SpringBoot源码系列（10）：@Async原理

布鸽不鸽 Lv4
  2023-06-12 22:49:56 2023-06-12 22:49:56 创建   2023-06-25 22:57:36 2023-06-25 22:57:36 更新    2.1k 字  9 分钟  

前言

在SpringBoot中，我们可以使用异步操作来处理耗时的任务。通常我们需要在想异步执行的方法上标注@Async，然后在主启动类上标注@EnableAsync开启异步功能。关于其应用，可以查看我之前的文章：SpringBoot中的@Asnyc注解。本文将讨论以下问题：

  • @EnableAsync和@Async的原理是什么？
  • SpringBoot中默认的线程池/执行器是如何被指定的？
  • @Async为什么能通过value属性指定使用的执行器？

原文地址：https://xuedongyun.cn/post/24765/

异步处理的原理（@EnableAsync）

EnableAsync

我们在主启动类上标注了@EnableAsync注解。该注解通过@Import注解，导入了AsyncConfigurationSelector

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

AsyncConfigurationSelector

继承关系：ImportSelector<-AdviceModeImportSelector<EnableAsync><-AsyncConfigurationSelector

我们先看一看父类中的方法：

在@Configuration配置类上，使用@Import注解导入一个实现了ImportSelector接口的类时：会根据@Configuration配置类的AnnotationMetadata，返回需要导入到容器中的类的名称

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public interface ImportSelector {
	
    // 根据@Configuration配置类的AnnotationMetadata，返回一个class名称列表，来决定向容器中导入哪些类
	String[] selectImports(AnnotationMetadata importingClassMetadata);

	@Nullable
	default Predicate<String> getExclusionFilter() {
		return null;
	}

}

而AdviceModeImportSelector<EnableAsync>类，实现了selectImports方法。该方法用于获取@EnableAsync注解中的mode对应的值，并调用selectImports(adviceMode)方法。最终返回需要导入的类。这正是我们AsyncConfigurationSelector实现的方法。

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// 代码有删减
public abstract class AdviceModeImportSelector<A extends Annotation> implements ImportSelector {

	public static final String DEFAULT_ADVICE_MODE_ATTRIBUTE_NAME = "mode";

	protected String getAdviceModeAttributeName() {
		return DEFAULT_ADVICE_MODE_ATTRIBUTE_NAME;
	}

	@Override
	public final String[] selectImports(AnnotationMetadata importingClassMetadata) {
        // 用于当前类，泛型的具体类型（此处是EnableAsync）
		Class<?> annType = GenericTypeResolver.resolveTypeArgument(getClass(), AdviceModeImportSelector.class);
        Assert.state(annType != null, "...");
        
        // 获取@Configuration配置类的@EnableAsync的属性
		AnnotationAttributes attributes = AnnotationConfigUtils.attributesFor(importingClassMetadata, annType);
        if (attributes == null) { throw new IllegalArgumentException("..."); }
        
        // 此处getAdviceModeAttributeName()返回"mode"，@EnableAsync注解的"mode"属性默认值是"PROXY"
		AdviceMode adviceMode = attributes.getEnum(getAdviceModeAttributeName());
        
        // 调用了selectImports(adviceMode)方法
		String[] imports = selectImports(adviceMode);
		return imports;
	}

	@Nullable
	protected abstract String[] selectImports(AdviceMode adviceMode);

}

此时，我们将向容器中导入ProxyAsyncConfiguration

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public class AsyncConfigurationSelector extends AdviceModeImportSelector<EnableAsync> {

	@Override
	@Nullable
	public String[] selectImports(AdviceMode adviceMode) {
		switch (adviceMode) {
            // 向容器中导入ProxyAsyncConfiguration
			case PROXY:
				return new String[] {ProxyAsyncConfiguration.class.getName()};
			case ASPECTJ:
				return new String[] {ASYNC_EXECUTION_ASPECT_CONFIGURATION_CLASS_NAME};
			default:
				return null;
		}
	}

}

ProxyAsyncConfiguration

继承关系：AbstractAsyncConfiguration<-ProxyAsyncConfiguration

导入的ProxyAsyncConfiguration是一个@Configuration配置类，它通过@Bean注入了AsyncAnnotationBeanPostProcessor类。

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// 代码有删改
@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() {
		
        // 创建了一个PostProcessor，放到容器中
		AsyncAnnotationBeanPostProcessor bpp = new AsyncAnnotationBeanPostProcessor();
        
        // 设置了执行器和错误处理器
        bpp.configure(this.executor, this.exceptionHandler);
        
