TestNG Interview Questions

TestNG questions show up in almost every Java automation interview, usually next to Selenium or Appium. Interviewers want to know whether you can make a suite run in parallel without the tests interfering with each other. Examples target TestNG 7.12, which requires Java 11 or later.

Junior level#

In what order do TestNG configuration methods run?#

@BeforeSuite
  @BeforeTest          <- once per <test> tag in testng.xml
    @BeforeClass
      @BeforeMethod
        @Test
      @AfterMethod
    @AfterClass
  @AfterTest
@AfterSuite

@BeforeGroups and @AfterGroups run before the first and after the last test method of a group.

• Follow-up: "Where do you create the WebDriver?" In @BeforeMethod for full isolation (a fresh browser per test), or @BeforeClass when a class's tests deliberately share one session.
• Red flag: Assuming @BeforeTest runs before every test method. It runs once per <test> element in testng.xml, which can contain many classes. This confusion is behind a lot of "the browser was already closed" bugs.

Hard assert or soft assert?#

Assert.assertEquals() stops the test on the first failure. SoftAssert collects failures and reports them all when you call assertAll():

SoftAssert softly = new SoftAssert();
softly.assertEquals(page.title(), "Order confirmed");
softly.assertTrue(page.hasInvoiceLink(), "invoice link");
softly.assertAll(); // forget this and the test always passes

Soft asserts suit checking several independent fields on one page. Use hard asserts when later steps make no sense after a failure.

How do you group and select tests?#

@Test(groups = {"smoke", "checkout"})
public void guestCanCheckOut() { ... }

<test name="Smoke">
  <groups><run><include name="smoke"/></run></groups>
  <packages><package name="com.example.tests"/></packages>
</test>

With Maven Surefire you can also select groups on the command line with -Dgroups=smoke.

Mid level#

How do you run TestNG tests in parallel?#

Set parallel and thread-count on the suite (or on a <test>):

<!DOCTYPE suite SYSTEM "https://testng.org/testng-1.0.dtd">
<suite name="Regression" parallel="methods" thread-count="8">
  <test name="Chrome">
    <classes>
      <class name="com.example.tests.CheckoutTest"/>
      <class name="com.example.tests.SearchTest"/>
    </classes>
  </test>
</suite>

parallel accepts methods, classes, tests and instances. tests is a common way to run the same classes on several browsers at once, with one <test> per browser and a <parameter name="browser" value="firefox"/> read via @Parameters("browser").

• Follow-up: "Now tests close each other's browsers. Why?" The driver is stored in a shared field. With parallel="methods" every method runs on its own thread, so the driver must be held in a ThreadLocal<WebDriver> (see the pattern in the advanced Selenium chapter) or created per method.
• Red flag: A static WebDriver driver in a base class combined with parallel="methods".

What is a DataProvider, and can it run in parallel?#

A method that supplies rows of arguments to a test:

@DataProvider(name = "logins", parallel = true)
public Object[][] logins() {
    return new Object[][] {
        {"standard_user", true},
        {"locked_user", false},
    };
}

@Test(dataProvider = "logins")
public void login(String user, boolean shouldSucceed) { ... }

With parallel = true the rows run on separate threads; the pool size is the suite's data-provider-thread-count (default 10), not thread-count.

How do you take a screenshot on failure?#

Implement ITestListener (its methods have default implementations, so override only what you need):

public class ScreenshotListener implements ITestListener {
    @Override
    public void onTestFailure(ITestResult result) {
        WebDriver driver = DriverFactory.get();
        if (driver != null) {
            byte[] png = ((TakesScreenshot) driver).getScreenshotAs(OutputType.BYTES);
            // attach to your report, named after result.getName()
        }
    }
}

Register it with @Listeners(ScreenshotListener.class) on a class or a <listeners> block in testng.xml. See what TestNG listeners are for the other listener interfaces.

Senior level#

How do you retry failed tests, and should you?#

Implement IRetryAnalyzer:

public class RetryOnce implements IRetryAnalyzer {
    private int attempts = 0;

    @Override
    public boolean retry(ITestResult result) {
        return attempts++ < 1;
    }
}

@Test(retryAnalyzer = RetryOnce.class)
public void flakyCheckout() { ... }

To apply it to every test without touching annotations, set it from an IAnnotationTransformer listener.

• What the interviewer wants to hear: retries are a reporting tool. Track which tests needed a retry and treat that list as a backlog; otherwise retries hide real defects, including race conditions in the product.

dependsOnMethods and priority: when would you use them?#

dependsOnMethods skips a test when the method it depends on fails, and priority orders tests within a class. Both are legitimate, and both create coupling: a dependent test cannot run alone, and it cannot be spread across threads freely. A senior answer prefers independent tests that set up their own state through APIs, and uses dependencies only for genuinely sequential flows such as a long multi-step migration test.

TestNG or JUnit 5?#

Both are capable. TestNG's strengths are suite-level XML configuration, built-in parallel modes, groups, dependencies and parallel data providers, which is why many Selenium frameworks use it. JUnit 5 has the larger general Java ecosystem, extensions, and parallel execution configured in junit-platform.properties. Pick by team familiarity and existing tooling, and say so.

How does a TestNG suite talk to a remote grid?#

Nothing in TestNG changes: the @BeforeMethod creates a RemoteWebDriver pointing at the grid URL, with the browser chosen from a @Parameters value. The Selenium with TestNG course builds this up step by step, and the TestNG documentation for TestingBot shows a parallel suite on a cloud grid.