Spring Boot 异步任务

川长思鸟来 2022-05-22 03:20 182阅读 0赞

Spring 异步任务涉及到两个注解

*  @EnableAsync 标记在配置类上，开启异步任务
 *  @Async 标记方法，将方法增强为异步方法

###### 简单示例 ######

1.  异步任务执行类

@Slf4j
    @Component
    public class SimpleAsyncTask { 
    
        @Async
        public void execute() { 
            log.info("【任务执行】当前线程：{}", Thread.currentThread().getName());
        }
    
    }

1.  引导程序

@Slf4j
    @EnableAsync
    @SpringBootApplication
    public class SimpleAsyncTaskApplication { 
    
        public static void main(String[] args) { 
            ApplicationContext applicationContext = SpringApplication.run(SimpleAsyncTaskApplication.class, args);
            log.info("【程序启动】当前线程：{}", Thread.currentThread().getName());
            applicationContext.getBean(SimpleAsyncTask.class).execute();
        }
    
    }

1.  运行程序

控制台打印出如下日志，说明任务执行并没有使用主线程执行，异步任务开启成功

2019-06-03 21:26:33.862  INFO 1150 --- [           main] c.g.l.s.t.a.s.SimpleAsyncTaskApplication : 【程序启动】当前线程：main
    2019-06-03 21:26:33.885  INFO 1150 --- [         task-1] c.g.l.s.t.async.simple.SimpleAsyncTask   : 【任务执行】当前线程：task-1

> 示例代码 [https://github.com/ly641921791/knowledge-examples/tree/master/spring-example/spring-task-async-simple][https_github.com_ly641921791_knowledge-examples_tree_master_spring-example_spring-task-async-simple]

###### 复杂示例 ######

Spring在没有找到线程池时会创建默认的线程池用于处理异常任务，对于异常的任务默认不处理

Spring提供了`AsyncConfigurer`接口用于配置异步任务使用的线程池和异常处理规则，代码如下

public interface AsyncConfigurer { 
    	@Nullable
    	default Executor getAsyncExecutor() { 
    		return null;
    	}
    
    	@Nullable
    	default AsyncUncaughtExceptionHandler getAsyncUncaughtExceptionHandler() { 
    		return null;
    	}
    }

实现`AsyncConfigurer`接口，并将实现类注册到Spring容器就可以达到配置的目的，示例如下

@EnableAsync
    @Configuration
    public class AsyncConfig implements AsyncConfigurer { 
    	// 实现略
    }

> 示例代码 [https://github.com/ly641921791/knowledge-examples/tree/master/spring-example/spring-task-async-complex][https_github.com_ly641921791_knowledge-examples_tree_master_spring-example_spring-task-async-complex]

###### 常见问题 ######

1.  异步任务失效

原因1：通过this调用的异步方法会失效，因为异步任务的原理是Aop拦截，this调用无法被拦截，可以通过修改配置Aop配置，通过AopContext得到当前对象的代理对象

原因2：通过new得到的对象异步方法失效，因为没有被Spring容器增强

原因3：`AsyncConfigurer`及子类中的异步方法失效，因为这些类在异步任务处理之前已经创建完成，无法得到二次处理

###### 源码解析 ######

1.  @EnableAsync

@EnableAsync通过@Import引入了`AsyncConfigurationSelector`，代码如下

@Import(AsyncConfigurationSelector.class)
    public @interface EnableAsync { 
    }

1.  AsyncConfigurationSelector

public class AsyncConfigurationSelector extends AdviceModeImportSelector<EnableAsync> { 
    
    	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;
    		}
    	}
    
    }

AdviceMode是@EnableAsync的一个属性，默认值是`AdviceMode.PROXY`，也就是默认情况下，会引入`ProxyAsyncConfiguration`

1.  ProxyAsyncConfiguration

`ProxyAsyncConfiguration`做了三件事，代码如下

*  通过`setImportMetadata`方法得到了@EnableAsync注解的属性信息，
 *  通过`setConfigurers`方法得到了`AsyncConfigurer`配置的线程池和异常处理器，
 *  通过`asyncAdvisor`方法配置并引入了`AsyncAnnotationBeanPostProcessor`

@Configuration
    @Role(BeanDefinition.ROLE_INFRASTRUCTURE)
    public class ProxyAsyncConfiguration extends AbstractAsyncConfiguration { 
    
    	/* 继承自 AbstractAsyncConfiguration @Nullable protected AnnotationAttributes enableAsync; @Nullable protected Supplier<Executor> executor; @Nullable protected Supplier<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()); } } @Autowired(required = false) void setConfigurers(Collection<AsyncConfigurer> 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; } */
    
    	@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;
    	}
    
    }

1.  AsyncAnnotationBeanPostProcessor

`AsyncAnnotationBeanPostProcessor`

*  通过`setBeanFactory`方法创建了`AsyncAnnotationAdvisor`
 *  通过`postProcessAfterInitialization`创建了Bean的代理对象，并将`AsyncAnnotationAdvisor`作为Advisor，

`AsyncAnnotationAdvisor`将`AnnotationAsyncExecutionInterceptor`作为方法拦截器，拦截目标方法执行，将其交过线程池执行实现异步调用

[https_github.com_ly641921791_knowledge-examples_tree_master_spring-example_spring-task-async-simple]: https://github.com/ly641921791/knowledge-examples/tree/master/spring-example/spring-task-async-simple
[https_github.com_ly641921791_knowledge-examples_tree_master_spring-example_spring-task-async-complex]: https://github.com/ly641921791/knowledge-examples/tree/master/spring-example/spring-task-async-complex