Spring @Async 注解的使用以及原理（二）

       在上一篇中《Spring @Async 注解的使用以及原理（一）》简单介绍了@Async的使用，本篇简单分析一下原理，源码版本：spring-context-5.0.5.RELEASE.

@EnableAsync注解：

/**
 * Enables Spring's asynchronous method execution capability, similar to functionality
 * found in Spring's {@code <task:*>} XML namespace.
 *
 * <p>To be used together with @{@link Configuration Configuration} classes as follows,
 * enabling annotation-driven async processing for an entire Spring application context:
 *
 * <pre class="code">
 * &#064;Configuration
 * &#064;EnableAsync
 * public class AppConfig {
 *
 * }</pre>
 *
 * {@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">
 * &#064;Configuration
 * public class AnotherAppConfig {
 *
 *     &#064;Bean
 *     public MyAsyncBean asyncBean() {
 *         return new MyAsyncBean();
 *     }
 * }</pre>
 * 
 * （以上部分展示了与 @Configuration 注解搭配使用的场景）
 *
 * <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.
 *
 * (默认情况下，Spring寻找一个唯一的TaskExecutor类型的bean 或者 bean名称是“taskExecutor”的Executor类型的bean。
 * 如果二者都不存在，则使用SimpleAsyncTaskExecutor进行异步方法的执行.) 
 * 返回类型为void 无法将任何异常传送回调用方。 默认情况下，仅记录此类未捕获的异常。
 *
 * <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>
 *
 * <pre class="code">
 * &#064;Configuration
 * &#064;EnableAsync
 * public class AppConfig implements AsyncConfigurer {
 *
 *     &#064;Override
 *     public Executor getAsyncExecutor() {
 *         ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
 *         executor.setCorePoolSize(7);
 *         executor.setMaxPoolSize(42);
 *         executor.setQueueCapacity(11);
 *         executor.setThreadNamePrefix("MyExecutor-");
 *         executor.initialize();
 *         return executor;
 *     }
 *
 *     &#064;Override
 *     public AsyncUncaughtExceptionHandler getAsyncUncaughtExceptionHandler() {
 *         return MyAsyncUncaughtExceptionHandler();
 *     }
 * }</pre>
 *
 * <p>If only one item needs to be customized, {@code null} can be returned to