModeAttributeName()
	 * advice mode attribute} for the annotation specified via generics.
	 * @return array containing classes to import (empty array if none;
	 * {@code null} if the given {@code AdviceMode} is unknown)
	 */
	@Nullable
	protected abstract String[] selectImports(AdviceMode adviceMode);

}
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selectImports(AnnotationMetadata importingClassMetadata)是主要函数，实现了ImportSelector interface的主要逻辑，子类主要实现selectImports(AdviceMode adviceMode)添加自己的额外配置
1， 获取子类具体的annotationType。Class annType = GenericTypeResolver.resolveTypeArgument(getClass(), AdviceModeImportSelector.class);

getClass： AsyncConfigurationSelector
annType：EnableAsync

ProxyAsyncConfiguration

@Configuration(proxyBeanMethods = false)
@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;
	}

}


@Configuration(proxyBeanMethods = false)
public abstract class AbstractAsyncConfiguration implements ImportAware {

	@Nullable
	protected AnnotationAttributes enableAsync;

	@Nullable
	protected Supplier<Executor> executor;

	@Nullable
	protected Supplier<AsyncUncaughtExceptionHandler> exceptionHandler;


	@Override
	public void setImportMetadata(AnnotationMetadata importMetadata) {
		this.enableAsync = AnnotationAttributes.fromMap(
				importMetadata.getAnnotationAttributes(EnableAsync.class.getName()));
		if (this.enableAsync == null) {
			throw new IllegalArgumentException(
					"@EnableAsync is not present on importing class " + importMetadata.getClassName());
		}
	}

	/**
	 * Collect any {@link AsyncConfigurer} beans through autowiring.
	 */
	@Autowired
	void setConfigurers(ObjectProvider<AsyncConfigurer> configurers) {
		Supplier<AsyncConfigurer> configurer = SingletonSupplier.of(() -> {
			List<AsyncConfigurer> candidates = configurers.stream().collect(Collectors.toList());
			if (CollectionUtils.isEmpty(candidates)) {
				return null;
			}
			if (candidates.size() > 1) {
				throw new IllegalStateException("Only one AsyncConfigurer may exist");
			}
			return candidates.get(0);
		});
		this.executor = adapt(configurer, AsyncConfigurer::getAsyncExecutor);
		this.exceptionHandler = adapt(configurer, AsyncConfigurer::getAsyncUncaughtExceptionHandler);
	}

	private <T> Supplier<T> adapt(Supplier<AsyncConfigurer> supplier, Function<AsyncConfigurer, T> provider) {
		return () -> {
			AsyncConfigurer configurer = supplier.get();
			return (configurer != null ? provider.apply(configurer) : null);
		};
	}

}

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就是实例化一个AsyncAnnotationBeanPostProcessor并且设置配置值

AsyncAnnotationAdvisor

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

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

	/**
	 * 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) {
			Pointcut cpc = new AnnotationMatchingPointcut(asyncAnnotationType, true);
			Pointcut mpc = new AnnotationMatchingPointcut(null, asyncAnnotationType, true);
			if (result == null) {
				result = new ComposablePointcut(cpc);
			}
			else {
				result.union(cpc);
			}
			result = result.union(mpc);
		}
		return (result != null ? result : Pointcut.TRUE);
	}

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主要就是advice和Pointcut 满足了PointcutAdvisor和Advisor接口，那么advisor又是在哪被调用的呢？在AsyncAnnotationBeanPostProcessor的继承树里面

org.springframework.aop.framework.AbstractAdvisingBeanPostProcessor#postProcessAfterInitialization

	@Override
	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);

			// Use original ClassLoader if bean class not locally loaded in overriding class loader
			ClassLoader classLoader = getProxyClassLoader();
			if (classLoader instanceof SmartClassLoader && classLoader != bean.getClass().getClassLoader()) {
				classLoader = ((SmartClassLoader) classLoader).getOriginalClassLoader();
			}
			return proxyFactory.getProxy(classLoader);
		}

		// No proxy needed.
		return bean;
	}
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那么pointCut又是什么时候用到的呢？根据常理，pointCut会决定这个bean是否需被proxy。

在org.springframework.aop.framework.AbstractAdvisingBeanPostProcessor#isEligible(java.lang.Class)

	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;
	}
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在org.springframework.aop.support.AopUtils#canApply(org.springframework.aop.Advisor, java.lang.Class, boolean)

	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;
		}
	}
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可以看到根据PointcutAdvisor来决定是否应该创建proxy。

至此，advice和Pointcut 的作用就都分析到了。

AnnotationAsyncExecutionInterceptor

	@Override
	@Nullable
	protected String getExecutorQualifier(Method method) {
		// Maintainer's note: changes made here should also be made in
		// AnnotationAsyncExecutionAspect#getExecutorQualifier
		Async async = AnnotatedElementUtils.findMergedAnnotation(method, Async.class);
		if (async == null) {
			async = AnnotatedElementUtils.findMergedAnnotation(method.getDeclaringClass(), Async.class);
		}
		return (async != null ? async.value() : null);
	}
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比起AsyncExecutionInterceptor，他就提供了一个功能：根据Async的value获取特定的executor。

AsyncExecutionInterceptor

public class AsyncExecutionInterceptor extends AsyncExecutionAspectSupport implements MethodInterceptor, Ordered {

