使用Awaitility替代Thread.sleep()触发最小等待时长报错的原因

当使用Thread.sleep()时，以下测试用例可正常运行：

@Test
public void closeableTimer() throws InterruptedException {
  Timer timer = new DefaultTimer(TimeUnit.NANOSECONDS);
  Assertions.assertThat(timer.count()).isEqualTo(0);
  Assertions.assertThat(timer.totalDuration()).isEqualTo(Duration.ZERO);
  try (Timer.Timed sample = timer.start()) {
    Thread.sleep(500L);
  }
  Assertions.assertThat(timer.count()).isEqualTo(1);
  Assertions.assertThat(timer.totalDuration()).isGreaterThan(Duration.ZERO);
} 

但改用Awaitility并设置atLeast(Duration.ofMillis(500L))执行相同测试时，却抛出如下错误：

Condition was evaluated in 108 milliseconds which is earlier than
expected minimum timeout 500 milliseconds
org.awaitility.core.ConditionTimeoutException

对应测试代码如下：

@Test
public void closeableTimer() {
  Timer timer = new DefaultTimer(TimeUnit.NANOSECONDS);
  Assertions.assertThat(timer.count()).isEqualTo(0);
  Assertions.assertThat(timer.totalDuration()).isEqualTo(Duration.ZERO);
  try (Timer.Timed ignored = timer.start()) {
    await().atLeast(Duration.ofMillis(500L)).until(() -> true);
  }
  Assertions.assertThat(timer.count()).isEqualTo(1);
  Assertions.assertThat(timer.totalDuration()).isGreaterThan(Duration.ZERO);
}

atLeast()的作用是强制要求等待时长至少达到指定值，但你传入的条件() -> true是一个会立即返回true的表达式——Awaitility会第一时间判定条件满足，此时实际等待时长远小于你设置的500毫秒，触发了Awaitility的校验逻辑，因此抛出ConditionTimeoutException。

简单来说：你要求必须等够500毫秒，但你的条件根本不需要等待就满足了，两者冲突导致报错。

如果想用Awaitility实现类似Thread.sleep()的固定等待效果，应该改用pollDelay()方法，代码示例：

await().pollDelay(Duration.ofMillis(500L)).until(() -> true);

内容的提问来源于stack exchange，提问作者kar