Java Spring之@Async原理

@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.
	 * 此处说明的是方法执行变成异步，扫描的是哪个注解，目前默认的是Async和Asynchronous，开发者也可以自定义
	 */
	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被设置成true，那么spring的所有proxy都会通过CGLIB方式实现，不再使用Java默认的基于接口的代理实现方式；而且此处如果设置，不仅仅是会影响添加了@Async注解的类的proxy方式，加了@Transactional的类也会变成CGLIB代理，不推荐修改；这个注解只有mode是默认的PROXY，才有意义
	 */
	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，默认的是使用Spring的proxy方式，也可以换成原生的AspectJ的proxy方式。
	 * 这两个的区别作用还是很明显的
	 */
	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.
	 * 因为在beanPostProcessor执行的时候，会根据order值进行排序，此处设置为最低值，就是想让其最后执行
	 * 其实即使不设置这个值，因为AsyncAnnotationBeanPostProcessor继承了ProxyProcessorSupport，ProxyProcessorSupport中的order默认也是最小优先级
	 * 
	 */
	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 {

    // 删除了一些无关紧要，或者默认不会设置的属性
	public AsyncAnnotationBeanPostProcessor() {
		setBeforeExistingAdvisors(true);
	}

	/**
	 * 因为AsyncAnnotationBeanPostProcessor实现了BeanFactoryAware接口
	 * 所以在实例化的过程中执行到initializeBean步骤的时候，里面第一步就是执行各种实现了Aware接口的接口方法
	 * 在此处new了一个advisor。advisor简单理解就是：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);
		/**
		 * 这儿设置符合pointCut需要的注解
		 * 此处的executor就是一个扩展点，如果不想用spring的默认单线程线程池，可以自定义一个线程池
		 * exceptionHandler，顾名思义，就是我们的方法在线程池中执行时抛出exception该如何handle使用的
		 * advice也就是咱们的interceptor
		 * pointCut就不多解释了，就是把设置符合什么条件会进行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了一个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);
		/**获取一个任务执行器
		 * 1. 从@Async注解里面获取配置的任务执行器
		 * 2. 从Spring容器中找TaskExecutor类的bean
		 * 3. 从spring容器中获取名为"taskExecutor"的bean，
		 * 4. 如果还没有，new SimpleAsyncTaskExecutor()）
		 */
		AsyncTaskExecutor executor = determineAsyncExecutor(userDeclaredMethod);
		if (executor == null) {
			throw new IllegalStateException(
					"No executor specified and no default executor set on AsyncExecutionInterceptor either");
		}

		//将当前方法执行封装成一个callable对象，然后放入到线程池里
		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;
		};

		//任务提交
		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) {
			// 就是根据这两个匹配器进行匹配的
			// 检查类上是否有@Async注解
			Pointcut cpc = new AnnotationMatchingPointcut(asyncAnnotationType, true);
			//检查方法上是否有@Async注解
			Pointcut mpc = new AnnotationMatchingPointcut(null, asyncAnnotationType, true);
			if (result == null) {
				result = new ComposablePointcut(cpc);
			}
			else {
			// 取并集：类上加了@Async或者类的方法上加了@Async
				result.union(cpc);
			}
			result = result.union(mpc);
		}
		return (result != null ? result : Pointcut.TRUE);
	}

@Override
	public Object postProcessAfterInitialization(Object bean, String beanName) {
		// 没有通知，或者是AOP的基础设施类，那么不进行代理
		if (this.advisor == null || bean instanceof AopInfrastructureBean) {
			// Ignore AOP infrastructure such as scoped proxies.
			return bean;
		}

		// 对已经被代理的类，不再生成代理，只是将通知添加到代理类的逻辑中
		// 这里通过beforeExistingAdvisors决定是将通知添加到所有通知之前还是添加到所有通知之后
		// 在使用@Async注解的时候，beforeExistingAdvisors被设置成了true，
		// @Async注解之所以把beforeExistingAdvisors设置为true，是因为该advisor和其他的advisor差别太大了，从情理上讲，也应该第一个执行
		// 意味着整个方法及其拦截逻辑都会异步执行
		if (bean instanceof Advised) {
			Advised advised = (Advised) bean;
			// 判断bean是否符合该advisor的使用范围，通过pointcut来判断
			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;
			}
		}

		// 如果还不是一个代理类，也需要通过eligible来判断是否符合使用该advisor的条件
		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;
		}
		// 其实就是判断类是否可以进行添加该advisor，也就是判断是否符合该advisor的使用条件
		// 就是把advisor的pointCut拿出来，pointCut里的classMatcher和methodMatcher拿出来对类及其方法进行判断
		eligible = AopUtils.canApply(this.advisor, targetClass);
		this.eligibleBeans.put(targetClass, eligible);
		return eligible;
	}