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学习注解，从注释和源码入手
部分关键注释，我自己标注了一些中文注释便于理解：
/**
* Enables Spring's asynchronous method execution capability, similar to functionality
* found in Spring's {@code <task:*>} XML namespace.
* <p>
* -- 与@Configuration注解配合使用，在Spring应用程序上下文中实现注解驱动的异步处理功能
* <p>To be used together with @{@link Configuration Configuration} classes as follows,
* enabling annotation-driven async processing for an entire Spring application context:
* <p>
* <pre class="code">
* @Configuration
* @EnableAsync
* public class AppConfig {<!-- -->
*
* }</pre>
* <p>
* {<!-- -->@code MyAsyncBean} is a user-defined type with one or more methods annotated with
* either Spring's {<!-- -->@code @Async} annotation, the EJB 3.1 {<!-- -->@code @javax.ejb.Asynchronous}
* annotation, or any custom annotation specified via the {<!-- -->@link #annotation} attribute.
* The aspect is added transparently for any registered bean, for instance via this
* configuration:
*
* <pre class="code">
* @Configuration
* public class AnotherAppConfig {<!-- -->
*
* @;Bean
* public MyAsyncBean asyncBean() {<!-- -->
* return new MyAsyncBean();
* }
* }</pre>
* <p>
* <p>By default, Spring will be searching for an associated thread pool definition:
* either a unique {<!-- -->@link org.springframework.core.task.TaskExecutor} bean in the context,
* or an {<!-- -->@link java.util.concurrent.Executor} bean named "taskExecutor" otherwise. If
* neither of the two is resolvable, a {<!-- -->@link org.springframework.core.task.SimpleAsyncTaskExecutor}
* will be used to process async method invocations. Besides, annotated methods having a
* {<!-- -->@code void} return type cannot transmit any exception back to the caller. By default,
* such uncaught exceptions are only logged.
* <p>
* <p>To customize all this, implement {<!-- -->@link AsyncConfigurer} and provide:
* <ul>
* <li>your own {<!-- -->@link java.util.concurrent.Executor Executor} through the
* {<!-- -->@link AsyncConfigurer#getAsyncExecutor getAsyncExecutor()} method, and</li>
* <li>your own {<!-- -->@link org.springframework.aop.interceptor.AsyncUncaughtExceptionHandler
* AsyncUncaughtExceptionHandler} through the {<!-- -->@link AsyncConfigurer#getAsyncUncaughtExceptionHandler
* getAsyncUncaughtExceptionHandler()}
* method.</li>
* </ul>
* <p><b>NOTE: {<!-- -->@link AsyncConfigurer} configuration classes get initialized early
* in the application context bootstrap. If you need any dependencies on other beans
* there, make sure to declare them 'lazy' as far as possible in order to let them
* go through other post-processors as well.</b>
* <p>
* <pre class="code">
* @Configuration
* @EnableAsync
* public class AppConfig implements AsyncConfigurer {<!-- -->
*
* @Override
* public Executor getAsyncExecutor() {<!-- -->
* ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
* executor.setCorePoolSize(7);
* executor.setMaxPoolSize(42);
* executor.setQueueCapacity(11);
* executor.setThreadNamePrefix("MyExecutor-");
* executor.initialize();
* return executor;
* }
*
* @Override
* public AsyncUncaughtExceptionHandler getAsyncUncaughtExceptionHandler() {<!-- -->
* return new MyAsyncUncaughtExceptionHandler();
* }
* }</pre>
*/
@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.
*/
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.
*/
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}.
*/
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.
*/
int order() default Ordered.LOWEST_PRECEDENCE;
点进@EnableAsync的@Import(AsyncConfigurationSelector.class)
AsyncConfigurationSelector中，selectImports方法在spring容器扫描bean的时候，根据通知模型判断要导入哪一个类
/**
* 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;
}
}
默认是proxy
返回的类为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;
}
}
这个类是一个配置类，实现了BeanPostProcessor接口，它向spring容器中添加了一个AsyncAnnotationBeanPostProcessor的bean后置处理器，其父类AbstractAdvisingBeanPostProcessor中定义了处理器
@Override
public Object postProcessBeforeInitialization(Object bean, String beanName) {<!-- -->
return bean;
}
@Override
public Object postProcessAfterInitialization(Object bean, String beanName) {<!-- -->
if (this.advisor == null || bean instanceof AopInfrastructureBean) {<!-- -->
// Ignore AOP infrastructure such as scoped proxies.
return bean;
}
// 类中加@Async的bean会走到这个if里，添加advisor
if (bean instanceof Advised) {<!-- -->
Advised advised = (Advised) bean;
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;
}
}
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;
}
可以看出前置处理器未做处理，后置处理器对添加了@Async的bean进行了addAdvisor，后续使用bean时，debug看一下，该bean中会有响应的advisor
@EnableAsync通过向Spring引入后置处理器AsyncAnnotationBeanPostProcessor，在bean的创建过程中对bean进行advisor增强，对@Async标识的bean增强异步功能