		// ...
		return bpp;
	}
}

在其父类AbstractAsyncConfiguration中，通过@Autowired拿到了容器中实现了AsyncConfigurer的组件（获取了线程池）。this.executor就是在此获取值的。

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@Configuration(proxyBeanMethods = false)
public abstract class AbstractAsyncConfiguration implements ImportAware {
    
    // 通过@Autowired拿到了容器中实现了AsyncConfigurer的组件（获取了线程池）
    @Autowired
	void setConfigurers(ObjectProvider<AsyncConfigurer> configurers) {
        Supplier<AsyncConfigurer> configurer = SingletonSupplier.of(() -> {
			List<AsyncConfigurer> candidates = configurers.stream().collect(Collectors.toList());
			if (CollectionUtils.isEmpty(candidates)) {
                // 没有就返回空
				return null;
			}
			if (candidates.size() > 1) {
                // 只能有一个实现AsyncConfigurer接口的组件
				throw new IllegalStateException("Only one AsyncConfigurer may exist");
			}
			return candidates.get(0);
		});
        
		// this.executor在此处获取
		this.executor = adapt(configurer, AsyncConfigurer::getAsyncExecutor);
		this.exceptionHandler = adapt(configurer, AsyncConfigurer::getAsyncUncaughtExceptionHandler);
	}
}

小结1：我们可以向容器中放入实现了AsyncConfigurer接口的组件，来向容器中放入线程池

AsyncAnnotationBeanPostProcessor

继承关系：BeanFactoryAware<-AbstractBeanFactoryAwareAdvisingPostProcessor<-AsyncAnnotationBeanPostProcessor

此时AsyncAnnotationBeanPostProcessor往容器中创建了一个增强器AsyncAnnotationAdvisor

在Spring中，如果Bean实现了BeanNameAware 接口，则会默认调用setBeanFactory方法，获取BeanFactory实例

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// 代码有删改
public class AsyncAnnotationBeanPostProcessor extends AbstractBeanFactoryAwareAdvisingPostProcessor {

	@Override
	public void setBeanFactory(BeanFactory beanFactory) {
		super.setBeanFactory(beanFactory);
		//创建一个增强器
		AsyncAnnotationAdvisor advisor = new AsyncAnnotationAdvisor(this.executor, this.exceptionHandler);
		if (this.asyncAnnotationType != null) {
			advisor.setAsyncAnnotationType(this.asyncAnnotationType);
		}
		advisor.setBeanFactory(beanFactory);
		this.advisor = advisor;
	}
    
    public void configure(@Nullable Supplier<Executor> executor, 
                          @Nullable Supplier<AsyncUncaughtExceptionHandler> exceptionHandler) {
        
		this.executor = executor;
		this.exceptionHandler = exceptionHandler;
	}

}

AsyncAnnotationAdvisor

在AsyncAnnotationAdvisor中，创建了增强方法this.advice，它其实也实现了BeanNameAware接口

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// 代码有删改
public class AsyncAnnotationAdvisor extends AbstractPointcutAdvisor implements BeanFactoryAware {

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

		// 增强方法this.advice真实类型：AnnotationAsyncExecutionInterceptor
		this.advice = buildAdvice(executor, exceptionHandler);
		this.pointcut = buildPointcut(asyncAnnotationTypes);
	}
    
    protected Advice buildAdvice(@Nullable Supplier<Executor> executor, 
                                 @Nullable Supplier<AsyncUncaughtExceptionHandler> exceptionHandler) {
		
        // 拦截器
		AnnotationAsyncExecutionInterceptor interceptor = new AnnotationAsyncExecutionInterceptor(null);
        //配置此切面的执行器和异常处理器
		interceptor.configure(executor, exceptionHandler);
		return interceptor;
	}
    
    // 配置此切面的执行器和异常处理器
    public void configure(@Nullable Supplier<Executor> defaultExecutor,
			@Nullable Supplier<AsyncUncaughtExceptionHandler> exceptionHandler) {

		this.defaultExecutor = new SingletonSupplier<>(defaultExecutor, () -> getDefaultExecutor(this.beanFactory));
		this.exceptionHandler = new SingletonSupplier<>(exceptionHandler, SimpleAsyncUncaughtExceptionHandler::new);
	}
    
    // 此处this.advice也实现了BeanNameAware接口
    @Override
	public void setBeanFactory(BeanFactory beanFactory) {
		if (this.advice instanceof BeanFactoryAware) {
			((BeanFactoryAware) this.advice).setBeanFactory(beanFactory);
		}
	}
}

