一、任务调度

注解类：@Scheduled

核心处理类：ScheduledAnnotationBeanPostProcessor

使用的线程池：从容器中查询TaskScheduler。

• 首先在容器中通过类型查找TaskScheduler Bean，如果没有则抛出NoSuchBeanDefinitionException异常。
• 在这一步中，如果找到多个，那么会在通过beanName=taskScheduler在容器中查找
• 在上一步中抛出异常后会继续查找java.util.concurrent.ScheduledExecutorService 类型的Bean。
• 在这一步中，如果找到多个，那么会在通过beanName=taskScheduler在容器中查找
• 在上一步中还是没有则结束（程序并不会报错）

如果上面流程都没有找到，则会通过如下方式创建一个。

this.localExecutor = Executors.newSingleThreadScheduledExecutor();
 this.taskScheduler = new ConcurrentTaskScheduler(this.localExecutor);

在Springboot中有个自动配置类会配置一个TaskSchedulingAutoConfiguration。

public class TaskSchedulingAutoConfiguration {
   @Bean
   @ConditionalOnBean(name = TaskManagementConfigUtils.SCHEDULED_ANNOTATION_PROCESSOR_BEAN_NAME)
   @ConditionalOnMissingBean({ SchedulingConfigurer.class, TaskScheduler.class, ScheduledExecutorService.class })
   public ThreadPoolTaskScheduler taskScheduler(TaskSchedulerBuilder builder) {
     return builder.build();
   }
   
   @Bean
   @ConditionalOnMissingBean
   public TaskSchedulerBuilder taskSchedulerBuilder(TaskSchedulingProperties properties,
       ObjectProvider taskSchedulerCustomizers) {
     TaskSchedulerBuilder builder = new TaskSchedulerBuilder();
     builder = builder.poolSize(properties.getPool().getSize());
     Shutdown shutdown = properties.getShutdown();
     builder = builder.awaitTermination(shutdown.isAwaitTermination());
     builder = builder.awaitTerminationPeriod(shutdown.getAwaitTerminationPeriod());
     builder = builder.threadNamePrefix(properties.getThreadNamePrefix());
     builder = builder.customizers(taskSchedulerCustomizers);
     return builder;
   }
 }

二、异步任务

• 注解类：Async。
• 核心处理类：AsyncAnnotationBeanPostProcessor。

通过ProxyAsyncConfiguration配置，该类继承AbstractAsyncConfiguration。

在父类中会初始化，下面两个成员变量：

@Configuration(proxyBeanMethods = false)
 public abstract class AbstractAsyncConfiguration implements ImportAware {
   @Nullable
   protected Supplier executor;
   @Nullable
   protected Supplier exceptionHandler;
   // 在容器中查找AsyncConfigurer Bean 且只能有一个
   @Autowired
   void setConfigurers(ObjectProvider configurers) {
     Supplier configurer = SingletonSupplier.of(() -> {
       List candidates = configurers.stream().collect(Collectors.toList());
       if (CollectionUtils.isEmpty(candidates)) {
         return null;
       }
       if (candidates.size() > 1) {
         throw new IllegalStateException("Only one AsyncConfigurer may exist");
       }
       return candidates.get(0);
     });
     this.executor = adapt(configurer, AsyncConfigurer::getAsyncExecutor);
     this.exceptionHandler = adapt(configurer, AsyncConfigurer::getAsyncUncaughtExceptionHandler);
   }
 
   private  Supplier adapt(Supplier supplier, Function provider) {
     return () -> {
       AsyncConfigurer configurer = supplier.get();
       return (configurer != null ? provider.apply(configurer) : null);
     };
   }
 }

