spring源码解读：@Async的原理——默认配置

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已于 2022-05-18 08:15:58 修改 · 1.9k 阅读

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文章标签：

#spring #Async #aop

于 2021-08-22 21:38:29 首次发布

本文详细介绍了Spring中@Async注解的使用、源码分析，包括@EnableAsync配置、异步执行机制、自定义任务执行器和advisor的构建过程。重点讲解了如何判断添加advice时机以及异步拦截器的工作原理。

文章目录

前言
一、如何使用@Async
二、源码解读
总结

前言

用过Spring的人多多少少也都用过@Async注解，至于作用嘛，看注解名，大概能猜出来，就是在方法执行的时候进行异步执行。

一、如何使用@Async

使用@Async注解主要分两步：

在配置类上添加@EnableAsync注解

@ComponentScan(value = "com.wang")
@Configuration
@EnableAsync
public class AppConfig {
   
   
}

在想要异步执行的方法上面加上@Async

@Service
public class CycleService2 {
   
   

	@Autowired
	private CycleService1 cycleService1;

	@Async
	public void alsoDo() {
   
   
		System.out.println("create cycleService2");
	}

}

二、源码解读

1.@EnableAsync的作用

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

	/**
	 * Indicate the 'async' annotation type to be detected at either class
	 * or method level.
	 * <p>By default, both Spring's @{@link Async} annotation and the EJB 3.1
	 * {@code @javax.ejb.Asynchronous} annotation will be detected.
	 * <p>This attribute exists so that developers can provide their own
	 * custom annotation type to indicate that a method (or all methods of
	 * a given class) should be invoked asynchronously.
	 * 此处说明的是方法执行变成异步，扫描的是哪个注解，目前默认的是Async和Asynchronous，开发者也可以自定义
	 */
	Class<? extends Annotation> annotation() default Annotation.class;

	/**
	 * Indicate whether subclass-based (CGLIB) proxies are to be created as opposed
	 * to standard Java interface-based proxies.
	 * <p><strong>Applicable only if the {@link #mode} is set to {@link AdviceMode#PROXY}</strong>.
	 * <p>The default is {@code false}.
	 * <p>Note that setting this attribute to {@code true} will affect <em>all</em>
	 * Spring-managed beans requiring proxying, not just those marked with {@code @Async}.
	 * For example, other beans marked with Spring's {@code @Transactional} annotation
	 * will be upgraded to subclass proxying at the same time. This approach has no
	 * negative impact in practice unless one is explicitly expecting one type of proxy
	 * vs. another &mdash; for example, in tests.
	 * 如何proxyTargetClass被设置成true，那么spring的所有proxy都会通过CGLIB方式实现，不再使用Java默认的基于接口的代理实现方式；而且此处如果设置，不仅仅是会影响添加了@Async注解的类的proxy方式，加了@Transactional的类也会变成CGLIB代理，不推荐修改；这个注解只有mode是默认的PROXY，才有意义
	 */
	boolean proxyTargetClass() default false;

	/**
	 * Indicate how async advice should be applied.
	 * <p><b>The default is {@link AdviceMode#PROXY}.</b>
	 * Please note that proxy mode allows for interception of calls through the proxy
	 * only. Local calls within the same class cannot get intercepted that way; an
	 * {@link Async} annotation on such a method within a local call will be ignored
	 * since Spring's interceptor does not even kick in for such a runtime scenario.
	 * For a more advanced mode of interception, consider switching this to
	 * {@link AdviceMode#ASPECTJ}.
	 * 代理方式的不同，默认的是使用Spring的proxy方式，也可以换成原生的AspectJ的proxy方式。
	 * 这两个的区别作用还是很明显的
	 */
	AdviceMode mode() default AdviceMode.PROXY;

	/**
	 * Indicate the order in which the {@link AsyncAnnotationBeanPostProcessor}
	 * should be applied.
	 * <p>The default is {@link Ordered#LOWEST_PRECEDENCE} in order to run
	 * after all other post-processors, so that it can add an advisor to
	 * existing proxies rather than double-proxy.
	 * 因为在beanPostProcessor执行的时候，会根据order值进行排序，此处设置为最低值，就是想让其最后执行
	 * 其实即使不设置这个值，因为AsyncAnnotationBeanPostProcessor继承了ProxyProcessorSupport，ProxyProcessorSupport中的order默认也是最小优先级
	 * 
	 */
	int order() default Ordered.LOWEST_PRECEDENCE;

}

2. AsyncConfigurationSelector的作用

public class AsyncConfigurationSelector extends AdviceModeImportSelector<EnableAsync> {
   
   

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


	/**
	 * Returns {@link ProxyAsyncConfiguration} or {@code AspectJAsyncConfiguration}
	 * for {@code PROXY} and {@code ASPECTJ} values of {@link EnableAsync#mode()},
	 * respectively.
	 */
	@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;
		}
	}

}