一、例子

我们先来看下面这个Demo。

pom.xml中maven依赖：

    	
     
      org.springframework.boot
     	
     
      spring-boot-starter-parent
     	
     
      1.5.14.RELEASE
     	
      
     
    
    	
     		
      
       org.springframework.boot
      		
      
       spring-boot-starter-web
      	
     	
     		
      
       org.projectlombok
      		
      
       lombok
      		
      
       1.16.18
      	
     	
     		
      
       org.springframework.boot
      		
      
       spring-boot-starter-test
      		
      
       test
      	
     
    

启动类SpringBootAsyncApplication.java

@SpringBootApplication@EnableAsyncpublic class SpringBootAsyncApplication {    public static void main(String[] args) {        SpringApplication.run(SpringBootAsyncApplication.class, args);    }}

DemoController.java

@RestController@RequestMapping(value = "/demos")@Slf4jpublic class DemoController {    @Autowired    private IDemoService demoService;    @GetMapping("")    public String test(@RequestParam String name) {        long start = System.currentTimeMillis();        log.info("start send. ThreadName: {}", Thread.currentThread().getName());        demoService.send(name);        long end = System.currentTimeMillis();        log.info("send end, time: {}", (end - start));        return "success";    }}

IDemoService接口实现类DemoServiceImpl.java

@Service@Slf4jpublic class DemoServiceImpl implements IDemoService {    @Async    @Override    public void send(String name) {        try {            Thread.sleep(5000);        } catch (InterruptedException e) {            e.printStackTrace();        }        log.info("async name={}, ThreadName: {}", name, Thread.currentThread().getName());    }}

启动项目后，访问GET http://127.0.0.1:8002/demos?name=test，得到如下结果：

2019-04-12 17:17:32.462 INFO 12 --- [nio-8002-exec-1] c.l.d.s.controller.DemoController        : start send. ThreadName: http-nio-8002-exec-12019-04-12 17:17:32.466 INFO 12 --- [nio-8002-exec-1] .s.a.AnnotationAsyncExecutionInterceptor : No task executor bean found for async processing: no bean of type TaskExecutor and no bean named 'taskExecutor' either2019-04-12 17:17:32.467 INFO 12 --- [nio-8002-exec-1] c.l.d.s.controller.DemoController        : send end, time: 52019-04-12 17:17:37.468 INFO 12 --- [cTaskExecutor-1] c.l.d.s.service.impl.DemoServiceImpl     : async name=test, ThreadName: SimpleAsyncTaskExecutor-1

通过控制台日志打印，我们可以看到有两个线程，一个是主线程，一个是异步的线程。没等异步的线程执行完，主线程就直接执行完毕，返回响应结果了，这就是异步的效果。

二、结论

2.1 实现异步方式

• 开启异步配置，即在启动类或者配置类上加@EnableAsync注解；
• 在方法或类上加@Async注解。

2.2 @Async注解

• 用@Async注解的方法，将使它在一个单独的线程（例子中SimpleAsyncTaskExecutor-1线程）中执行，调用者不用等待被调用方法完成。
• 用@Async注解的方法，必须只应用于public方法上（只有public修饰的方法才能被进行代理）。
• @Async注解不能自调用，即在同一个类中调用异步方法，否则不起作用（同一个类中调用方法的话会略过代理进行直接调用）。

三、疑问

为什么会抛出下面这段日志？

[nio-8002-exec-1] .s.a.AnnotationAsyncExecutionInterceptor : No task executor bean found for async processing: no bean of type TaskExecutor and no bean named 'taskExecutor' either

没有发现用于异步处理的任务执行器，既没有TaskExecutor类型的bean，也没有名为“taskExecutor”的bean。什么意思？而且异步开启的线程为啥前缀是SimpleAsyncTaskExecutor？带着这些疑问，我们深入到源码中，看看到底发生了什么。

四、源码解析

spring-context-4.3.18.RELEASE.jar!org.springframework.scheduling.annotation.Async.java

