【源码解析】实现异步功能的注解 @Async 的源码解析

使用方式

启动类上添加注解@EnableAsync()
在方法或者类上添加@Async

源码解析

初始化配置

@EnableAsync注入了AsyncConfigurationSelector

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Import(AsyncConfigurationSelector.class)
public @interface EnableAsync {

	Class<? extends Annotation> annotation() default Annotation.class;


	boolean proxyTargetClass() default false;

	AdviceMode mode() default AdviceMode.PROXY;

	int order() default Ordered.LOWEST_PRECEDENCE;

}

AsyncConfigurationSelector注入了ProxyAsyncConfiguration

public class AsyncConfigurationSelector extends AdviceModeImportSelector<EnableAsync> {

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

	@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;
		}
	}

}

ProxyAsyncConfiguration该类初始化了AsyncAnnotationBeanPostProcessor，并注入到Spring容器中。

@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;
	}

}

AsyncAnnotationBeanPostProcessor继承了AbstractBeanFactoryAwareAdvisingPostProcessor，AbstractBeanFactoryAwareAdvisingPostProcessor实现了BeanFactoryAware，所以系统启动会执行AsyncAnnotationBeanPostProcessor#setBeanFactory。

	@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;
	}

AsyncAnnotationAdvisor构造方法中，初始化了advice和pointcut，AsyncAnnotationAdvisor#buildAdvice拦截类指定了AnnotationAsyncExecutionInterceptor，AsyncAnnotationAdvisor#buildPointcut指定了可以根据方法和类上的注解进行拦截。

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

		Set<Class<? extends Annotation>> asyncAnnotationTypes = new LinkedHashSet<>(2);
		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) {

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

	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);
	}

拦截处理

AnnotationAsyncExecutionInterceptor继承了AsyncExecutionInterceptor，真正的拦截处理在AsyncExecutionInterceptor#invoke内执行。核心逻辑就是获取指定的线程池，在线程池中执行任务。

	@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());
	}

	@Nullable
	protected Object doSubmit(Callable<Object> task, AsyncTaskExecutor executor, Class<?> returnType) {
		if (CompletableFuture.class.isAssignableFrom(returnType)) {
			return CompletableFuture.supplyAsync(() -> {
				try {
					return task.call();
				}
				catch (Throwable ex) {
					throw new CompletionException(ex);
				}
			}, executor);
		}
		else if (ListenableFuture.class.isAssignableFrom(returnType)) {
			return ((AsyncListenableTaskExecutor) executor).submitListenable(task);
		}
		else if (Future.class.isAssignableFrom(returnType)) {
			return executor.submit(task);
		}
		else {
			executor.submit(task);
			return null;
		}
	}

获取线程池

AsyncExecutionAspectSupport#determineAsyncExecutor获取线程池，如果指定了线程池，获取指定的。如果没有指定，获取默认的线程池。

	@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;
	}

AsyncExecutionAspectSupport#getDefaultExecutor，从Spring容器中获取。

	public AsyncExecutionAspectSupport(@Nullable Executor defaultExecutor) {
		this.defaultExecutor = new SingletonSupplier<>(defaultExecutor, () -> getDefaultExecutor(this.beanFactory));
		this.exceptionHandler = SingletonSupplier.of(SimpleAsyncUncaughtExceptionHandler::new);
	}

	@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);
			}
        }
    }

在TaskExecutionAutoConfiguration中，通过生成ThreadPoolTaskExecutor的Bean，来提供默认的Executor。

    @Bean
	@ConditionalOnMissingBean
	public TaskExecutorBuilder taskExecutorBuilder(TaskExecutionProperties properties,
			ObjectProvider<TaskExecutorCustomizer> taskExecutorCustomizers,
			ObjectProvider<TaskDecorator> taskDecorator) {
		TaskExecutionProperties.Pool pool = properties.getPool();
		TaskExecutorBuilder builder = new TaskExecutorBuilder();
		builder = builder.queueCapacity(pool.getQueueCapacity());
		builder = builder.corePoolSize(pool.getCoreSize());
		builder = builder.maxPoolSize(pool.getMaxSize());
		builder = builder.allowCoreThreadTimeOut(pool.isAllowCoreThreadTimeout());
		builder = builder.keepAlive(pool.getKeepAlive());
		Shutdown shutdown = properties.getShutdown();
		builder = builder.awaitTermination(shutdown.isAwaitTermination());
		builder = builder.awaitTerminationPeriod(shutdown.getAwaitTerminationPeriod());
		builder = builder.threadNamePrefix(properties.getThreadNamePrefix());
		builder = builder.customizers(taskExecutorCustomizers.orderedStream()::iterator);
		builder = builder.taskDecorator(taskDecorator.getIfUnique());
		return builder;
	}

	@Lazy
    @Bean(
        name = {"applicationTaskExecutor", "taskExecutor"}
    )
    @ConditionalOnMissingBean({Executor.class})
    public ThreadPoolTaskExecutor applicationTaskExecutor(TaskExecutorBuilder builder) {
        return builder.build();
    }

如果没有进行对应的配置，线程池阻塞队列的大小是Integer.MAX_VALUE。

@ConfigurationProperties("spring.task.execution")
public class TaskExecutionProperties {

	private final Pool pool = new Pool();

	private final Shutdown shutdown = new Shutdown();

	/**
	 * Prefix to use for the names of newly created threads.
	 */
	private String threadNamePrefix = "task-";

	public static class Pool {

		/**
		 * Queue capacity. An unbounded capacity does not increase the pool and therefore
		 * ignores the "max-size" property.
		 */
		private int queueCapacity = Integer.MAX_VALUE;

		/**
		 * Core number of threads.
		 */
		private int coreSize = 8;

		/**
		 * Maximum allowed number of threads. If tasks are filling up the queue, the pool
		 * can expand up to that size to accommodate the load. Ignored if the queue is
		 * unbounded.
		 */
		private int maxSize = Integer.MAX_VALUE;

		/**
		 * Whether core threads are allowed to time out. This enables dynamic growing and
		 * shrinking of the pool.
		 */
		private boolean allowCoreThreadTimeout = true;

		/**
		 * Time limit for which threads may remain idle before being terminated.
		 */
		private Duration keepAlive = Duration.ofSeconds(60);
    }
}

由于默认的线程池队列大小是Integer.MAX_VALUE，当有很多线程进来，而系统没有及时处理掉，就会将队列塞满，从而系统会出现内存溢出。所以使用@Async需要指定自己设定的线程池。

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