在使用异步注解之前，我们需要先了解，什么是异步调用？

异步调用对应的事同步调用，同步调用是值程序按照我们定义的顺序依次执行，每一行程序都必须等待上一行的程序执行完成之后才执行，而异步是指在顺序执行的时候，不等待异步调用的语句返回结果就执行后面的程序，例如，我们常常进行对报表的导出，因为数据导出需要时间，为了避免用户在界面等待时间过长，我们就会采用异步进行导出。

我们来对比一下同步导出和异步导出的案例：

@Slf4j
@Component
public class AsyncTasks {

    public static Random random = new Random();

    //@Async
    public CompletableFuture<String> doTaskOne() throws Exception {
        log.info("开始做任务一");
        long start = System.currentTimeMillis();
        Thread.sleep(random.nextInt(10000));
        long end = System.currentTimeMillis();
        log.info("完成任务一，耗时：" + (end - start) + "毫秒");
        return CompletableFuture.completedFuture("任务一完成");
    }

    //@Async
    public CompletableFuture<String> doTaskTwo() throws Exception {
        log.info("开始做任务二");
        long start = System.currentTimeMillis();
        Thread.sleep(random.nextInt(10000));
        long end = System.currentTimeMillis();
        log.info("完成任务二，耗时：" + (end - start) + "毫秒");
        return CompletableFuture.completedFuture("任务二完成");
    }

    //@Async
    public CompletableFuture<String>  doTaskThree() throws Exception {
        log.info("开始做任务三");
        long start = System.currentTimeMillis();
        Thread.sleep(random.nextInt(10000));
        long end = System.currentTimeMillis();
        log.info("完成任务三，耗时：" + (end - start) + "毫秒");
        return CompletableFuture.completedFuture("任务三完成");
    }

}

在我们的测试类中执行三个函数方法，

@Slf4j
@SpringBootTest
public class ApplicationTests {

    @Autowired
    private AsyncTasks asyncTasks;

    @Test
    public void test() throws Exception {
        asyncTasks.doTaskOne();
        asyncTasks.doTaskTwo();
        asyncTasks.doTaskThree();
    }

}

异步调用：

@Slf4j
@Component
public class AsyncTasks {

    public static Random random = new Random();

    @Async
    public CompletableFuture<String> doTaskOne() throws Exception {
        log.info("开始做任务一");
        long start = System.currentTimeMillis();
        Thread.sleep(random.nextInt(10000));
        long end = System.currentTimeMillis();
        log.info("完成任务一，耗时：" + (end - start) + "毫秒");
        return CompletableFuture.completedFuture("任务一完成");
    }

    @Async
    public CompletableFuture<String> doTaskTwo() throws Exception {
        log.info("开始做任务二");
        long start = System.currentTimeMillis();
        Thread.sleep(random.nextInt(10000));
        long end = System.currentTimeMillis();
        log.info("完成任务二，耗时：" + (end - start) + "毫秒");
        return CompletableFuture.completedFuture("任务二完成");
    }

    @Async
    public CompletableFuture<String>  doTaskThree() throws Exception {
        log.info("开始做任务三");
        long start = System.currentTimeMillis();
        Thread.sleep(random.nextInt(10000));
        long end = System.currentTimeMillis();
        log.info("完成任务三，耗时：" + (end - start) + "毫秒");
        return CompletableFuture.completedFuture("任务三完成");
    }

}

@Slf4j
@SpringBootTest
public class ApplicationTests {

    @Autowired
    private AsyncTasks asyncTasks;

    @Test
    public void test() throws Exception {
        long start = System.currentTimeMillis();

        CompletableFuture<String> task1 = asyncTasks.doTaskOne();
        CompletableFuture<String> task2 = asyncTasks.doTaskTwo();
        CompletableFuture<String> task3 = asyncTasks.doTaskThree();

        CompletableFuture.allOf(task1, task2, task3).join();

        long end = System.currentTimeMillis();

        log.info("任务全部完成，总耗时：" + (end - start) + "毫秒");
    }

}

注意，异步调用的结果是随机的，你的结果和我这边的结果不一样很正常。