/** * Annotation that marks a method as a candidate for asynchronous execution. * Can also be used at the type level, in which case all of the type's methods are * considered as asynchronous. * * 
In terms of target method signatures, any parameter types are supported. * However, the return type is constrained to either {@code void} or * {@link java.util.concurrent.Future}. In the latter case, you may declare the * more specific {@link org.springframework.util.concurrent.ListenableFuture} or * {@link java.util.concurrent.CompletableFuture} types which allow for richer * interaction with the asynchronous task and for immediate composition with * further processing steps. * * 
A {@code Future} handle returned from the proxy will be an actual asynchronous * {@code Future} that can be used to track the result of the asynchronous method * execution. However, since the target method needs to implement the same signature, * it will have to return a temporary {@code Future} handle that just passes a value * through: e.g. Spring's {@link AsyncResult}, EJB 3.1's {@link javax.ejb.AsyncResult}, * or {@link java.util.concurrent.CompletableFuture#completedFuture(Object)}. * * @author Juergen Hoeller * @author Chris Beams * @since 3.0 * @see AnnotationAsyncExecutionInterceptor * @see AsyncAnnotationAdvisor */@Target({ElementType.METHOD, ElementType.TYPE})@Retention(RetentionPolicy.RUNTIME)@Documentedpublic @interface Async {	/**     * 指定异步操作的限定符值	 * A qualifier value for the specified asynchronous operation(s).	 * 
May be used to determine the target executor to be used when executing this	 * method, matching the qualifier value (or the bean name) of a specific	 * {@link java.util.concurrent.Executor Executor} or	 * {@link org.springframework.core.task.TaskExecutor TaskExecutor}	 * bean definition.	 * 
When specified on a class level {@code @Async} annotation, indicates that the	 * given executor should be used for all methods within the class. Method level use	 * of {@code Async#value} always overrides any value set at the class level.	 * @since 3.1.2	 */	String value() default "";}

spring-context-4.3.18.RELEASE.jar!org.springframework.scheduling.annotation.EnableAsync.java

package org.springframework.scheduling.annotation;import java.lang.annotation.Annotation;import java.lang.annotation.Documented;import java.lang.annotation.ElementType;import java.lang.annotation.Retention;import java.lang.annotation.RetentionPolicy;import java.lang.annotation.Target;import org.springframework.context.annotation.AdviceMode;import org.springframework.context.annotation.Configuration;import org.springframework.context.annotation.Import;import org.springframework.core.Ordered;/** * Enables Spring's asynchronous method execution capability, similar to functionality * found in Spring's {@code 
    
     } XML namespace. * * 
     
To be used together with @{@link Configuration Configuration} classes as follows, * enabling annotation-driven async processing for an entire Spring application context: * * 
     
 * @Configuration * @EnableAsync * public class AppConfig { * * }
 * * {@code MyAsyncBean} is a user-defined type with one or more methods annotated with * either Spring's {@code @Async} annotation, the EJB 3.1 {@code @javax.ejb.Asynchronous} * annotation, or any custom annotation specified via the {@link #annotation} attribute. * The aspect is added transparently for any registered bean, for instance via this * configuration: * * 
     
 * @Configuration * public class AnotherAppConfig { * *     @Bean *     public MyAsyncBean asyncBean() { *         return new MyAsyncBean(); *     } * }
 * * 
     
By default, Spring will be searching for an associated thread pool definition: * either a unique {@link org.springframework.core.task.TaskExecutor} bean in the context, * or an {@link java.util.concurrent.Executor} bean named "taskExecutor" otherwise. If * neither of the two is resolvable, a {@link org.springframework.core.task.SimpleAsyncTaskExecutor} * will be used to process async method invocations. Besides, annotated methods having a * {@code void} return type cannot transmit any exception back to the caller. By default, * such uncaught exceptions are only logged. * * 
     
To customize all this, implement {@link AsyncConfigurer} and provide: * 
     
       * 
      
your own {@link java.util.concurrent.Executor Executor} through the * {@link AsyncConfigurer#getAsyncExecutor getAsyncExecutor()} method, and
 * 
      
your own {@link org.springframework.aop.interceptor.AsyncUncaughtExceptionHandler * AsyncUncaughtExceptionHandler} through the {@link AsyncConfigurer#getAsyncUncaughtExceptionHandler * getAsyncUncaughtExceptionHandler()} * method.
 * 
     
 * * 
     
 * @Configuration * @EnableAsync * public class AppConfig implements AsyncConfigurer { * *     @Override *     public Executor getAsyncExecutor() { *         ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor(); *         executor.setCorePoolSize(7); *         executor.setMaxPoolSize(42); *         executor.setQueueCapacity(11); *         executor.setThreadNamePrefix("MyExecutor-"); *         executor.initialize(); *         return executor; *     } * *     @Override *     public AsyncUncaughtExceptionHandler getAsyncUncaughtExceptionHandler() { *         return MyAsyncUncaughtExceptionHandler(); *     } * }
 * * 
     