AnnotationAsyncExecutionInterceptor

继承关系：MethodInterceptor&AsyncExecutionAspectSupport<-AsyncExecutionInterceptor<-AnnotationAsyncExecutionInterceptor

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public abstract class AsyncExecutionAspectSupport implements BeanFactoryAware {

	public void configure(@Nullable Supplier<Executor> defaultExecutor,
			@Nullable Supplier<AsyncUncaughtExceptionHandler> exceptionHandler) {
		// 拿到了默认的执行器/线程池，
		this.defaultExecutor = new SingletonSupplier<>(defaultExecutor, () -> getDefaultExecutor(this.beanFactory));
		this.exceptionHandler = new SingletonSupplier<>(exceptionHandler, SimpleAsyncUncaughtExceptionHandler::new);
	}
    
    @Nullable
	protected Executor getDefaultExecutor(@Nullable BeanFactory beanFactory) {
		if (beanFactory != null) {
			try {
				// 拿唯一的类型为TaskExecutor的组件
				return beanFactory.getBean(TaskExecutor.class);
			}
			catch (NoUniqueBeanDefinitionException ex) {
				try {
                    // 容器里有多个TaskExecutor，拿名字叫"taskExecutor"的，类型为Executor的组件
					return beanFactory.getBean(DEFAULT_TASK_EXECUTOR_BEAN_NAME, Executor.class);
				}
			}
			catch (NoSuchBeanDefinitionException ex) {
				try {
                    // 容器里没有TaskExecutor，拿名字叫"taskExecutor"的，类型为Executor的组件
					return beanFactory.getBean(DEFAULT_TASK_EXECUTOR_BEAN_NAME, Executor.class);
				}
			}
		}
		return null;
	}
}

更进一步的，AsyncExecutionInterceptor继续重写了getDefaultExecutor，加入一个新的逻辑：如果没有默认执行器，就用SimpleAsyncTaskExecutor

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public class AsyncExecutionInterceptor extends AsyncExecutionAspectSupport implements MethodInterceptor, Ordered {
    
    @Override
    @Nullable
    protected Executor getDefaultExecutor(@Nullable BeanFactory beanFactory) {
        Executor defaultExecutor = super.getDefaultExecutor(beanFactory);
        return (defaultExecutor != null ? defaultExecutor : new SimpleAsyncTaskExecutor());
    }
}

小结2：若容器中只有一个TaskExecutor组件，其为默认执行器；若不唯一，拿名字叫”taskExecutor”的，类型为Executor的组件。若都不满足，使用SimpleAsyncTaskExecutor作为默认执行器（每次执行被注解方法时，单独创建一个Thread来执行）

在AsyncExecutionInterceptor中，实现了invoke方法，把原来方法的调用提交到新的线程池执行，从而实现了方法的异步。

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// 代码有删改
public class AsyncExecutionInterceptor extends AsyncExecutionAspectSupport implements MethodInterceptor, Ordered {
    
    public AsyncExecutionInterceptor(@Nullable Executor defaultExecutor) {
		super(defaultExecutor);
	}

	@Override
	@Nullable
	public Object invoke(final MethodInvocation invocation) throws Throwable {
        
        // 决定使用哪个executor
        AsyncTaskExecutor executor = determineAsyncExecutor(userDeclaredMethod);
		
		// 创建task（去执行原来方法）
		Callable<Object> task = () -> {
			try {
				Object result = invocation.proceed();
				if (result instanceof Future) {
					return ((Future<?>) result).get();
				}
			}
			return null;
		};
		
        // 提交到新的线程池执行
		return doSubmit(task, executor, invocation.getMethod().getReturnType());
	}

}

异步处理的原理（@Async）

AnnotationAsyncExecutionInterceptor

继承关系：MethodInterceptor&AsyncExecutionAspectSupport<-AsyncExecutionInterceptor<-AnnotationAsyncExecutionInterceptor

我们继续上面一节的内容，AsyncExecutionAspectSupport中实现了determineAsyncExecutor方法，用于确定使用哪个executor

