关于@Async 注解

| 简介

应用场景

[同步： 同步就是整个处理过程顺序执行，当各个过程都执行完毕，并返回结果。]

[异步： 异步调用则是只是发送了调用的指令，调用者无需等待被调用的方法完全执行完毕；而是继续执行下面的流程。]

[例如，在某个调用中，需要顺序调用 A, B, C三个过程方法；如他们都是同步调用，则需要将他们都顺序执行完毕之后，方算作过程执行完毕；如B为一个异步的调用方法，则在执行完A之后，调用B，并不等待B完成，而是执行开始调用C，待C执行完毕之后，就意味着这个过程执行完毕了。在Java中，一般在处理类似的场景之时，都是基于创建独立的线程去完成相应的异步调用逻辑，通过主线程和不同的业务子线程之间的执行流程，从而在启动独立的线程之后，主线程继续执行而不会产生停滞等待的情况。]

Spring 已经实现的线程池

[SimpleAsyncTaskExecutor：不是真的线程池，这个类不重用线程，默认每次调用都会创建一个新的线程。]

[SyncTaskExecutor：这个类没有实现异步调用，只是一个同步操作。只适用于不需要多线程的地方。]

[ConcurrentTaskExecutor：Executor的适配类，不推荐使用。如果ThreadPoolTaskExecutor不满足要求时，才用考虑使用这个类。]

[SimpleThreadPoolTaskExecutor：是Quartz的SimpleThreadPool的类。线程池同时被quartz和非quartz使用，才需要使用此类。

[ThreadPoolTaskExecutor ：最常使用，推荐。其实质是对java.util.concurrent.ThreadPoolExecutor的包装。]

异步的方法有：

[最简单的异步调用，返回值为void。]
[带参数的异步调用，异步方法可以传入参数。]
[存在返回值，常调用返回Future。]

| Spring中启用@Async

<pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="" cid="n24" mdtype="fences" style="font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-indent: 0px; text-transform: none; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; box-sizing: border-box; overflow: visible; font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace; margin-top: 0px; margin-bottom: 20px; font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; white-space: normal; background-color: rgb(51, 51, 51); padding: 10px 10px 10px 30px; width: inherit; caret-color: rgb(184, 191, 198); color: rgb(184, 191, 198); position: relative !important;">// 基于Java配置的启用方式：
@Configuration
@EnableAsync
public class SpringAsyncConfig { ... }

// Spring boot启用：
@EnableAsync
@EnableTransactionManagement
public class SettlementApplication {
public static void main(String[] args) {
SpringApplication.run(SettlementApplication.class, args);
}
}</pre>

| @Async应用默认线程池

[Spring应用默认的线程池，指在@Async注解在使用时，不指定线程池的名称。查看源码，@Async的默认线程池为SimpleAsyncTaskExecutor。]

[无返回值调用]

[基于@Async无返回值调用，直接在使用类，使用方法（建议在使用方法）上，加上注解。若需要抛出异常，需手动new一个异常抛出。]

<pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="" cid="n29" mdtype="fences" style="font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-indent: 0px; text-transform: none; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; box-sizing: border-box; overflow: visible; font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace; margin-top: 0px; margin-bottom: 20px; font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; white-space: normal; background-color: rgb(51, 51, 51); padding: 10px 10px 10px 30px; width: inherit; caret-color: rgb(184, 191, 198); color: rgb(184, 191, 198); position: relative !important;">/**
* 带参数的异步调用异步方法可以传入参数
* 对于返回值是void，异常会被AsyncUncaughtExceptionHandler处理掉
* @param s
*/
@Async
public void asyncInvokeWithException(String s) {
log.info("asyncInvokeWithParameter, parementer={}", s);
throw new IllegalArgumentException(s);
}</pre>

