Spring : AsyncAnnotationBeanPostProcessor

概述

AsyncAnnotationBeanPostProcessor是一个AbstractBeanFactoryAwareAdvisingPostProcessor/BeanPostProcessor,它在容器启动时被使用，检测每个bean是否符合在类级别或者方法级别使用异步注解，如果使用了，则会使用一个AsyncAnnotationAdvisor包裹该bean,从而该bean上通过异步注解所定义的方法在调用时会被真正地异步调用起来。

上面所提到的"异步注解"可以通过AsyncAnnotationBeanPostProcessor指定，如果不指定，则缺省是指Spring @Async注解和EJB 3.1 javax.ejb.Asynchronous注解。

另外在真正执行异步调用的方面，可以为AsyncAnnotationBeanPostProcessor指定一个任务执行器TaskExecutor，如果不指定，AsyncAnnotationBeanPostProcessor会这么决定一个任务执行器:

1. 如果容器中存在唯一的一个TaskExecutor bean则使用它;
2. 否则从容器中查找名字为taskExecutor类型为Executor的bean;
3. 再不然，就自己创建一个SimpleAsyncTaskExecutor使用。

以上该逻辑，可以参考类源代码AsyncExecutionInterceptor。

在异步执行的异常处理方面，AsyncAnnotationBeanPostProcessor也做了考虑。方法调用的异步执行根据返回结果类型分两种情况：

1. 返回结果类型为void
2. 返回结果类型为Future
   对于返回结果类型为Future的情况，不用做任何设置，调用者就可以使用try-catch机制捕捉异步执行产生的异常。而对于返回结果类型为void的情况，Spring支持设置一个AsyncUncaughtExceptionHandler来支持调用者处理其中发生的异常。这个AsyncUncaughtExceptionHandler可以通过AsyncAnnotationBeanPostProcessor来设置。

另外AsyncAnnotationBeanPostProcessor会将AsyncAnnotationAdvisor包裹到代理对象的最外面，也就是在调用链中会被优先调用到。

关于AsyncAnnotationBeanPostProcessor的使用，可以参考如下类 :

1. 注解@EnableAsync – 导入了 AsyncConfigurationSelector
2. AsyncConfigurationSelector – 导入了 ProxyAsyncConfiguration
3. ProxyAsyncConfiguration – 使用AsyncAnnotationBeanPostProcessor定义基础设施bean asyncAdvisor

源代码

源代码版本 : spring-context-5.1.5.RELEASE

package org.springframework.scheduling.annotation;

import java.lang.annotation.Annotation;
import java.util.concurrent.Executor;
import java.util.function.Supplier;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import org.springframework.aop.framework.autoproxy.AbstractBeanFactoryAwareAdvisingPostProcessor;
import org.springframework.aop.interceptor.AsyncUncaughtExceptionHandler;
import org.springframework.beans.factory.BeanFactory;
import org.springframework.core.task.TaskExecutor;
import org.springframework.lang.Nullable;
import org.springframework.util.Assert;
import org.springframework.util.function.SingletonSupplier;

/**
 * Bean post-processor that automatically applies asynchronous invocation
 * behavior to any bean that carries the @Async annotation at class or
 * method-level by adding a corresponding AsyncAnnotationAdvisor to the
 * exposed proxy (either an existing AOP proxy or a newly generated proxy that
 * implements all of the target's interfaces).
 *
 * The TaskExecutor responsible for the asynchronous execution may
 * be provided as well as the annotation type that indicates a method should be
 * invoked asynchronously. If no annotation type is specified, this post-
 * processor will detect both Spring's Async @Async annotation as well
 * as the EJB 3.1 javax.ejb.Asynchronous annotation.
 *
 * For methods having a void return type, any exception thrown
 * during the asynchronous method invocation cannot be accessed by the
 * caller. An AsyncUncaughtExceptionHandler can be specified to handle
 * these cases.
 *
 * Note: The underlying async advisor applies before existing advisors by default,
 * in order to switch to async execution as early as possible in the invocation chain.
 *
 * @author Mark Fisher
 * @author Juergen Hoeller
 * @author Stephane Nicoll
 * @since 3.0
 * @see Async
 * @see AsyncAnnotationAdvisor
 * @see #setBeforeExistingAdvisors
 * @see ScheduledAnnotationBeanPostProcessor
 */
@SuppressWarnings("serial")
public class AsyncAnnotationBeanPostProcessor 
					extends AbstractBeanFactoryAwareAdvisingPostProcessor {

