TestNG — The Complete Guide

A single-read, end-to-end reference for TestNG: enough to wire up a Java test suite from scratch, configure parallel/cross-browser execution, or walk into an SDET interview. Organized as a lookup you can also read top-to-bottom.

📋 Quick reference: TestNG →

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🧭 Mental model

TestNG's annotations aren't a flat list — they nest like Russian dolls: suite-level setup wraps test-level setup, which wraps class-level setup, which wraps the actual @Test method run, and each layer tears back down in reverse order once its inner layer finishes.

1. What TestNG Is and Why It Exists

TestNG ("Testing, Next Generation") is a Java testing framework inspired by JUnit and NUnit but built to remove their early limitations — it was the first mainstream Java framework with built-in annotations, flexible test configuration, parallel execution, data-driven testing, and dependency-aware test ordering, all without external add-ons.

It's the dominant framework in the Selenium/Java SDET ecosystem specifically because of features JUnit historically lacked: native parallelism, suite-level XML configuration, and first-class support for grouping and ordering tests that model real-world test suite needs (smoke → regression → sanity, or "skip dependents if setup fails").

<!-- Minimal Maven dependency -->
<dependency>
    <groupId>org.testng</groupId>
    <artifactId>testng</artifactId>
    <version>7.10.2</version>
    <scope>test</scope>
</dependency>

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2. Core Annotations

import org.testng.annotations.*;

public class OrderServiceTest {

    @BeforeSuite
    public void beforeSuite() { /* runs once, before any test in the <suite> */ }

    @BeforeTest
    public void beforeTest() { /* once per <test> tag in testng.xml */ }

    @BeforeClass
    public void beforeClass() { /* once per test class, before its first @Test */ }

    @BeforeMethod
    public void beforeMethod() { /* before EVERY @Test method in this class */ }

    @Test
    public void orderTotalsAreCorrect() {
        // the actual test
    }

    @AfterMethod
    public void afterMethod() { /* after every @Test — good place to screenshot on failure */ }

    @AfterClass
    public void afterClass() { /* once per class, after its last @Test */ }

    @AfterTest
    public void afterTest() { /* once per <test> tag */ }

    @AfterSuite
    public void afterSuite() { /* once, after everything — teardown shared resources */ }
}

Execution order (outermost to innermost)

@BeforeSuite
  @BeforeTest
    @BeforeClass
      @BeforeMethod → @Test → @AfterMethod   (repeats per @Test)
    @AfterClass
  @AfterTest
@AfterSuite

This nesting is the single most-tested TestNG concept — know it cold. It maps directly onto testng.xml's <suite> → <test> → <class> → <methods> hierarchy: a @BeforeTest fires once per <test> block, not once per class inside it.

@Test attributes worth knowing

Attribute	Purpose
enabled = false	Skip this test without deleting it
priority = 1	Lower runs first (default 0); ties run in declaration order
timeOut = 5000	Fail the test if it exceeds 5000ms
expectedExceptions = {NullPointerException.class}	Test passes only if this exception is thrown
invocationCount = 5	Run this test method 5 times
dependsOnMethods = {"login"}	Run only after login() passes (see §6)
groups = {"smoke", "regression"}	Tag for selective execution (see §6)

@Test(priority = 1, timeOut = 3000, expectedExceptions = IllegalArgumentException.class)
public void rejectsNegativeQuantity() {
    orderService.createOrder(-1);
}

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3. testng.xml — Suite Configuration

testng.xml is TestNG's suite-level configuration file — it decides what runs, in what order, how parallel, and with what parameters, entirely outside the Java code.

<!DOCTYPE suite SYSTEM "https://testng.org/testng-1.0.dtd">
<suite name="RegressionSuite" parallel="classes" thread-count="4" verbose="1">

    <parameter name="environment" value="staging"/>

    <listeners>
        <listener class-name="com.company.listeners.RetryListener"/>
        <listener class-name="com.company.listeners.ExtentReportListener"/>
    </listeners>

    <test name="SmokeTests">
        <groups>
            <run>
                <include name="smoke"/>
            </run>
        </groups>
        <classes>
            <class name="com.company.tests.LoginTest"/>
            <class name="com.company.tests.CheckoutTest"/>
        </classes>
    </test>

    <test name="RegressionTests">
        <classes>
            <class name="com.company.tests.OrderServiceTest">
                <methods>
                    <exclude name="slowReconciliationTest"/>
                </methods>
            </class>
        </classes>
    </test>

</suite>

• <suite> is the root — one XML file can define multiple <test> blocks, each with its own classes, groups, and parallelism.
• Run it from Maven with the Surefire plugin:

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-surefire-plugin</artifactId>
    <version>3.2.5</version>
    <configuration>
        <suiteXmlFiles>
            <suiteXmlFile>testng.xml</suiteXmlFile>
        </suiteXmlFiles>
    </configuration>
</plugin>

mvn test                      # picks up testng.xml via Surefire config
mvn test -Dgroups=smoke        # override groups from the CLI

Passing parameters from XML to test code

@Parameters({"environment"})
@Test
public void verifyBaseUrl(String environment) {
    Assert.assertEquals(config.getEnv(), environment);
}

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4. Parallel Execution

TestNG can parallelize at four granularities via the parallel attribute on <suite> or <test>:

parallel value	Parallelizes	Typical use
methods	Every @Test method, even across classes	Maximum throughput; requires fully independent, thread-safe tests
classes	Each class runs in its own thread; methods within a class run sequentially	Most common for UI suites — isolates WebDriver instances per class
tests	Each <test> XML block runs in its own thread	Coarse-grained — good for separating unrelated suites (API vs UI)
instances	Each instance of a test class (with @Factory) runs in parallel	Data-driven object instantiation scenarios

<suite name="Suite" parallel="methods" thread-count="10">

The #1 pitfall: shared mutable state

public class LoginTest {
    private WebDriver driver;   // BUG if parallel="methods" — shared across threads

    @BeforeMethod
    public void setUp() {
        driver = new ChromeDriver();   // fine ONLY if parallel="classes" or each
    }                                    // class gets its own instance per thread
}

Fix: use ThreadLocal<WebDriver> (the standard Selenium+TestNG parallel pattern) so each thread gets its own driver instance, or ensure parallel="classes"/"tests" so a class's @BeforeMethod-created driver is never shared across threads in the first place.

private static ThreadLocal<WebDriver> driverThread = new ThreadLocal<>();

public static WebDriver getDriver() { return driverThread.get(); }

@BeforeMethod
public void setUp() {
    driverThread.set(new ChromeDriver());
}

@AfterMethod
public void tearDown() {
    driverThread.get().quit();
    driverThread.remove();
}

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5. Data-Driven Testing: @DataProvider

@DataProvider(name = "loginCredentials")
public Object[][] loginData() {
    return new Object[][] {
        {"validUser", "correctPass", true},
        {"validUser", "wrongPass", false},
        {"", "anyPass", false},
    };
}

@Test(dataProvider = "loginCredentials")
public void testLogin(String username, String password, boolean expectedResult) {
    boolean actual = loginPage.login(username, password);
    Assert.assertEquals(actual, expectedResult);
}

Each row runs as a separate, independently reported test — a failure on row 2 doesn't stop rows 1 and 3.

Parallel data providers

@DataProvider(name = "loginCredentials", parallel = true)

Runs each data row in its own thread — combine with thread-count on the <suite> tag to control the pool size.

DataProviders from external sources

@DataProvider(name = "csvData")
public Object[][] fromCsv() throws IOException {
    // read a CSV/Excel/JSON file and map rows -> Object[][]
    return CsvUtils.readAsObjectArray("testdata/users.csv");
}

This is the standard way to decouple test logic from test data — QA teams edit the CSV without touching Java code.

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6. Groups & Dependencies

Groups — tag-based selective execution

@Test(groups = {"smoke"})
public void homePageLoads() { ... }

@Test(groups = {"regression", "checkout"})
public void fullCheckoutFlow() { ... }

<groups>
    <run>
        <include name="smoke"/>
        <exclude name="flaky"/>
    </run>
</groups>

mvn test -Dgroups=smoke,checkout -DexcludedGroups=flaky

Groups model a CI pipeline naturally: run smoke on every PR, regression nightly, flaky never (until fixed and reclassified).

dependsOnMethods / dependsOnGroups

@Test
public void login() { ... }

@Test(dependsOnMethods = {"login"})
public void addToCart() { ... }   // skipped (not failed) if login() fails

@Test(dependsOnMethods = {"addToCart"}, alwaysRun = true)
public void checkout() { ... }    // runs even if a dependency fails, if alwaysRun=true

If login() fails, addToCart() is reported as SKIPPED, not run at all — this matters for correct pass/fail metrics; TestNG distinguishes "didn't run because a prerequisite failed" from an actual failure. Use dependencies sparingly — they couple tests together and hurt parallelism (dependent tests serialize even under parallel="methods").

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7. Assertions: Soft vs. Hard

Hard assertions (Assert) — default, fail-fast

import org.testng.Assert;

Assert.assertEquals(actualTotal, 99.99, "Cart total mismatch");
Assert.assertTrue(user.isActive());
Assert.assertNotNull(response.getBody());

A hard assertion failure immediately stops the test method — any assertions after it never run.