If only one item needs to be customized, {@code null} can be returned to * keep the default settings. Consider also extending from {@link AsyncConfigurerSupport} * when possible. * * 
     
Note: In the above example the {@code ThreadPoolTaskExecutor} is not a fully managed * Spring bean. Add the {@code @Bean} annotation to the {@code getAsyncExecutor()} method * if you want a fully managed bean. In such circumstances it is no longer necessary to * manually call the {@code executor.initialize()} method as this will be invoked * automatically when the bean is initialized. * * 
     
For reference, the example above can be compared to the following Spring XML * configuration: * * 
     
 * {@code * 
      
        * *     
        * *     
        * *     
        * *     
        * * 
       * }
 * * The above XML-based and JavaConfig-based examples are equivalent except for the * setting of the 
     thread name prefix of the {@code Executor}; this is because * the {@code 
     
      } element does not expose such an attribute. This * demonstrates how the JavaConfig-based approach allows for maximum configurability * through direct access to actual componentry. * * 
      
The {@link #mode} attribute controls how advice is applied: If the mode is * {@link AdviceMode#PROXY} (the default), then the other attributes control the behavior * of the proxying. 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. * * 
      
Note that if the {@linkplain #mode} is set to {@link AdviceMode#ASPECTJ}, then the * value of the {@link #proxyTargetClass} attribute will be ignored. Note also that in * this case the {@code spring-aspects} module JAR must be present on the classpath, with * compile-time weaving or load-time weaving applying the aspect to the affected classes. * There is no proxy involved in such a scenario; local calls will be intercepted as well. * * @author Chris Beams * @author Juergen Hoeller * @author Stephane Nicoll * @author Sam Brannen * @since 3.1 * @see Async * @see AsyncConfigurer * @see AsyncConfigurationSelector */@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
        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;}
     
    

@EnableAsync启用Spring异步方法执行功能，类似于Spring的<task:*>XML命名空间，和@Configuration注解类一起使用，为整个Spring应用程序上下文启用注释驱动的异步处理。

默认情况下，Spring将会搜索一个关联的线程池定义：要么是一个在上下文中唯一的TaskExecutor类型的bean，要么是一个名叫"taskExecutor"的Executor类型的bean；如果两者都无法解决，SimpleAsyncTaskExecutor将用于处理异步方法调用。此外，具有void返回类型的带注释的方法不能将任何异常传回调用者。默认情况下，此类未捕获异常只会被记录下来。

为了定制所有这些，需要实现AsyncConfigurer类，并重写AsyncConfigurer类中getAsyncExecutor()方法和getAsyncUncaughtExceptionHandler()方法。

@Configuration@EnableAsyncpublic class AppConfig implements AsyncConfigurer {    @Override    public Executor getAsyncExecutor() {        ThreadPoolTaskExecutor executor= new ThreadPoolTaskExecutor();        executor.setCorePoolSize(7);        executor.setMaxPoolSize(42);        executor.setQueueCapacity(11);        executor.setThreadNamePrefix("MyExecutor-");        executor.initialize();        return executor;    }    @Override    public AsyncUncaughtExceptionHandler getAsyncUncaughtExceptionHandler() {        return MyAsyncUncaughtExceptionHandler();    }}

如果只需要定制一个项目，则可以返回null以保持默认设置，如果可能的话还可以考虑从AsyncConfigurerSupport扩展。

注意：在上面的示例中，ThreadPoolTaskExecutor不是一个完全受管理Spring bean。如果你想要一个完全受管理的bean，可以将@Bean注解添加到getAsyncExecutor()方法上；在这种情况下，就没有必要再手动调用executor.initialize()方法进行bean初始化了。

@Bean@Overridepublic Executor getAsyncExecutor() {	ThreadPoolTaskExecutor executor= new ThreadPoolTaskExecutor();	executor.setCorePoolSize(7);	executor.setMaxPoolSize(42);	executor.setQueueCapacity(11);	executor.setThreadNamePrefix("MyExecutor-");	return executor;}

为了便于参考，可以将上面的示例与下面的Spring XML配置进行比较：

上述基于xml和基于Java配置的示例除了设置执行器的线程名前缀外，其余都是等价的，这是因为<task:executor>元素没有暴露这样的属性。这演示了基于Java配置的方法如何通过直接访问实际组件来实现最大的可配置性。

#mode属性控制应用如何被通知：如果mode是AdviceMode#PROXY（默认值），那么其他属性控制代理的行为。请注意，代理模式只允许拦截通过代理进行的调用，同一类内的本地调用不能以这种方式被拦截。

