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Java Spring之@Async原理

liangsheng_g 人气:0

前言

用过Spring的人多多少少也都用过@Async注解,至于作用嘛,看注解名,大概能猜出来,就是在方法执行的时候进行异步执行。

一、如何使用@Async

使用@Async注解主要分两步:

1.在配置类上添加@EnableAsync注解

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

2.在想要异步执行的方法上面加上@Async

@Service
public class CycleService2 {

	@Autowired
	private CycleService1 cycleService1;

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

}

二、源码解读

1.@EnableAsync的作用

@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;

}

2. AsyncConfigurationSelector的作用

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;
		}
	}

}

看过我之前博客的同学应该知道,其实此处就是往Spring容器中增加一个新的需要扫描的类,很明显可以看到差别主要集中在adviceMode的差别上。

3. adviceMode:PROXY(默认值)

引入了ProxyAsyncConfiguration配置类

3.1 ProxyAsyncConfiguration

@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;
	}

}

作用也很明显,就是往spring容器中添加了AsyncAnnotationBeanPostProcessor类

3.2 AsyncAnnotationBeanPostProcessor

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;
	}

}

其实可以看到最重要的方法,就是setBeanFactory了,该方法是在AsyncAnnotationBeanPostProcessor的生命周期最后一步initializeBean里面的第一小步,也就是执行所有Aware接口的时候执行。
对于AOP来说,其实最主要的就是advice+pointcut,也就是advisor,在生命周期的这一步,也创建了advisor。

3.3 AsyncAnnotationAdvisor

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);
	}

可以看到最主要的工作就是buildAdvice和buildPointcut。advice的作用是定义在方法执行方面,该如何执行;pointcut的作用是定义方法的范围

3.3.1 buildAdvice

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;
	}

可以看到advice主要就是定义了一个烂机器interceptor,在方法执行的时候进行一些拦截,至于executor,是方法执行器,默认为null,exceptionHandler也默认是null。

3.3.1.1 AnnotationAsyncExecutionInterceptor,异步执行的原理

在AnnotationAsyncExecutionInterceptor的父类AsyncExecutionInterceptor中,实现了拦截器的接口方法invoke,也就是真实的方法执行逻辑。

/**
	 * 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());
	}

可以看到主要做的事情是:

  1. 寻找任务执行器:
  2. 从@Async注解里面获取配置的任务执行器
  3. 从Spring容器中找TaskExecutor类的bean
  4. 从spring容器中获取名为"taskExecutor"的bean,
  5. 如果还没有,new SimpleAsyncTaskExecutor())可以看到其实我们是可以给@Async进行任务执行器的配置的。
  6. 将具体的方法封装成callable的对象,然后doSubmit
  7. 此处我们就看一下默认的doSumit,使用的SimpleAsyncTaskExecutor是如何实现的
  8. 最终会执行到下面这个doExecute方法,默认情况下threadFactory是null,所以默认情况下,我们的方法,每次都是被创建了一个新的守护线程来进行方法的执行。
	protected void doExecute(Runnable task) {
		Thread thread = (this.threadFactory != null ? this.threadFactory.newThread(task) : createThread(task));
		thread.start();
	}

3.3.1.2 自定义任务执行器

  1. 可以在配置类里new SimpleAsyncTaskExecutor(),然后setThreadFactory,这样修改了默认线程的产生方式
  2. 比较主流的方式是,定义一个ThreadPoolTaskExecutor,也就是线程池任务执行器,可以进行线程复用

3.3.2 buildPointcut

/**
	 * 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);
	}

主要方法就是定义了一个类匹配pointcut和一个方法匹配pointcut。

4 什么时候判断进行advice的添加呢

当然就是在对某个bean进行proxy的判断的时候,也就是bean的生命周期最后一步,也是initializeBean里最后的一步,对于BeanPostProcessor的执行

3.4.1 AsyncAnnotationBeanPostProcessor#postProcessAfterInitialization

要注意的是AsyncAnnotationBeanPostProcessor的postProcessAfterInitialization方法其实是继承的是父类AbstractAdvisingBeanPostProcessor的。

@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;
	}

而在isEligible中,就是判断当前执行生命周期的bean是否满足我们的@Async注解的使用范围,主要是通过其class来判断

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;
	}

具体的AopUtils.canApply(this.advisor, targetClass)逻辑就不写了,就是根据pointcut里设置的classFilter和methodMatcher类判断当前bean的class是否需要进行该advisor的使用。

总结

发现@Async注解还是挺麻烦的,特别是要写一篇简单易懂的博客,更难。
默认配置实现原理:在执行的时候将method最终封装成一个Runable对象,然后new一个线程,通过线程的start方法,进行method的执行,来实现异步。

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