有返回值Future调用

<pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="" cid="n31" mdtype="fences" style="font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-indent: 0px; text-transform: none; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; box-sizing: border-box; overflow: visible; font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace; margin-top: 0px; margin-bottom: 20px; font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; white-space: normal; background-color: rgb(51, 51, 51); padding: 10px 10px 10px 30px; width: inherit; caret-color: rgb(184, 191, 198); color: rgb(184, 191, 198); position: relative !important;">/**
* 异常调用返回Future
* 对于返回值是Future，不会被AsyncUncaughtExceptionHandler处理，需要我们在方法中捕获异常并处理
* 或者在调用方在调用Futrue.get时捕获异常进行处理
*
* @param I
* @return
*/
@Async
public Future<String> asyncInvokeReturnFuture(int i) {
log.info("asyncInvokeReturnFuture, parementer={}", i);
Future<String> future;
try {
Thread.sleep(1000 * 1);
future = new AsyncResult<String>("success:" + i);
throw new IllegalArgumentException("a");
} catch (InterruptedException e) {
future = new AsyncResult<String>("error");
} catch(IllegalArgumentException e){
future = new AsyncResult<String>("error-IllegalArgumentException");
}
return future;
}</pre>

有返回值CompletableFuture调用

[CompletableFuture 并不使用@Async注解，可达到调用系统线程池处理业务的功能。]

[JDK5新增了Future接口，用于描述一个异步计算的结果。虽然 Future 以及相关使用方法提供了异步执行任务的能力，但是对于结果的获取却是很不方便，只能通过阻塞或者轮询的方式得到任务的结果。阻塞的方式显然和我们的异步编程的初衷相违背，轮询的方式又会耗费无谓的 CPU 资源，而且也不能及时地得到计算结果。]

[CompletionStage代表异步计算过程中的某一个阶段，一个阶段完成以后可能会触发另外一个阶段]

[一个阶段的计算执行可以是一个Function，Consumer或者Runnable。比如：stage.thenApply(x -> square(x)).thenAccept(x -> System.out.print(x)).thenRun(() -> System.out.println())。]

[一个阶段的执行可能是被单个阶段的完成触发，也可能是由多个阶段一起触发。]

[在Java8中，CompletableFuture提供了非常强大的Future的扩展功能，可以帮助我们简化异步编程的复杂性，并且提供了函数式编程的能力，可以通过回调的方式处理计算结果，也提供了转换和组合 CompletableFuture 的方法。]

[它可能代表一个明确完成的Future，也有可能代表一个完成阶段（ CompletionStage ），它支持在计算完成以后触发一些函数或执行某些动作。]
[它实现了Future和CompletionStage接口。]

<pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="" cid="n44" mdtype="fences" style="font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-indent: 0px; text-transform: none; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; box-sizing: border-box; overflow: visible; font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace; margin-top: 0px; margin-bottom: 20px; font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; white-space: normal; background-color: rgb(51, 51, 51); padding: 10px 10px 10px 30px; width: inherit; caret-color: rgb(184, 191, 198); color: rgb(184, 191, 198); position: relative !important;">/**
* 数据查询线程池
*/
private static final ThreadPoolExecutor SELECT_POOL_EXECUTOR = new ThreadPoolExecutor(10, 20, 5000,
TimeUnit.MILLISECONDS, new LinkedBlockingQueue<>(1024), new ThreadFactoryBuilder().setNameFormat("selectThreadPoolExecutor-%d").build());

// tradeMapper.countTradeLog(tradeSearchBean)方法表示，获取数量，返回值为int
// 获取总条数
CompletableFuture<Integer> countFuture = CompletableFuture
.supplyAsync(() -> tradeMapper.countTradeLog(tradeSearchBean), SELECT_POOL_EXECUTOR);
// 同步阻塞
CompletableFuture.allOf(countFuture).join();
// 获取结果
int count = countFuture.get();</pre>

默认线程池的弊端

[在线程池应用中，参考阿里巴巴java开发规范：线程池不允许使用Executors去创建，不允许使用系统默认的线程池，推荐通过ThreadPoolExecutor的方式，这样的处理方式让开发的工程师更加明确线程池的运行规则，规避资源耗尽的风险。]

Executors各个方法的弊端：]

[newFixedThreadPool和newSingleThreadExecutor：主要问题是堆积的请求处理队列可能会耗费非常大的内存，甚至OOM。]
[newCachedThreadPool和newScheduledThreadPool：要问题是线程数最大数是Integer.MAX_VALUE，可能会创建数量非常多的线程，甚至OOM。]

