Automation Testing TestNG Guide
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What is TestNG?

TestNG is an open-source testing framework for Java, inspired by JUnit and NUnit, with extra features that suit end-to-end and integration testing. It provides annotations to mark test methods and setup steps, assertions to check results, flexible grouping and ordering, data-driven testing and built-in support for running tests in parallel. TestNG itself does not drive a browser; it is typically paired with Selenium WebDriver, which controls the browser, while TestNG structures the tests and reports the results. Many automation testing job descriptions for Java still mention TestNG, so it is worth understanding even if you later learn newer tools.

Common TestNG annotations

A simple TestNG test with Selenium

Illustrative structure only. It assumes Selenium and TestNG are on the project classpath and uses a public demo site; adapt locators to your application.

import org.openqa.selenium.By;
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.chrome.ChromeDriver;
import org.testng.Assert;
import org.testng.annotations.*;

public class LoginTest {
    private WebDriver driver;

    @BeforeMethod
    public void setup() {
        driver = new ChromeDriver();
        driver.get("https://example.com/login");
    }

    @Test(groups = "smoke")
    public void validateLoginShowsDashboard() {
        driver.findElement(By.id("email")).sendKeys("test.user@example.com");
        driver.findElement(By.id("password")).sendKeys("your-test-password");
        driver.findElement(By.id("login")).click();
        Assert.assertEquals(
            driver.getCurrentUrl().contains("dashboard"),
            true,
            "User should land on the dashboard"
        );
    }

    @AfterMethod(alwaysRun = true)
    public void tearDown() {
        if (driver != null) driver.quit();
    }
}

Assertions

Data-driven testing with a DataProvider

A data provider supplies rows of data, and the test method runs once for each row.

@DataProvider(name = "loginData")
public Object[][] loginData() {
    return new Object[][] {
        {"valid.user@example.com", "ValidPass#1", true},
        {"valid.user@example.com", "wrong-pass",   false},
        {"",                        "",             false}
    };
}

@Test(dataProvider = "loginData")
public void loginScenarios(String email, String password, boolean shouldSucceed) {
    // enter credentials, submit, then assert the expected outcome
}

testng.xml suite file

The suite file selects which tests, groups or classes run, and can set parallel execution.

<!DOCTYPE suite SYSTEM "https://testng.org/testng-1.0.dtd">
<suite name="RegressionSuite" parallel="classes" thread-count="3">
  <test name="Smoke">
    <groups>
      <run>
        <include name="smoke"/>
      </run>
    </groups>
    <classes>
      <class name="tests.LoginTest"/>
    </classes>
  </test>
</suite>

Useful TestNG features

• → Groups: tag tests such as smoke or regression and run only those.
• → Dependencies: dependsOnMethods can order tests, though independent tests are preferable.
• → Priority: controls order within a class; use sparingly.
• → Parallel execution: run methods, classes or tests in parallel to save time; keep tests independent and drivers thread-safe.
• → Listeners: react to events such as failure, for example to capture a screenshot.
• → Retry analyzer: re-run failed tests a limited number of times, visibly.

Parallel execution and thread safety

Running tests in parallel can cut execution time sharply, but only if tests do not share mutable state. A single static WebDriver shared across threads will cause tests to interfere with each other. A common approach is to store the driver in a ThreadLocal so each thread has its own browser, and to keep test data unique per test. Start with a small thread count, watch for instability and increase carefully.

Reports

Common mistakes

Selenium waits compared

Interview-style quick answers

• → Difference between @BeforeTest and @BeforeMethod: the first runs once before the testng.xml test tag, the second before every test method.
• → How to skip a test: use enabled=false or throw SkipException.
• → How to run only some tests: use groups or select classes in testng.xml.

Folder structure that scales

• → src/main/java/pages: page objects.
• → src/test/java/tests: test classes.
• → src/test/resources: testng.xml, configuration and test data.
• → utils: driver factory, readers, listeners.