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Technology

TestNG

Difficulty

Advanced

Experience

3-5 Years

Type

Technical

AutomateQA Team

QA Automation Engineers

June 15, 2026

Interview Question

What is singleThreaded in TestNG and when do you use it?

Q: What is singleThreaded in TestNG and when do you use it?

singleThreaded in TestNG When a suite runs in parallel="methods" mode, individual @Test methods from the same class can run in different threads. @Test(singleThreaded = true) at class level overrides this — forcing all methods of that class to run in ONE thread. The Problem Without singleThreaded java // parallel="methods" — each method runs in its own thread // LoginTest.validLogin → Thread-1 // LoginTest.invalidLogin → Thread-2 ← CONFLICT: both share driver! // LoginTest.sessionTimeout → Thread-3 Without ThreadLocal, multiple threads interfere with each other''s driver. Solution 1 — singleTh

Answer

singleThreaded in TestNG

When a suite runs in parallel="methods" mode, individual @Test methods from the same class can run in different threads. @Test(singleThreaded = true) at class level overrides this — forcing all methods of that class to run in ONE thread.

The Problem Without singleThreaded

Java

// parallel="methods" — each method runs in its own thread
// LoginTest.validLogin     → Thread-1
// LoginTest.invalidLogin   → Thread-2   ← CONFLICT: both share driver!
// LoginTest.sessionTimeout → Thread-3

Without ThreadLocal, multiple threads interfere with each other''s driver.

Solution 1 — singleThreaded (Keep Class Sequential)

Java

@Test(singleThreaded = true) // ALL methods run in same thread
public class LoginTest extends BaseTest {

    @Test
    public void openLoginPage() {
        driver.get("https://automateqa.online/login");
    }

    @Test(dependsOnMethods = "openLoginPage")
    public void enterCredentials() {
        driver.findElement(By.id("username")).sendKeys("admin");
    }

    @Test(dependsOnMethods = "enterCredentials")
    public void submitAndVerify() {
        driver.findElement(By.id("loginBtn")).click();
        Assert.assertTrue(driver.getTitle().contains("Dashboard"));
    }
}

All 3 methods run sequentially in the same thread → same driver instance → no conflict.

testng.xml Configuration

XML

<suite name="Suite" parallel="methods" thread-count="5">
    <test name="Tests">
        <classes>
            <class name="tests.LoginTest"/>      <!-- singleThreaded=true in class -->
            <class name="tests.CartTest"/>       <!-- can run methods in parallel -->
            <class name="tests.ProductTest"/>
        </classes>
    </test>
</suite>

LoginTest runs its methods sequentially, while CartTest and ProductTest methods run in parallel.

Solution 2 — ThreadLocal (Better for Parallel Methods)

Rather than using singleThreaded, the modern approach is:

Java

// No singleThreaded needed — each thread gets its OWN driver
public class LoginTest extends BaseTest {
    // BaseTest uses ThreadLocal<WebDriver>

    @Test
    public void validLogin() {
        DriverManager.getDriver().get("..."); // Thread-safe
    }

    @Test
    public void invalidLogin() {
        DriverManager.getDriver().get("..."); // Different thread, different driver
    }
}

When to Use singleThreaded

Scenario	Use singleThreaded?
Tests share browser session (login → cart → checkout)	Yes
Tests are fully independent	No — use ThreadLocal
Methods have `dependsOnMethods` chain	Yes (or use `parallel="classes"`)
Complex state machine tests	Yes
Stateless API tests	No

singleThreaded vs parallel="classes"

✓singleThreaded=true on a class: that specific class is sequential; others may be parallel
✓parallel="classes" in XML: all classes run in parallel, but each class''s methods are sequential

Best modern approach: Use ThreadLocal<WebDriver> + parallel="methods" rather than singleThreaded.