注意，如果#mode被设置为AdviceMode#ASPECTJ，那么#proxyTargetClass属性值将被忽略。还要注意，在这种情况下，spring-aspects模块JAR必须出现在类路径上，编译时或加载时应用切面到受影响的类上。在这种情况下不涉及代理，本地调用也会被拦截。

上面这些都是EnableAsync类注释，具体是如何实现的？且看下面。

有没有发现，EnableAsync类唯一的核心注解就是@Import(AsyncConfigurationSelector.class)，我们来看下它的源码：

spring-context-4.3.18.RELEASE.jar!org.springframework.scheduling.annotation.AsyncConfigurationSelector.java

/** * Selects which implementation of {@link AbstractAsyncConfiguration} should be used based * on the value of {@link EnableAsync#mode} on the importing {@code @Configuration} class. * * @author Chris Beams * @since 3.1 * @see EnableAsync * @see ProxyAsyncConfiguration */public class AsyncConfigurationSelector extends AdviceModeImportSelector
    
      {	private static final String ASYNC_EXECUTION_ASPECT_CONFIGURATION_CLASS_NAME =			"org.springframework.scheduling.aspectj.AspectJAsyncConfiguration";	/**	 * {@inheritDoc}	 * @return {@link ProxyAsyncConfiguration} or {@code AspectJAsyncConfiguration} for	 * {@code PROXY} and {@code ASPECTJ} values of {@link EnableAsync#mode()}, respectively	 */	@Override	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;		}	}}
    

根据导入@Configuration类上EnableAsync#mode的值选择AbstractAsyncConfiguration的哪个实现类应该被使用。

• 默认配置PROXY，使用ProxyAsyncConfiguration。

spring-context-4.3.18.RELEASE.jar!org.springframework.scheduling.annotation.ProxyAsyncConfiguration.java

/** * {@code @Configuration} class that registers the Spring infrastructure beans necessary * to enable proxy-based asynchronous method execution. * * @author Chris Beams * @author Stephane Nicoll * @since 3.1 * @see EnableAsync * @see AsyncConfigurationSelector */@Configuration@Role(BeanDefinition.ROLE_INFRASTRUCTURE)public class ProxyAsyncConfiguration extends AbstractAsyncConfiguration {    /**     * 定义了一个AsyncAnnotationBeanPostProcessor类bean     */	@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");        // 新建一个异步注解bean后处理器		AsyncAnnotationBeanPostProcessor bpp = new AsyncAnnotationBeanPostProcessor();        // 获取@EnableAsync中用户自定义annotation		Class
     customAsyncAnnotation = this.enableAsync.getClass("annotation");        // 如果不是默认注解，则设置异步注解配置		if (customAsyncAnnotation != AnnotationUtils.getDefaultValue(EnableAsync.class, "annotation")) {			bpp.setAsyncAnnotationType(customAsyncAnnotation);		}        // 设置线程任务执行器		if (this.executor != null) {			bpp.setExecutor(this.executor);		}        // 设置异常处理器		if (this.exceptionHandler != null) {			bpp.setExceptionHandler(this.exceptionHandler);		}        // 设置是否升级到CGLIB子类代理，默认不开启		bpp.setProxyTargetClass(this.enableAsync.getBoolean("proxyTargetClass"));        // 设置执行优先级，默认最后执行		bpp.setOrder(this.enableAsync.
    
     getNumber("order"));		return bpp;	}}
    

spring-context-4.3.18.RELEASE.jar!org.springframework.scheduling.annotation.AbstractAsyncConfiguration.java

/** * Abstract base {@code Configuration} class providing common structure for enabling * Spring's asynchronous method execution capability. * * @author Chris Beams * @author Stephane Nicoll * @since 3.1 * @see EnableAsync */@Configurationpublic abstract class AbstractAsyncConfiguration implements ImportAware {    // 注解属性	protected AnnotationAttributes enableAsync;    // 线程任务执行器	protected Executor executor;    // 异常处理器	protected AsyncUncaughtExceptionHandler exceptionHandler;	@Override	public void setImportMetadata(AnnotationMetadata importMetadata) {		this.enableAsync = AnnotationAttributes.fromMap(				importMetadata.getAnnotationAttributes(EnableAsync.class.getName(), false));		if (this.enableAsync == null) {			throw new IllegalArgumentException(					"@EnableAsync is not present on importing class " + importMetadata.getClassName());		}	}	/**	 * Collect any {@link AsyncConfigurer} beans through autowiring.	 */	@Autowired(required = false)	void setConfigurers(Collection
    