使用的线程池：

• 首先在容器中通过类型查找AsyncConfigurer Bean。
• 如果没有则设置默认的AsyncConfigurer::getAsyncExecutor 该方法是接口中默认方法，返回的是null。
• 在上一步中如果容器中没有AsyncConfigurer，那么设置到AsyncAnnotationBeanPostProcessor中也将就是null。
• 初始化AsyncAnnotationBeanPostProcessor。

public class AsyncAnnotationBeanPostProcessor extends AbstractBeanFactoryAwareAdvisingPostProcessor {
   @Nullable
   private Supplier executor;
   @Nullable
   private Supplier exceptionHandler;
   @Override
   public void setBeanFactory(BeanFactory beanFactory) {
     super.setBeanFactory(beanFactory);
     // 构建切面Advisor
     AsyncAnnotationAdvisor advisor = new AsyncAnnotationAdvisor(this.executor, this.exceptionHandler);
     if (this.asyncAnnotationType != null) {
       advisor.setAsyncAnnotationType(this.asyncAnnotationType);
     }
     advisor.setBeanFactory(beanFactory);
     this.advisor = advisor;
   }
 }
 public class AsyncAnnotationAdvisor extends AbstractPointcutAdvisor implements BeanFactoryAware {
   public AsyncAnnotationAdvisor(
       @Nullable Supplier executor, @Nullable Supplier exceptionHandler) {
 
     Set> asyncAnnotationTypes = new LinkedHashSet<>(2);
     asyncAnnotationTypes.add(Async.class);
     try {
       asyncAnnotationTypes.add((Class)
           ClassUtils.forName("javax.ejb.Asynchronous", AsyncAnnotationAdvisor.class.getClassLoader()));
     }
     // 构建通知类
     this.advice = buildAdvice(executor, exceptionHandler);
     this.pointcut = buildPointcut(asyncAnnotationTypes);
   }
   protected Advice buildAdvice(
       @Nullable Supplier executor, @Nullable Supplier exceptionHandler) {
 
     AnnotationAsyncExecutionInterceptor interceptor = new AnnotationAsyncExecutionInterceptor(null);
     // 调用父类方法
     interceptor.configure(executor, exceptionHandler);
     return interceptor;
   }
 }
 public abstract class AsyncExecutionAspectSupport implements BeanFactoryAware { 
   public void configure(@Nullable Supplier defaultExecutor,
       @Nullable Supplier exceptionHandler) {
 
     // 如果defaultExecutor则调用getDefaultExecutor方法，该方法在子类重写了
     this.defaultExecutor = new SingletonSupplier<>(defaultExecutor, () -> getDefaultExecutor(this.beanFactory));
     this.exceptionHandler = new SingletonSupplier<>(exceptionHandler, SimpleAsyncUncaughtExceptionHandler::new);
   }
   protected Executor getDefaultExecutor(@Nullable BeanFactory beanFactory) {
     if (beanFactory != null) {
       try {
         // 容器中查找TaskExecutor类型的Bean
         return beanFactory.getBean(TaskExecutor.class);
       } catch (NoUniqueBeanDefinitionException ex) {
         try {
           return beanFactory.getBean(DEFAULT_TASK_EXECUTOR_BEAN_NAME, Executor.class);
         } catch (NoSuchBeanDefinitionException ex2) {
         }
       } catch (NoSuchBeanDefinitionException ex) {
         try {
           return beanFactory.getBean(DEFAULT_TASK_EXECUTOR_BEAN_NAME, Executor.class);
         } catch (NoSuchBeanDefinitionException ex2) {
         }
         // Giving up -> either using local default executor or none at all...
       }
     }
     return null;
   }
 }
 public class AsyncExecutionInterceptor extends AsyncExecutionAspectSupport implements MethodInterceptor, Ordered {
   protected Executor getDefaultExecutor(@Nullable BeanFactory beanFactory) {
     // 先通过父类获取
     Executor defaultExecutor = super.getDefaultExecutor(beanFactory);
     // 如果父类获取不到，则创建默认的SimpleAsyncTaskExecutor
     return (defaultExecutor != null ? defaultExecutor : new SimpleAsyncTaskExecutor());
   }
 }

