Java Spring之@Async原理案例详解

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：

1.在配置类上添加@EnableAsync注解

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

2.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.
	 * 
By default, both Spring's @{@link Async} annotation and the EJB 3.1
	 * {@code @javax.ejb.Asynchronous} annotation will be detected.
	 * 
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 annotation() default Annotation.class;

	/**
	 * Indicate whether subclass-based (CGLIB) proxies are to be created as opposed
	 * to standard Java interface-based proxies.
	 * 
Applicable only if the {@link #mode} is set to {@link AdviceMode#PROXY}.
	 * 
The default is {@code false}.
	 * 
Note that setting this attribute to {@code true} will affect all
	 * 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 — 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.
	 * 
The default is {@link AdviceMode#PROXY}.
	 * 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.
	 * 
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 {

	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;
		}
	}

}

看过我之前博客的同学应该知道，其实此处就是往Spring容器中增加一个新的需要扫描的类，很明显可以看到差别主要集中在adviceMode的差别上。

3. adviceMode:PROXY(默认值)

引入了ProxyAsyncConfiguration配置类

3.1 ProxyAsyncConfiguration

@Configuration
@Role(BeanDefinition.ROLE_INFRASTRUCTURE)
public class ProxyAsyncConfiguration extends AbstractAsyncConfiguration {

	@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 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.getNumber("order"));
		return bpp;
	}

}

作用也很明显，就是往spring容器中添加了AsyncAnnotationBeanPostProcessor类

3.2 AsyncAnnotationBeanPostProcessor

public class AsyncAnnotationBeanPostProcessor extends AbstractBeanFactoryAwareAdvisingPostProcessor {

    // 删除了一些无关紧要，或者默认不会设置的属性
	public AsyncAnnotationBeanPostProcessor() {
		setBeforeExistingAdvisors(true);
	}

	/**
	 * 因为AsyncAnnotationBeanPostProcessor实现了BeanFactoryAware接口
	 * 所以在实例化的过程中执行到initializeBean步骤的时候，里面第一步就是执行各种实现了Aware接口的接口方法
	 * 在此处new了一个advisor。advisor简单理解就是：advice+pointcut
	 * @param beanFactory
	 */
	@Override
	public void setBeanFactory(BeanFactory beanFactory) {
		super.setBeanFactory(beanFactory);
		AsyncAnnotationAdvisor advisor = new AsyncAnnotationAdvisor(this.executor, this.exceptionHandler);
		if (this.asyncAnnotationType != null) {
			advisor.setAsyncAnnotationType(this.asyncAnnotationType);
		}
		advisor.setBeanFactory(beanFactory);
		this.advisor = advisor;
	}

}

其实可以看到最重要的方法，就是setBeanFactory了，该方法是在AsyncAnnotationBeanPostProcessor的生命周期最后一步initializeBean里面的第一小步，也就是执行所有Aware接口的时候执行。
对于AOP来说，其实最主要的就是advice+pointcut，也就是advisor，在生命周期的这一步，也创建了advisor。

3.3 AsyncAnnotationAdvisor

public AsyncAnnotationAdvisor(
			@Nullable Supplier executor, @Nullable Supplier exceptionHandler) {

		Set> asyncAnnotationTypes = new LinkedHashSet<>(2);
		/**
		 * 这儿设置符合pointCut需要的注解
		 * 此处的executor就是一个扩展点，如果不想用spring的默认单线程线程池，可以自定义一个线程池
		 * exceptionHandler，顾名思义，就是我们的方法在线程池中执行时抛出exception该如何handle使用的
		 * advice也就是咱们的interceptor
		 * pointCut就不多解释了，就是把设置符合什么条件会进行interceptor的invoke方法
		 */
		asyncAnnotationTypes.add(Async.class);
		try {
			asyncAnnotationTypes.add((Class)
					ClassUtils.forName("javax.ejb.Asynchronous", AsyncAnnotationAdvisor.class.getClassLoader()));
		}
		catch (ClassNotFoundException ex) {
			// If EJB 3.1 API not present, simply ignore.
		}
		this.advice = buildAdvice(executor, exceptionHandler);
		this.pointcut = buildPointcut(asyncAnnotationTypes);
	}

可以看到最主要的工作就是buildAdvice和buildPointcut。advice的作用是定义在方法执行方面，该如何执行；pointcut的作用是定义方法的范围

3.3.1 buildAdvice

protected Advice buildAdvice(
			@Nullable Supplier executor, @Nullable Supplier exceptionHandler) {
		// new了一个interceptor
		AnnotationAsyncExecutionInterceptor interceptor = new AnnotationAsyncExecutionInterceptor(null);
		interceptor.configure(executor, exceptionHandler);
		return interceptor;
	}

可以看到advice主要就是定义了一个烂机器interceptor，在方法执行的时候进行一些拦截，至于executor，是方法执行器，默认为null，exceptionHandler也默认是null。

3.3.1.1 AnnotationAsyncExecutionInterceptor，异步执行的原理

在AnnotationAsyncExecutionInterceptor的父类AsyncExecutionInterceptor中，实现了拦截器的接口方法invoke，也就是真实的方法执行逻辑。

/**
	 * Intercept the given method invocation, submit the actual calling of the method to
	 * the correct task executor and return immediately to the caller.
	 * @param invocation the method to intercept and make asynchronous
	 * @return {@link Future} if the original method returns {@code Future}; {@code null}
	 * otherwise.
	 */
	@Override
	@Nullable
	public Object invoke(final MethodInvocation invocation) throws Throwable {
		Class targetClass = (invocation.getThis() != null ? AopUtils.getTargetClass(invocation.getThis()) : null);
		Method specificMethod = ClassUtils.getMostSpecificMethod(invocation.getMethod(), targetClass);
		final Method userDeclaredMethod = BridgeMethodResolver.findBridgedMethod(specificMethod);
		/**获取一个任务执行器
		 * 1. 从@Async注解里面获取配置的任务执行器
		 * 2. 从Spring容器中找TaskExecutor类的bean
		 * 3. 从spring容器中获取名为"taskExecutor"的bean，
		 * 4. 如果还没有，new SimpleAsyncTaskExecutor()）
		 */
		AsyncTaskExecutor executor = determineAsyncExecutor(userDeclaredMethod);
		if (executor == null) {
			throw new IllegalStateException(
					"No executor specified and no default executor set on AsyncExecutionInterceptor either");
		}

		//将当前方法执行封装成一个callable对象，然后放入到线程池里
		Callable