Java Spring之@Async原理案例详解

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.
* <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 — 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;
}
}

}

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<? 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;
}

}

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> executor, @Nullable Supplier<AsyncUncaughtExceptionHandler> exceptionHandler) {

Set<Class<? extends Annotation>> 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<? extends Annotation>)
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> executor, @Nullable Supplier<AsyncUncaughtExceptionHandler> 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 * 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<Object> task = () -> {
try {
Object result = invocation.proceed();
if (result instanceof Future) {
return ((Future<?>) result).get();
}
}
catch (ExecutionException ex) {
handleError(ex.getCause(), userDeclaredMethod, invocation.getArguments());
}
catch (Throwable ex) {
handleError(ex, userDeclaredMethod, invocation.getArguments());
}
return null;
};

//任务提交
return doSubmit(task, executor, invocation.getMethod().getReturnType());
}

可以看到主要做的事情是:

1. 寻找任务执行器：

2. 从@Async注解里面获取配置的任务执行器

3. 从Spring容器中找TaskExecutor类的bean

4. 从spring容器中获取名为"taskExecutor"的bean，

5. 如果还没有，new SimpleAsyncTaskExecutor()）可以看到其实我们是可以给@Async进行任务执行器的配置的。

6. 将具体的方法封装成callable的对象，然后doSubmit

7. 此处我们就看一下默认的doSumit，使用的SimpleAsyncTaskExecutor是如何实现的

8. 最终会执行到下面这个doExecute方法，默认情况下threadFactory是null，所以默认情况下，我们的方法，每次都是被创建了一个新的守护线程来进行方法的执行。

protected void doExecute(Runnable task) {
Thread thread = (this.threadFactory != null ? this.threadFactory.newThread(task) : createThread(task));
thread.start();
}

3.3.1.2 自定义任务执行器

1. 可以在配置类里new SimpleAsyncTaskExecutor()，然后setThreadFactory，这样修改了默认线程的产生方式

2. 比较主流的方式是，定义一个ThreadPoolTaskExecutor，也就是线程池任务执行器，可以进行线程复用

3.3.2 buildPointcut

/**
* Calculate a pointcut for the given async annotation types, if any.
* @param asyncAnnotationTypes the async annotation types to introspect
* @return the applicable Pointcut object, or {@code null} if none
*/
protected Pointcut buildPointcut(Set<Class<? extends Annotation>> asyncAnnotationTypes) {
ComposablePointcut result = null;
for (Class<? extends Annotation> asyncAnnotationType : asyncAnnotationTypes) {
// 就是根据这两个匹配器进行匹配的
// 检查类上是否有@Async注解
Pointcut cpc = new AnnotationMatchingPointcut(asyncAnnotationType, true);
//检查方法上是否有@Async注解
Pointcut mpc = new AnnotationMatchingPointcut(null, asyncAnnotationType, true);
if (result == null) {
result = new ComposablePointcut(cpc);
}
else {
// 取并集：类上加了@Async或者类的方法上加了@Async
result.union(cpc);
}
result = result.union(mpc);
}
return (result != null ? result : Pointcut.TRUE);
}

主要方法就是定义了一个类匹配pointcut和一个方法匹配pointcut。

4 什么时候判断进行advice的添加呢

当然就是在对某个bean进行proxy的判断的时候，也就是bean的生命周期最后一步，也是initializeBean里最后的一步，对于BeanPostProcessor的执行

3.4.1 AsyncAnnotationBeanPostProcessor#postProcessAfterInitialization

要注意的是AsyncAnnotationBeanPostProcessor的postProcessAfterInitialization方法其实是继承的是父类AbstractAdvisingBeanPostProcessor的。

@Override
public Object postProcessAfterInitialization(Object bean, String beanName) {
// 没有通知，或者是AOP的基础设施类，那么不进行代理
if (this.advisor == null || bean instanceof AopInfrastructureBean) {
// Ignore AOP infrastructure such as scoped proxies.
return bean;
}

// 对已经被代理的类，不再生成代理，只是将通知添加到代理类的逻辑中
// 这里通过beforeExistingAdvisors决定是将通知添加到所有通知之前还是添加到所有通知之后
// 在使用@Async注解的时候，beforeExistingAdvisors被设置成了true，
// @Async注解之所以把beforeExistingAdvisors设置为true，是因为该advisor和其他的advisor差别太大了，从情理上讲，也应该第一个执行
// 意味着整个方法及其拦截逻辑都会异步执行
if (bean instanceof Advised) {
Advised advised = (Advised) bean;
// 判断bean是否符合该advisor的使用范围，通过pointcut来判断
if (!advised.isFrozen() && isEligible(AopUtils.getTargetClass(bean))) {
// Add our local Advisor to the existing proxy's Advisor chain...
if (this.beforeExistingAdvisors) {
advised.addAdvisor(0, this.advisor);
}
else {
advised.addAdvisor(this.advisor);
}
return bean;
}
}

// 如果还不是一个代理类，也需要通过eligible来判断是否符合使用该advisor的条件
if (isEligible(bean, beanName)) {
ProxyFactory proxyFactory = prepareProxyFactory(bean, beanName);
if (!proxyFactory.isProxyTargetClass()) {
evaluateProxyInterfaces(bean.getClass(), proxyFactory);
}
proxyFactory.addAdvisor(this.advisor);
customizeProxyFactory(proxyFactory);
return proxyFactory.getProxy(getProxyClassLoader());
}

// No proxy needed.
return bean;
}

而在isEligible中，就是判断当前执行生命周期的bean是否满足我们的@Async注解的使用范围，主要是通过其class来判断

protected boolean isEligible(Class<?> targetClass) {
Boolean eligible = this.eligibleBeans.get(targetClass);
if (eligible != null) {
return eligible;
}
if (this.advisor == null) {
return false;
}
// 其实就是判断类是否可以进行添加该advisor，也就是判断是否符合该advisor的使用条件
// 就是把advisor的pointCut拿出来，pointCut里的classMatcher和methodMatcher拿出来对类及其方法进行判断
eligible = AopUtils.canApply(this.advisor, targetClass);
this.eligibleBeans.put(targetClass, eligible);
return eligible;
}

具体的AopUtils.canApply(this.advisor, targetClass)逻辑就不写了，就是根据pointcut里设置的classFilter和methodMatcher类判断当前bean的class是否需要进行该advisor的使用。

来源：https://blog.csdn.net/liangsheng_g/article/details/119852868