	/**
	 * The default name of the TaskExecutor bean to pick up: "taskExecutor".
	 * Note that the initial lookup happens by type; this is just the fallback
	 * in case of multiple executor beans found in the context.
	 * @since 4.2
	 * @see AnnotationAsyncExecutionInterceptor#DEFAULT_TASK_EXECUTOR_BEAN_NAME
	 */
	public static final String DEFAULT_TASK_EXECUTOR_BEAN_NAME =
			AnnotationAsyncExecutionInterceptor.DEFAULT_TASK_EXECUTOR_BEAN_NAME;


	protected final Log logger = LogFactory.getLog(getClass());

    // 用于记录外部指定的执行器
	@Nullable
	private Supplier<Executor> executor;

    // 用于记录外部指定的异常处理器
	@Nullable
	private Supplier<AsyncUncaughtExceptionHandler> exceptionHandler;

    // 用于记录外部指定的异步注解类型
	@Nullable
	private Class<? extends Annotation> asyncAnnotationType;



	public AsyncAnnotationBeanPostProcessor() {
        // 设置将自己的 Advisor 包裹到已存在 Advisor 的外面，也就是设置为拦截器调用链中的高优先级
		setBeforeExistingAdvisors(true);
	}


	/**
	 * Configure this post-processor with the given executor and exception handler suppliers,
	 * applying the corresponding default if a supplier is not resolvable.
	 * @since 5.1
	 */
	public void configure(
			@Nullable Supplier<Executor> executor, 
			@Nullable Supplier<AsyncUncaughtExceptionHandler> exceptionHandler) {

		this.executor = executor;
		this.exceptionHandler = exceptionHandler;
	}

	/**
	 * Set the Executor to use when invoking methods asynchronously.
	 * If not specified, default executor resolution will apply: searching for a
	 * unique TaskExecutor bean in the context, or for an Executor
	 * bean named "taskExecutor" otherwise. If neither of the two is resolvable,
	 * a local default executor will be created within the interceptor.
	 * @see AnnotationAsyncExecutionInterceptor#getDefaultExecutor(BeanFactory)
	 * @see #DEFAULT_TASK_EXECUTOR_BEAN_NAME
	 */
	public void setExecutor(Executor executor) {
		this.executor = SingletonSupplier.of(executor);
	}

	/**
	 * Set the AsyncUncaughtExceptionHandler to use to handle uncaught
	 * exceptions thrown by asynchronous method executions.
	 * @since 4.1
	 */
	public void setExceptionHandler(AsyncUncaughtExceptionHandler exceptionHandler) {
		this.exceptionHandler = SingletonSupplier.of(exceptionHandler);
	}

	/**
	 * Set the 'async' annotation type to be detected at either class or method
	 * level. By default, both the @Async annotation and the EJB 3.1
	 * javax.ejb.Asynchronous annotation will be detected.
	 * This setter property exists so that developers can provide their own
	 * (non-Spring-specific) annotation type to indicate that a method (or all
	 * methods of a given class) should be invoked asynchronously.
	 * @param asyncAnnotationType the desired annotation type
	 */
	public void setAsyncAnnotationType(Class<? extends Annotation> asyncAnnotationType) {
		Assert.notNull(asyncAnnotationType, "'asyncAnnotationType' must not be null");
		this.asyncAnnotationType = asyncAnnotationType;
	}


	@Override
	public void setBeanFactory(BeanFactory beanFactory) {
		super.setBeanFactory(beanFactory);

       // 创建自己将要包括到符合条件的bean上的Advisor 
		AsyncAnnotationAdvisor advisor = new AsyncAnnotationAdvisor(this.executor, 
			this.exceptionHandler);
		if (this.asyncAnnotationType != null) {
			advisor.setAsyncAnnotationType(this.asyncAnnotationType);
		}
		advisor.setBeanFactory(beanFactory);
		this.advisor = advisor;
	}

}