Tests Interfere When Run in Parallel

The Scenario

To speed up the suite, someone set parallel="methods" with four threads in testng.xml. Now tests fail with NoSuchSessionException, browsers close mid-test, and one test’s assertions see another test’s page. Setting the thread count back to one makes everything pass.

Your Task

Identify the shared state and restructure driver management so every test thread owns its own browser, with no leaks between tests.

What Good Looks Like

• No static WebDriver field.
• A ThreadLocal (or equivalent) behind a small manager class.
• quit() and remove() both happen in teardown.
• Test code changes minimally; page objects receive the driver by constructor.

public class BaseTest {
    protected static WebDriver driver;

    @BeforeMethod
    public void start() {
        driver = new ChromeDriver();
    }

    @AfterMethod
    public void stop() {
        driver.quit();
    }
}

// testng.xml: <suite name="e2e" parallel="methods" thread-count="4">

public class CheckoutTest extends BaseTest {
    @Test public void appliesCoupon() { driver.get("https://app.example.com/checkout"); /* ... */ }
    @Test public void placesOrder()   { driver.get("https://app.example.com/checkout"); /* ... */ }
}