	/**
	 * Create a new instance with a default {@link AsyncUncaughtExceptionHandler}.
	 * @param defaultExecutor the {@link Executor} (typically a Spring {@link AsyncTaskExecutor}
	 * or {@link java.util.concurrent.ExecutorService}) to delegate to;
	 * as of 4.2.6, a local executor for this interceptor will be built otherwise
	 */
	public AsyncExecutionInterceptor(@Nullable Executor defaultExecutor) {
		super(defaultExecutor);
	}

	/**
	 * Create a new {@code AsyncExecutionInterceptor}.
	 * @param defaultExecutor the {@link Executor} (typically a Spring {@link AsyncTaskExecutor}
	 * or {@link java.util.concurrent.ExecutorService}) to delegate to;
	 * as of 4.2.6, a local executor for this interceptor will be built otherwise
	 * @param exceptionHandler the {@link AsyncUncaughtExceptionHandler} to use
	 */
	public AsyncExecutionInterceptor(@Nullable Executor defaultExecutor, AsyncUncaughtExceptionHandler exceptionHandler) {
		super(defaultExecutor, exceptionHandler);
	}


	/**
	 * 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);

		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());
	}

	/**
	 * This implementation is a no-op for compatibility in Spring 3.1.2.
	 * Subclasses may override to provide support for extracting qualifier information,
	 * e.g. via an annotation on the given method.
	 * @return always {@code null}
	 * @since 3.1.2
	 * @see #determineAsyncExecutor(Method)
	 */
	@Override
	@Nullable
	protected String getExecutorQualifier(Method method) {
		return null;
	}

	/**
	 * This implementation searches for a unique {@link org.springframework.core.task.TaskExecutor}
	 * bean in the context, or for an {@link Executor} bean named "taskExecutor" otherwise.
	 * If neither of the two is resolvable (e.g. if no {@code BeanFactory} was configured at all),
	 * this implementation falls back to a newly created {@link SimpleAsyncTaskExecutor} instance
	 * for local use if no default could be found.
	 * @see #DEFAULT_TASK_EXECUTOR_BEAN_NAME
	 */
	@Override
	@Nullable
	protected Executor getDefaultExecutor(@Nullable BeanFactory beanFactory) {
		Executor defaultExecutor = super.getDefaultExecutor(beanFactory);
		return (defaultExecutor != null ? defaultExecutor : new SimpleAsyncTaskExecutor());
	}

	@Override
	public int getOrder() {
		return Ordered.HIGHEST_PRECEDENCE;
	}

}

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1, 提供默认的SimpleAsyncTaskExecutor
2，最高的优先级

determineAsyncExecutor

	@Nullable
	protected AsyncTaskExecutor determineAsyncExecutor(Method method) {
		AsyncTaskExecutor executor = this.executors.get(method);
		if (executor == null) {
			Executor targetExecutor;
			String qualifier = getExecutorQualifier(method);
			if (StringUtils.hasLength(qualifier)) {
				targetExecutor = findQualifiedExecutor(this.beanFactory, qualifier);
			}
			else {
				targetExecutor = this.defaultExecutor.get();
			}
			if (targetExecutor == null) {
				return null;
			}
			executor = (targetExecutor instanceof AsyncListenableTaskExecutor ?
					(AsyncListenableTaskExecutor) targetExecutor : new TaskExecutorAdapter(targetExecutor));
			this.executors.put(method, executor);
		}
		return executor;
	}

	@Nullable
	protected Executor getDefaultExecutor(@Nullable BeanFactory beanFactory) {
		if (beanFactory != null) {
			try {
				// Search for TaskExecutor bean... not plain Executor since that would
				// match with ScheduledExecutorService as well, which is unusable for
				// our purposes here. TaskExecutor is more clearly designed for it.
				return beanFactory.getBean(TaskExecutor.class);
			}
			catch (NoUniqueBeanDefinitionException ex) {
				logger.debug("Could not find unique TaskExecutor bean. " +
						"Continuing search for an Executor bean named 'taskExecutor'", ex);
				try {
					return beanFactory.getBean(DEFAULT_TASK_EXECUTOR_BEAN_NAME, Executor.class);
				}
				catch (NoSuchBeanDefinitionException ex2) {
					if (logger.isInfoEnabled()) {
						logger.info("More than one TaskExecutor bean found within the context, and none is named " +
								"'taskExecutor'. Mark one of them as primary or name it 'taskExecutor' (possibly " +
								"as an alias) in order to use it for async processing: " + ex.getBeanNamesFound());
					}
				}
			}
			catch (NoSuchBeanDefinitionException ex) {
				logger.debug("Could not find default TaskExecutor bean. " +
						"Continuing search for an Executor bean named 'taskExecutor'", ex);
				try {
					return beanFactory.getBean(DEFAULT_TASK_EXECUTOR_BEAN_NAME, Executor.class);
				}
				catch (NoSuchBeanDefinitionException ex2) {
					logger.info("No task executor bean found for async processing: " +
							"no bean of type TaskExecutor and no bean named 'taskExecutor' either");
				}
				// Giving up -> either using local default executor or none at all...
			}
		}
		return null;
	}
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由此我们可以看出executor的顺序：
1， findQualifiedExecutor，
2，return beanFactory.getBean(TaskExecutor.class);
3，return beanFactory.getBean(DEFAULT_TASK_EXECUTOR_BEAN_NAME, Executor.class);
4，return (defaultExecutor != null ? defaultExecutor : new SimpleAsyncTaskExecutor());

exceptionUncaughtHandler就比较简单， this.exceptionHandler = new SingletonSupplier<>(exceptionHandler, SimpleAsyncUncaughtExceptionHandler::new);
没有配置就直接SimpleAsyncUncaughtExceptionHandler。