[@Async默认异步配置使用的是SimpleAsyncTaskExecutor，该线程池默认来一个任务创建一个线程，若系统中不断的创建线程，最终会导致系统占用内存过高，引发OutOfMemoryError错误。针对线程创建问题，SimpleAsyncTaskExecutor提供了限流机制，通过concurrencyLimit属性来控制开关，当concurrencyLimit>=0时开启限流机制，默认关闭限流机制即concurrencyLimit=-1，当关闭情况下，会不断创建新的线程来处理任务。基于默认配置，SimpleAsyncTaskExecutor并不是严格意义的线程池，达不到线程复用的功能。]

| @Async应用自定义线程池

[自定义线程池，可对系统中线程池更加细粒度的控制，方便调整线程池大小配置，线程执行异常控制和处理。在设置系统自定义线程池代替默认线程池时，虽可通过多种模式设置，但替换默认线程池最终产生的线程池有且只能设置一个（不能设置多个类继承AsyncConfigurer）。自定义线程池有如下模式：]

[重新实现接口AsyncConfigurer；]
[继承AsyncConfigurerSupport；]
[配置由自定义的TaskExecutor替代内置的任务执行器。]

[通过查看Spring源码关于@Async的默认调用规则，会优先查询源码中实现AsyncConfigurer这个接口的类，实现这个接口的类为AsyncConfigurerSupport。但默认配置的线程池和异步处理方法均为空，所以，无论是继承或者重新实现接口，都需指定一个线程池。且重新实现 public Executor getAsyncExecutor()方法。]

实现接口AsyncConfigurer

<pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="" cid="n64" mdtype="fences" style="font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-indent: 0px; text-transform: none; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; box-sizing: border-box; overflow: visible; font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace; margin-top: 0px; margin-bottom: 20px; font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; white-space: normal; background-color: rgb(51, 51, 51); padding: 10px 10px 10px 30px; width: inherit; caret-color: rgb(184, 191, 198); color: rgb(184, 191, 198); position: relative !important;">@Configuration
public class AsyncConfiguration implements AsyncConfigurer {
@Bean("kingAsyncExecutor")
public ThreadPoolTaskExecutor executor() {
ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
int corePoolSize = 10;
executor.setCorePoolSize(corePoolSize);
int maxPoolSize = 50;
executor.setMaxPoolSize(maxPoolSize);
int queueCapacity = 10;
executor.setQueueCapacity(queueCapacity);
executor.setRejectedExecutionHandler(new ThreadPoolExecutor.CallerRunsPolicy());
String threadNamePrefix = "kingDeeAsyncExecutor-";
executor.setThreadNamePrefix(threadNamePrefix);
executor.setWaitForTasksToCompleteOnShutdown(true);
// 使用自定义的跨线程的请求级别线程工厂类19 int awaitTerminationSeconds = 5;
executor.setAwaitTerminationSeconds(awaitTerminationSeconds);
executor.initialize();
return executor;
}

@Override
public Executor getAsyncExecutor() {
    return executor();
}

@Override
public AsyncUncaughtExceptionHandler getAsyncUncaughtExceptionHandler() {
    return (ex, method, params) -> ErrorLogger.getInstance().log(String.format("执行异步任务'%s'", method), ex);
}

}</pre>

继承AsyncConfigurerSupport

<pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="" cid="n66" mdtype="fences" style="font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-indent: 0px; text-transform: none; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; box-sizing: border-box; overflow: visible; font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace; margin-top: 0px; margin-bottom: 20px; font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; white-space: normal; background-color: rgb(51, 51, 51); padding: 10px 10px 10px 30px; width: inherit; caret-color: rgb(184, 191, 198); color: rgb(184, 191, 198); position: relative !important;">@Configuration
@EnableAsync
class SpringAsyncConfigurer extends AsyncConfigurerSupport {