      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();	}}
    

可以看到接口org.springframework.scheduling.annotation.AsyncConfigurer的唯一实现类org.springframework.scheduling.annotation.AsyncConfigurerSupport：

/** * A convenience {@link AsyncConfigurer} that implements all methods * so that the defaults are used. Provides a backward compatible alternative * of implementing {@link AsyncConfigurer} directly. * * @author Stephane Nicoll * @since 4.1 */public class AsyncConfigurerSupport implements AsyncConfigurer {	@Override	public Executor getAsyncExecutor() {		return null;	}	@Override	public AsyncUncaughtExceptionHandler getAsyncUncaughtExceptionHandler() {		return null;	}}

这就是上面注释讲的，可以通过实现AsyncConfigurer接口实现默认线程池和异常处理的定制化。

回到ProxyAsyncConfiguration类，这是一个Configuration类，在其中通过@bean注入了AsyncAnnotationBeanPostProcessor 类。

AsyncAnnotationBeanPostProcessor这个类的Bean初始化时，重写了BeanFactoryAware接口setBeanFactory方法，对AsyncAnnotationAdvisor异步注解切面进行了构造。

@Overridepublic 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;}

它会在普通bean属性之后、初始化回调（如InitializingBean#afterPropertiesSet() 或者一个自定义初始化方法）之前被调用。

 

AsyncAnnotationBeanPostProcessor的后置bean处理是通过其父类AbstractAdvisingBeanPostProcessor来实现的，该类实现了BeanPostProcessor接口,重写postProcessAfterInitialization方法。

@Overridepublic Object postProcessAfterInitialization(Object bean, String beanName) {	if (bean instanceof AopInfrastructureBean) {		// Ignore AOP infrastructure such as scoped proxies.		return bean;	}    //把Advisor添加进bean  ProxyFactory-》AdvisedSupport-》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;		}	}       //构造ProxyFactory代理工厂，添加代理的接口，设置切面，最后返回代理类：AopProxy	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 async proxy needed.	return bean;}

JDK动态代理类JdkDynamicAopProxy实现AopProxy接口，最终执行的是InvocationHandler接口的invoke方法。

/** * Implementation of {@code InvocationHandler.invoke}. * 
Callers will see exactly the exception thrown by the target, * unless a hook method throws an exception. */@Overridepublic Object invoke(Object proxy, Method method, Object[] args) throws Throwable {	MethodInvocation invocation;	Object oldProxy = null;	boolean setProxyContext = false;	TargetSource targetSource = this.advised.targetSource;	Class
      targetClass = null;	Object target = null;	try {		if (!this.equalsDefined && AopUtils.isEqualsMethod(method)) {			// The target does not implement the equals(Object) method itself.			return equals(args[0]);		}		else if (!this.hashCodeDefined && AopUtils.isHashCodeMethod(method)) {			// The target does not implement the hashCode() method itself.			return hashCode();		}		else if (method.getDeclaringClass() == DecoratingProxy.class) {			// There is only getDecoratedClass() declared -> dispatch to proxy config.			return AopProxyUtils.ultimateTargetClass(this.advised);		}		else if (!this.advised.opaque && method.getDeclaringClass().isInterface() &&				method.getDeclaringClass().isAssignableFrom(Advised.class)) {			// Service invocations on ProxyConfig with the proxy config...			return AopUtils.invokeJoinpointUsingReflection(this.advised, method, args);		}		Object retVal;		if (this.advised.exposeProxy) {			// Make invocation available if necessary.			oldProxy = AopContext.setCurrentProxy(proxy);			setProxyContext = true;		}		// May be null. Get as late as possible to minimize the time we "own" the target,		// in case it comes from a pool.		target = targetSource.getTarget();		if (target != null) {			targetClass = target.getClass();		}		// Get the interception chain for this method.		List

@Async注解的拦截器是AsyncExecutionInterceptor，它继承了MethodInterceptor接口。而MethodInterceptor就是AOP规范中的Advice（切点的处理器）。chain不为空，执行第二个分支，构造ReflectiveMethodInvocation，然后执行proceed方法。

