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前言
@Async这个注解想必大家都用过，是用来实现异步调用的。一个方法加上这个注解以后，当被调用时会使用新的线程来调用。但其实这里面也有一个坑。
问题
这个注解使用时存在如下问题：在没有自定义线程池的场景下，默认会采用SimpleAsyncTaskExecutor创建线程，线程池的最大大小为Integer的MAX_VALUE，相当于调用一次创建一个线程，缺乏线程重用机制。在并发大的场景下可能引发严重性能问题。下面是他的源代码：
/*** {@link TaskExecutor} implementation that fires up a new Thread for each task,* executing it asynchronously.** <p>Supports limiting concurrent threads through the "concurrencyLimit"* bean property. By default, the number of concurrent threads is unlimited.** <p><b>NOTE: This implementation does not reuse threads!</b> Consider a* thread-pooling TaskExecutor implementation instead, in particular for* executing a large number of short-lived tasks.*/
public class SimpleAsyncTaskExecutor extends CustomizableThreadCreatorimplements AsyncListenableTaskExecutor, Serializable {//省略不重要的方法@Overridepublic void execute(Runnable task, long startTimeout) {Assert.notNull(task, "Runnable must not be null");Runnable taskToUse = (this.taskDecorator != null ? this.taskDecorator.decorate(task) : task);if (isThrottleActive() && startTimeout > TIMEOUT_IMMEDIATE) {this.concurrencyThrottle.beforeAccess();doExecute(new ConcurrencyThrottlingRunnable(taskToUse));}else {doExecute(taskToUse);}}/*** 模板方法，用于实际执行任务.* <p>默认实现创建一个新线程并启动它*/protected void doExecute(Runnable task) {//如果threadFactory为空则直接创建线程执行。Thread thread = (this.threadFactory != null ? this.threadFactory.newThread(task) : createThread(task));thread.start();}}
那么如何解决这个问题呢？可以采用下面的方法：
自定义线程池
有如下几种方式可以配置线程池，一种配置默认线程池，让所有@Async自动共享或者配置单独的线程池，使用@Async时指定线程池。
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使用配置文件中配置默认线程池
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application.properties参考配置，yml文件同理。
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# 线程池创建时的初始化线程数，默认为8 spring.task.execution.pool.core-size=1 # 线程池的最大线`在这里插入代码片`程数，默认为int最大值 spring.task.execution.pool.max-size=1 # 用来缓冲执行任务的队列，默认为int最大值 spring.task.execution.pool.queue-capacity=10 # 线程终止前允许保持空闲的时间 spring.task.execution.pool.keep-alive=60s # 是否允许核心线程超时 spring.task.execution.pool.allow-core-thread-timeout=true # 是否等待剩余任务完成后才关闭应用 spring.task.execution.shutdown.await-termination=false # 等待剩余任务完成的最大时间 spring.task.execution.shutdown.await-termination-period= # 线程名的前缀，设置好了之后可以方便我们在日志中查看处理任务所在的线程池 spring.task.execution.thread-name-prefix=asynctask-
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通过实现接口配置默认线程池
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实现AsyncConfigurer覆盖getAsyncExecutor()方法。注意：这个方法的优先级比配置文件高。
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@Configuration @EnableAsync public class AsyncConfig implements AsyncConfigurer {public Executor getAsyncExecutor() {ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();executor.setCorePoolSize(3); //核心线程数executor.setMaxPoolSize(3); //最大线程数executor.setQueueCapacity(1000); //队列大小executor.setKeepAliveSeconds(600); //线程最大空闲时间executor.setThreadNamePrefix("async-Executor-"); //指定用于新创建的线程名称的前缀。executor.setRejectedExecutionHandler(new ThreadPoolExecutor.CallerRunsPolicy()); // 拒绝策略（一共四种，此处省略）// 这一步千万不能忘了，否则报错： java.lang.IllegalStateException: ThreadPoolTaskExecutor not initializedexecutor.initialize();return executor;} }
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单独配置线程池，使用@Async指定线程池
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这种方式可以给每个async的方法指定单独的线程池，但缺点是开发得知道怎么去设置。
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/** * 独立线程池配置 */ @Configuration public class TaskExecutorConfig {@Beanpublic TaskExecutor taskExecutor() {ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();// 设置核心线程数executor.setCorePoolSize(1);// 设置最大线程数executor.setMaxPoolSize(1);// 设置队列容量executor.setQueueCapacity(20);// 设置线程活跃时间（秒）executor.setKeepAliveSeconds(60);// 设置默认线程名称executor.setThreadNamePrefix("task-");// 设置拒绝策略executor.setRejectedExecutionHandler(new ThreadPoolExecutor.CallerRunsPolicy());// 等待所有任务结束后再关闭线程池executor.setWaitForTasksToCompleteOnShutdown(true);return executor;} }public class AsyncService {@Async("taskExecutor")public void task1() throws InterruptedException {TimeUnit.SECONDS.sleep(1L);log.info("task1 complete");}@Async("taskExecutor")public void task2() throws InterruptedException {TimeUnit.SECONDS.sleep(2L);log.info("task2 complete");}@Async("taskExecutor")public void task3() throws InterruptedException {TimeUnit.SECONDS.sleep(3L);log.info("task3 complete");} } -
下面是测试代码，大家可以用这个代码分别测试上述3种方式。
@RestController @RequestMapping("/async") public class AsyncController {@AutowiredAsyncService asyncService;@RequestMapping("/test")public String test() throws InterruptedException {asyncService.task1();asyncService.task2();asyncService.task3();return "success";} }@Service @Slf4j public class AsyncService {@Asyncpublic void task1() throws InterruptedException {TimeUnit.SECONDS.sleep(1L);log.info("task1 complete");}@Asyncpublic void task2() throws InterruptedException {TimeUnit.SECONDS.sleep(2L);log.info("task2 complete");}@Asyncpublic void task3() throws InterruptedException {TimeUnit.SECONDS.sleep(3L);log.info("task3 complete");} }
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