本文带你快速了解@Async注解的用法,包括异步方法无返回值、有返回值,最后总结了@Async注解失效的几个坑。

在 SpringBoot 应用中,经常会遇到在一个接口中,同时做事情1,事情2,事情3,如果同步执行的话,则本次接口时间取决于事情1 2 3执行时间之和;如果三件事同时执行,则本次接口时间取决于事情1 2 3执行时间最长的那个,合理使用多线程,可以大大缩短接口时间。那么在 SpringBoot 应用中如何优雅的使用多线程呢?

Don"t bb, show me code.


(相关资料图)

快速使用

SpringBoot应用中需要添加@EnableAsync注解,来开启异步调用,一般还会配置一个线程池,异步的方法交给特定的线程池完成,如下:

@Configuration@EnableAsyncpublic class AsyncConfiguration {    @Bean("doSomethingExecutor")    public Executor doSomethingExecutor() {        ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();        // 核心线程数:线程池创建时候初始化的线程数        executor.setCorePoolSize(10);        // 最大线程数:线程池最大的线程数,只有在缓冲队列满了之后才会申请超过核心线程数的线程        executor.setMaxPoolSize(20);        // 缓冲队列:用来缓冲执行任务的队列        executor.setQueueCapacity(500);        // 允许线程的空闲时间60秒:当超过了核心线程之外的线程在空闲时间到达之后会被销毁        executor.setKeepAliveSeconds(60);        // 线程池名的前缀:设置好了之后可以方便我们定位处理任务所在的线程池        executor.setThreadNamePrefix("do-something-");        // 缓冲队列满了之后的拒绝策略:由调用线程处理(一般是主线程)        executor.setRejectedExecutionHandler(new ThreadPoolExecutor.DiscardPolicy());        executor.initialize();        return executor;    }}

使用的方式非常简单,在需要异步的方法上加@Async注解

@RestControllerpublic class AsyncController {    @Autowired    private AsyncService asyncService;    @GetMapping("/open/something")    public String something() {        int count = 10;        for (int i = 0; i < count; i++) {            asyncService.doSomething("index = " + i);        }        lon        return "success";    }}@Slf4j@Servicepublic class AsyncService {    // 指定使用beanname为doSomethingExecutor的线程池    @Async("doSomethingExecutor")    public String doSomething(String message) {        log.info("do something, message={}", message);        try {            Thread.sleep(1000);        } catch (InterruptedException e) {            log.error("do something error: ", e);        }        return message;    }}

访问:127.0.0.1:8080/open/something,日志如下

2020-04-19 23:42:42.486  INFO 21168 --- [io-8200-exec-17] x.g.b.system.controller.AsyncController  : do something end, time 8 milliseconds2020-04-19 23:42:42.488  INFO 21168 --- [ do-something-1] x.gits.boot.system.service.AsyncService  : do something, message=index = 02020-04-19 23:42:42.488  INFO 21168 --- [ do-something-5] x.gits.boot.system.service.AsyncService  : do something, message=index = 42020-04-19 23:42:42.488  INFO 21168 --- [ do-something-4] x.gits.boot.system.service.AsyncService  : do something, message=index = 32020-04-19 23:42:42.488  INFO 21168 --- [ do-something-6] x.gits.boot.system.service.AsyncService  : do something, message=index = 52020-04-19 23:42:42.488  INFO 21168 --- [ do-something-9] x.gits.boot.system.service.AsyncService  : do something, message=index = 82020-04-19 23:42:42.488  INFO 21168 --- [ do-something-8] x.gits.boot.system.service.AsyncService  : do something, message=index = 72020-04-19 23:42:42.488  INFO 21168 --- [do-something-10] x.gits.boot.system.service.AsyncService  : do something, message=index = 92020-04-19 23:42:42.488  INFO 21168 --- [ do-something-7] x.gits.boot.system.service.AsyncService  : do something, message=index = 62020-04-19 23:42:42.488  INFO 21168 --- [ do-something-2] x.gits.boot.system.service.AsyncService  : do something, message=index = 12020-04-19 23:42:42.488  INFO 21168 --- [ do-something-3] x.gits.boot.system.service.AsyncService  : do something, message=index = 2

由此可见已经达到异步执行的效果了,并且使用到了咱们配置的线程池。

获取异步方法返回值

当异步方法有返回值时,如何获取异步方法执行的返回结果呢?这时需要异步调用的方法带有返回值CompletableFuture。

CompletableFuture是对Feature的增强,Feature只能处理简单的异步任务,而CompletableFuture可以将多个异步任务进行复杂的组合。如下:

@RestControllerpublic class AsyncController {    @Autowired    private AsyncService asyncService;    @SneakyThrows    @ApiOperation("异步 有返回值")    @GetMapping("/open/somethings")    public String somethings() {        CompletableFuture createOrder = asyncService.doSomething1("create order");        CompletableFuture reduceAccount = asyncService.doSomething2("reduce account");        CompletableFuture saveLog = asyncService.doSomething3("save log");        // 等待所有任务都执行完        CompletableFuture.allOf(createOrder, reduceAccount, saveLog).join();        // 获取每个任务的返回结果        String result = createOrder.get() + reduceAccount.get() + saveLog.get();        return result;    }}@Slf4j@Servicepublic class AsyncService {    @Async("doSomethingExecutor")    public CompletableFuture doSomething1(String message) throws InterruptedException {        log.info("do something1: {}", message);        Thread.sleep(1000);        return CompletableFuture.completedFuture("do something1: " + message);    }    @Async("doSomethingExecutor")    public CompletableFuture doSomething2(String message) throws InterruptedException {        log.info("do something2: {}", message);        Thread.sleep(1000);        return CompletableFuture.completedFuture("; do something2: " + message);    }    @Async("doSomethingExecutor")    public CompletableFuture doSomething3(String message) throws InterruptedException {        log.info("do something3: {}", message);        Thread.sleep(1000);        return CompletableFuture.completedFuture("; do something3: " + message);    }}

访问接口

C:\Users\Administrator>curl -X GET "http://localhost:8200/open/somethings" -H "accept: */*"do something1: create order; do something2: reduce account; do something3: save log

控制台上关键日志如下:

2020-04-20 00:27:42.238  INFO 5672 --- [ do-something-3] x.gits.boot.system.service.AsyncService  : do something3: save log2020-04-20 00:27:42.238  INFO 5672 --- [ do-something-2] x.gits.boot.system.service.AsyncService  : do something2: reduce account2020-04-20 00:27:42.238  INFO 5672 --- [ do-something-1] x.gits.boot.system.service.AsyncService  : do something1: create order

注意事项

@Async注解会在以下几个场景失效,也就是说明明使用了@Async注解,但就没有走多线程。

异步方法使用static关键词修饰;异步类不是一个Spring容器的bean(一般使用注解@Component@Service,并且能被Spring扫描到);SpringBoot应用中没有添加@EnableAsync注解;在同一个类中,一个方法调用另外一个有@Async注解的方法,注解不会生效。原因是@Async注解的方法,是在代理类中执行的。

通过上边几个示例,@Async实际还是通过Future或CompletableFuture来异步执行的,Spring又封装了一下,让我们使用的更方便。

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