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@Nullable
protected AsyncTaskExecutor determineAsyncExecutor(Method method) {
    AsyncTaskExecutor executor = this.executors.get(method);
    if (executor == null) {
        Executor targetExecutor;
        // 根据@Async注解的value属性，查找对应类型为Executor的异步执行器
        String qualifier = getExecutorQualifier(method);
        if (StringUtils.hasLength(qualifier)) {
            targetExecutor = findQualifiedExecutor(this.beanFactory, qualifier);
        }
        // 如果没找到，获取defaultExecutor（之前已经获取过了）
        else {
            targetExecutor = this.defaultExecutor.get();
        }
        if (targetExecutor == null) {
            return null;
        }
        executor = (targetExecutor instanceof AsyncListenableTaskExecutor ?
                (AsyncListenableTaskExecutor) targetExecutor : new TaskExecutorAdapter(targetExecutor));
        // 缓存，为某个方法指定executor
        this.executors.put(method, executor);
    }
    return executor;
}

AnnotationAsyncExecutionInterceptor中实现了getExecutorQualifier方法

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@Override
@Nullable
protected String getExecutorQualifier(Method method) {
    // 核心：拿到@Async注解标注的value
    Async async = AnnotatedElementUtils.findMergedAnnotation(method, Async.class);
    if (async == null) {
        async = AnnotatedElementUtils.findMergedAnnotation(method.getDeclaringClass(), Async.class);
    }
    return (async != null ? async.value() : null);
}

小结3：我们可以通过@Async(value="xxx")来手动指定想要使用的执行器

SpringBoot中的默认线程池

TaskExecutionAutoConfiguration

SpringBoot中的线程池，是通过TaskExecutionAutoConfiguration这个自动配置类加载的。我们来看一下其中的源码。

  1. taskExecutorBuilder方法用于创建一个构造器
  2. 在applicationTaskExecutor方法中，使用该构造器创建了线程池
  3. TaskExecutionProperties.class属性类，绑定了所有的属性
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@ConditionalOnClass(ThreadPoolTaskExecutor.class)
@AutoConfiguration
@EnableConfigurationProperties(TaskExecutionProperties.class)
public class TaskExecutionAutoConfiguration {

	// Bean name of the application {@link TaskExecutor}.
	public static final String APPLICATION_TASK_EXECUTOR_BEAN_NAME = "applicationTaskExecutor";
	
    // 构造器，用于创建线程池
	@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;
	}
	
    // 使用构造器，创建线程池Bean，放到容器中
    // name = {applicationTaskExecutor, taskExecutor}
	@Lazy
	@Bean(name = { APPLICATION_TASK_EXECUTOR_BEAN_NAME,
			AsyncAnnotationBeanPostProcessor.DEFAULT_TASK_EXECUTOR_BEAN_NAME })
	@ConditionalOnMissingBean(Executor.class)
	public ThreadPoolTaskExecutor applicationTaskExecutor(TaskExecutorBuilder builder) {
		return builder.build();
	}

}

TaskExecutionProperties

该属性配置类绑定到spring.task.execution，我们可以定制SpringBoot线程池中的基本属性

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// 代码有删改
@ConfigurationProperties("spring.task.execution")
public class TaskExecutionProperties {

	private final Pool pool = new Pool();
	private final Shutdown shutdown = new Shutdown();
	private String threadNamePrefix = "task-";

	public static class Pool {
		private int queueCapacity = Integer.MAX_VALUE;
		private int coreSize = 8;
		private int maxSize = Integer.MAX_VALUE;
	}
	
    // ...
}
  • 标题: SpringBoot源码系列（10）：@Async原理
  • 作者: 布鸽不鸽
  • 创建于 : 2023-06-12 22:49:56
  • 更新于 : 2023-06-25 22:57:36
  • 链接: https://xuedongyun.cn//post/24765/
  • 版权声明: 本文章采用 CC BY-NC-SA 4.0 进行许可。
推荐阅读
SpringBoot中的@Asnyc注解 SpringBoot中的@Asnyc注解 JUC系列：（六）线程池 JUC系列：（六）线程池 SpringBoot源码系列（2）：SpringBootApplication注解 SpringBoot源码系列（2）：SpringBootApplication注解
推荐阅读
SpringBoot中的@Asnyc注解 SpringBoot中的@Asnyc注解 JUC系列：（六）线程池 JUC系列：（六）线程池
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目录
SpringBoot源码系列（10）：@Async原理
  1. 前言
  2. 异步处理的原理（@EnableAsync）
    1. EnableAsync
    2. AsyncConfigurationSelector
    3. ProxyAsyncConfiguration
    4. AsyncAnnotationBeanPostProcessor
    5. AsyncAnnotationAdvisor
    6. AnnotationAsyncExecutionInterceptor
  3. 异步处理的原理（@Async）
    1. AnnotationAsyncExecutionInterceptor
  4. SpringBoot中的默认线程池
    1. TaskExecutionAutoConfiguration
    2. TaskExecutionProperties