# Test Smells and Flaky Tests — JUnit 5 & Mockito

Source: https://www.skillbyai.com/en/java-testing/d-smells

> Recognise brittle, slow, unclear and flaky tests and fix them.

## When tests cause pain

Bad tests slow teams down. Common **test smells**: **brittle tests** that break on harmless refactors because they verify internal calls or exact log messages; **over-mocking**, where tests replicate the implementation line by line; **logic in tests** (loops, conditionals) that can itself be wrong; **mystery guests**, where tests depend on external files or data you cannot see; **giant tests** checking many behaviours at once; **assertion-free tests** that only check that no exception occurred; and **slow tests** that start Spring or databases for logic that needs neither. **Flaky tests** pass and fail without code changes, destroying trust in the suite. Typical causes: **`Thread.sleep`** and timing assumptions (use await libraries such as **Awaitility** to poll for conditions), **shared state** between tests, dependence on **test order**, real **time** and **time zones**, **randomness** without seeds, **network calls**, and **parallel execution** on shared resources. Track flaky tests, fix or quarantine them quickly, and never simply re-run CI until it turns green.

## Replacing sleep with Awaitility

Polling for a condition is faster and more reliable than a fixed sleep.

```java
// flaky: guesses how long the async work takes
@Test
void emailEventuallySent_flaky() throws Exception {
    service.placeOrderAsync(order);
    Thread.sleep(2_000);                       // too short on a slow CI machine, too long locally
    assertThat(outbox.sentEmails()).hasSize(1);
}

// reliable: wait until the condition holds, up to a timeout
@Test
void emailEventuallySent() {
    service.placeOrderAsync(order);
    await().atMost(Duration.ofSeconds(5))
           .pollInterval(Duration.ofMillis(50))
           .untilAsserted(() -> assertThat(outbox.sentEmails()).hasSize(1));
}
```

## A smoke alarm that goes off randomly

A flaky test is a smoke alarm that sometimes beeps for no reason. After a few false alarms, people start ignoring it, and then it is useless when there really is a fire.

**Quiz:** What is the most reliable replacement for Thread.sleep when waiting for asynchronous work in a test?

- [ ] A longer sleep
- [x] Polling for the expected condition with a timeout (for example Awaitility)
- [ ] Removing the assertion
- [ ] Running the test twice

*Answer:* Polling for the expected condition with a timeout (for example Awaitility). Polling waits exactly as long as needed, up to a bound, removing timing guesses.