@Overridepublic Object proceed() throws Throwable {	//	We start with an index of -1 and increment early.	if (this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size() - 1) {		return invokeJoinpoint();	}	Object interceptorOrInterceptionAdvice =			this.interceptorsAndDynamicMethodMatchers.get(++this.currentInterceptorIndex);	if (interceptorOrInterceptionAdvice instanceof InterceptorAndDynamicMethodMatcher) {		// Evaluate dynamic method matcher here: static part will already have		// been evaluated and found to match.		InterceptorAndDynamicMethodMatcher dm =				(InterceptorAndDynamicMethodMatcher) interceptorOrInterceptionAdvice;		if (dm.methodMatcher.matches(this.method, this.targetClass, this.arguments)) {			return dm.interceptor.invoke(this);		}		else {			// Dynamic matching failed.			// Skip this interceptor and invoke the next in the chain.			return proceed();		}	}	else {		// It's an interceptor, so we just invoke it: The pointcut will have		// been evaluated statically before this object was constructed.		return ((MethodInterceptor) interceptorOrInterceptionAdvice).invoke(this);	}}

核心方法是InterceptorAndDynamicMethodMatcher.interceptor.invoke(this)，实际就是执行了AsyncExecutionInterceptor.invoke。

/** * 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. */@Overridepublic 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

spring-aop-4.3.18.RELEASE-sources.jar!/org/springframework/aop/interceptor/AsyncExecutionAspectSupport.java

/** * Determine the specific executor to use when executing the given method. * Should preferably return an {@link AsyncListenableTaskExecutor} implementation. * @return the executor to use (or {@code null}, but just if no default executor is available) */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;			if (targetExecutor == null) {				synchronized (this.executors) {					if (this.defaultExecutor == null) {						this.defaultExecutor = getDefaultExecutor(this.beanFactory);					}					targetExecutor = this.defaultExecutor;				}			}		}		if (targetExecutor == null) {			return null;		}		executor = (targetExecutor instanceof AsyncListenableTaskExecutor ?				(AsyncListenableTaskExecutor) targetExecutor : new TaskExecutorAdapter(targetExecutor));		this.executors.put(method, executor);	}	return executor;}

@Aync注解有个value可以标注使用哪个executor，这里的getExecutorQualifier就是寻找这个标识。这里如果defaultExecutor为null的话，则获取找默认的executor。

/** * Retrieve or build a default executor for this advice instance. * An executor returned from here will be cached for further use. * 
The default implementation searches for a unique {@link TaskExecutor} bean * in the context, or for an {@link Executor} bean named "taskExecutor" otherwise. * If neither of the two is resolvable, this implementation will return {@code null}. * @param beanFactory the BeanFactory to use for a default executor lookup * @return the default executor, or {@code null} if none available * @since 4.2.6 * @see #findQualifiedExecutor(BeanFactory, String) * @see #DEFAULT_TASK_EXECUTOR_BEAN_NAME */protected Executor getDefaultExecutor(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", 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", 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;}

如果工程里头没有定义默认的task executor的话，则获取bean的时候会抛出NoSuchBeanDefinitionException。这就是为什么上面例子中会抛出如下错误的原因。

[nio-8002-exec-1] .s.a.AnnotationAsyncExecutionInterceptor : No task executor bean found for async processing: no bean of type TaskExecutor and no bean named 'taskExecutor' either

spring-aop-4.3.18.RELEASE-sources.jar!/org/springframework/aop/interceptor/AsyncExecutionInterceptor.java

/** * 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 */@Overrideprotected Executor getDefaultExecutor(BeanFactory beanFactory) {	Executor defaultExecutor = super.getDefaultExecutor(beanFactory);	return (defaultExecutor != null ? defaultExecutor : new SimpleAsyncTaskExecutor());}

AsyncExecutionInterceptor重写了getDefaultExecutor方法，先调用AsyncExecutionAspectSupport的getDefaultExecutor，如果默认的找不到，这里new一个SimpleAsyncTaskExecutor。这也是为什么上面例子中出现SimpleAsyncTaskExecutor线程前缀的原因。

五、总结

整体流程大体可梳理为两条线：

1.从注解开始：@EnableAsync--》ProxyAsyncConfiguration类构造一个bean（类型：AsyncAnnotationBeanPostProcessor）

2.从AsyncAnnotationBeanPostProcessor这个类的bean的生命周期走：AOP-Advisor切面初始化（setBeanFactory()）--》AOP-生成代理类AopProxy（postProcessAfterInitialization()）--》AOP-切点执行（InvocationHandler.invoke）

 

参考

https://segmentfault.com/a/1190000011339882

https://blog.csdn.net/qq_39470742/article/details/83382338

https://blog.csdn.net/fenglongmiao/article/details/82429460

https://www.baeldung.com/spring-async