总结：

• 从容器中查找TaskExecutor Bean。
• 上一步没有找到，则查找beanName=taskExecutor，类型为java.util.concurrent.Executor的Bean。
• 如果上一步还是没有找到，那么最终创建默认的SimpleAsyncTaskExecutor 这是个没有上限的线程池，来一个任务创建新线程。

如果执行的异步任务很多且线程池，线程有限则多的任务会等待。

三、Web异步接口

RequestMappingHandlerAdapter。

public class RequestMappingHandlerAdapter {
   // 默认是一个没有上限的线程池
   private AsyncTaskExecutor taskExecutor = new SimpleAsyncTaskExecutor("MvcAsync");
   protected ModelAndView invokeHandlerMethod(
     HttpServletRequest request,
     HttpServletResponse response, 
     HandlerMethod handlerMethod) throws Exception {
     // ...
     WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
     asyncManager.setTaskExecutor(this.taskExecutor);
     // ...
   }
 }

创建RequestMappingHandlerAdapter Bean对象。

继承关系。

public static class EnableWebMvcConfiguration extends DelegatingWebMvcConfiguration{}
 public class DelegatingWebMvcConfiguration extends WebMvcConfigurationSupport {
   private final WebMvcConfigurerComposite configurers = new WebMvcConfigurerComposite();
   // WebMvcConfigurer默认实现
   @Autowired(required = false)
   public void setConfigurers(List configurers) {
     if (!CollectionUtils.isEmpty(configurers)) {
       this.configurers.addWebMvcConfigurers(configurers);
     }
   }
   @Override
   protected void configureAsyncSupport(AsyncSupportConfigurer configurer) {
     this.configurers.configureAsyncSupport(configurer);
   }
 }
 public class WebMvcConfigurationSupport implements ApplicationContextAware, ServletContextAware {
   private AsyncSupportConfigurer asyncSupportConfigurer;
   @Bean
   public RequestMappingHandlerAdapter requestMappingHandlerAdapter(...) {
     // ...
     AsyncSupportConfigurer configurer = getAsyncSupportConfigurer();
     if (configurer.getTaskExecutor() != null) {
       adapter.setTaskExecutor(configurer.getTaskExecutor());
     }
     // ...
   }
   protected AsyncSupportConfigurer getAsyncSupportConfigurer() {
     if (this.asyncSupportConfigurer == null) {
       this.asyncSupportConfigurer = new AsyncSupportConfigurer();
       configureAsyncSupport(this.asyncSupportConfigurer);
     }
     return this.asyncSupportConfigurer;
   }
 }
 public class WebMvcAutoConfiguration {
   public static class WebMvcAutoConfigurationAdapter implements WebMvcConfigurer, ServletContextAware {
     public void configureAsyncSupport(AsyncSupportConfigurer configurer) {
       // 判断容器中是否有beanName = applicationTaskExecutor 的Bean
       if (this.beanFactory.containsBean(TaskExecutionAutoConfiguration.APPLICATION_TASK_EXECUTOR_BEAN_NAME)) {
         // 通过beanName = applicationTaskExecutor获取bean对象
         Object taskExecutor = this.beanFactory.getBean(TaskExecutionAutoConfiguration.APPLICATION_TASK_EXECUTOR_BEAN_NAME);
         // 判断是否AsyncTaskExecutor对象
         if (taskExecutor instanceof AsyncTaskExecutor) {
           configurer.setTaskExecutor(((AsyncTaskExecutor) taskExecutor));
         }
       }
       Duration timeout = this.mvcProperties.getAsync().getRequestTimeout();
       if (timeout != null) {
         configurer.setDefaultTimeout(timeout.toMillis());
       }
     }
   }
 }

总结：

• 默认使用SimpleAsyncTaskExecutor。
• 如果容器中存在以beanName = applicationTaskExecutor 且 类型是 AsyncTaskExecutor， 则使用该bean。

到这你应该知道了这三者在线程池方面该如何正确配置及使用了。