@Bean
public ThreadPoolTaskExecutor asyncExecutor() {
ThreadPoolTaskExecutor threadPool = new ThreadPoolTaskExecutor();
threadPool.setCorePoolSize(3);
threadPool.setMaxPoolSize(3);
threadPool.setWaitForTasksToCompleteOnShutdown(true);
threadPool.setAwaitTerminationSeconds(60 * 15);
return threadPool;
}

@Override
public Executor getAsyncExecutor() {
return asyncExecutor;
}

@Override
public AsyncUncaughtExceptionHandler getAsyncUncaughtExceptionHandler() {
return (ex, method, params) -> ErrorLogger.getInstance().log(String.format("执行异步任务'%s'", method), ex);
}
}</pre>

配置自定义的TaskExecutor

[由于AsyncConfigurer的默认线程池在源码中为空，Spring通过beanFactory.getBean(TaskExecutor.class)先查看是否有线程池，未配置时，通过beanFactory.getBean(DEFAULT_TASK_EXECUTOR_BEAN_NAME, Executor.class)，又查询是否存在默认名称为TaskExecutor的线程池。所以可以在项目中，定义名称为TaskExecutor的bean生成一个默认线程池。也可不指定线程池的名称，申明一个线程池，本身底层是基于TaskExecutor.class便可。比如：]

<pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="" cid="n69" mdtype="fences" style="font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-indent: 0px; text-transform: none; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; box-sizing: border-box; overflow: visible; font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace; margin-top: 0px; margin-bottom: 20px; font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; white-space: normal; background-color: rgb(51, 51, 51); padding: 10px 10px 10px 30px; width: inherit; caret-color: rgb(184, 191, 198); color: rgb(184, 191, 198); position: relative !important;">Executor.class:ThreadPoolExecutorAdapter->ThreadPoolExecutor->AbstractExecutorService->ExecutorService->Executor</pre>

这样的模式，最终底层为Executor.class，在替换默认的线程池时，需设置默认的线程池名称为TaskExecutor。

<pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="" cid="n71" mdtype="fences" style="font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-indent: 0px; text-transform: none; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; box-sizing: border-box; overflow: visible; font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace; margin-top: 0px; margin-bottom: 20px; font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; white-space: normal; background-color: rgb(51, 51, 51); padding: 10px 10px 10px 30px; width: inherit; caret-color: rgb(184, 191, 198); color: rgb(184, 191, 198); position: relative !important;">TaskExecutor.class:ThreadPoolTaskExecutor->SchedulingTaskExecutor->AsyncTaskExecutor->TaskExecutor</pre>

这样的模式，最终底层为TaskExecutor.class，在替换默认的线程池时，可不指定线程池名称。

<pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="" cid="n73" mdtype="fences" style="font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-indent: 0px; text-transform: none; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; box-sizing: border-box; overflow: visible; font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace; margin-top: 0px; margin-bottom: 20px; font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; white-space: normal; background-color: rgb(51, 51, 51); padding: 10px 10px 10px 30px; width: inherit; caret-color: rgb(184, 191, 198); color: rgb(184, 191, 198); position: relative !important;">@EnableAsync
@Configuration
public class TaskPoolConfig {
@Bean(name = AsyncExecutionAspectSupport.DEFAULT_TASK_EXECUTOR_BEAN_NAME)
public Executor taskExecutor() {
ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
//核心线程池大小
executor.setCorePoolSize(10);
//最大线程数
executor.setMaxPoolSize(20);
//队列容量
executor.setQueueCapacity(200);
//活跃时间
executor.setKeepAliveSeconds(60);
//线程名字前缀
executor.setThreadNamePrefix("taskExecutor-");
executor.setRejectedExecutionHandler(new ThreadPoolExecutor.CallerRunsPolicy());
return executor;
}
@Bean(name = "new_task")
public Executor taskExecutor() {
ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
//核心线程池大小
executor.setCorePoolSize(10);
//最大线程数
executor.setMaxPoolSize(20);
//队列容量
executor.setQueueCapacity(200);
//活跃时间
executor.setKeepAliveSeconds(60);
//线程名字前缀
executor.setThreadNamePrefix("taskExecutor-");
executor.setRejectedExecutionHandler(new ThreadPoolExecutor.CallerRunsPolicy());
return executor;
}
}</pre>