Soft assertions (SoftAssert) — collect-and-report

import org.testng.asserts.SoftAssert;

@Test
public void verifyOrderSummary() {
    SoftAssert softAssert = new SoftAssert();
    softAssert.assertEquals(order.getStatus(), "CONFIRMED");
    softAssert.assertEquals(order.getItemCount(), 3);
    softAssert.assertTrue(order.getTotal() > 0);
    softAssert.assertAll();   // MUST call this — throws if any assertion above failed
}

SoftAssert records every failure but keeps executing, then throws a single aggregated exception at assertAll() — invaluable for UI tests that verify many independent fields on one page: one broken layout element shouldn't hide five other broken fields in the same run. Forgetting assertAll() silently swallows all failures — the most common soft-assert bug.

	Hard Assert	SoftAssert
On failure	Stops test immediately	Records and continues
Best for	Sequential logic where step 2 depends on step 1 succeeding	Independent checks on one page/response
Gotcha	None — this is the safe default	Forgetting assertAll() = false positives

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8. Listeners & Reporting

Listeners hook into the test lifecycle for cross-cutting behavior — screenshots on failure, retry logic, custom reporting — without touching individual test methods.

public class TestListener implements ITestListener {

    @Override
    public void onTestFailure(ITestResult result) {
        String testName = result.getMethod().getMethodName();
        byte[] screenshot = ((TakesScreenshot) DriverFactory.getDriver())
                .getScreenshotAs(OutputType.BYTES);
        ExtentReportManager.attachScreenshot(testName, screenshot);
    }

    @Override
    public void onTestSuccess(ITestResult result) { ... }

    @Override
    public void onTestSkipped(ITestResult result) { ... }
}

Wire it up either in testng.xml (<listeners>, shown in §3) or via annotation on the test class:

@Listeners(TestListener.class)
public class CheckoutTest { ... }

Retry failed tests (IRetryAnalyzer)

public class RetryAnalyzer implements IRetryAnalyzer {
    private int count = 0;
    private static final int MAX_RETRIES = 2;

    @Override
    public boolean retry(ITestResult result) {
        if (count < MAX_RETRIES) {
            count++;
            return true;
        }
        return false;
    }
}

@Test(retryAnalyzer = RetryAnalyzer.class)
public void flakyNetworkCall() { ... }

Apply it suite-wide via an IAnnotationTransformer so you don't have to annotate every method — the standard pattern to tame flaky UI tests without masking genuinely broken ones (cap retries low, and track retry counts in reporting so "flaky" doesn't quietly become invisible).

Built-in & third-party reporting

TestNG auto-generates test-output/index.html and emailable-report.html after every run with pass/fail/skip counts and stack traces. Production suites typically layer ExtentReports or Allure on top via a listener for richer, screenshot-embedded, stakeholder-friendly HTML reports.

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9. TestNG vs. JUnit 5 — Quick Comparison

Capability	TestNG	JUnit 5
Parallel execution	Built-in, XML-configurable, fine-grained (methods/classes/tests)	Supported since 5.3 but requires explicit config (junit-platform.properties), less granular out of the box
Suite configuration	External testng.xml — no code change to reorganize suites	Code-centric (@Suite, tags) — JUnit 5 Platform Suite API is newer/less mature
Data-driven tests	@DataProvider — supports parallel data rows natively	@ParameterizedTest + @MethodSource/@CsvSource — clean but less flexible for programmatic data
Dependencies between tests	dependsOnMethods/dependsOnGroups — native	Not supported natively — discouraged in JUnit philosophy (tests should be independent)
Grouping/tagging	groups attribute + XML <groups>	@Tag + tag expressions
Soft assertions	Built-in SoftAssert	Not built-in — needs AssertJ's SoftAssertions or similar
Ecosystem/IDE support	Strong (Selenium/enterprise Java QA)	Broader (default for most non-QA Java projects, Spring Boot default)

Rule of thumb: TestNG tends to win for large Selenium/API automation suites needing fine-grained parallel control and XML-driven suite composition; JUnit 5 tends to win for general application unit/integration testing and Spring ecosystem projects. Many SDET teams use both — JUnit for unit tests inside the app, TestNG for the end-to-end automation suite.

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10. Interview-Ready Q&A

Q: Walk me through TestNG's annotation execution order. A: @BeforeSuite → @BeforeTest → @BeforeClass → (@BeforeMethod → @Test → @AfterMethod, repeated for every test method) → @AfterClass → @AfterTest → @AfterSuite. The Before/After pairs nest around the testng.xml hierarchy: suite contains tests, tests contain classes, classes contain methods.

Q: What's the difference between @BeforeMethod and @BeforeClass? A: @BeforeClass runs once per class, before its first @Test method. @BeforeMethod runs before every single @Test method in that class. Use @BeforeClass for expensive one-time setup (starting a browser session that's reused) and @BeforeMethod for per-test setup that must be fresh (resetting state, navigating to a start page).

Q: Explain soft vs. hard assertions and when you'd use each. A: A hard Assert failure throws immediately and halts the test method — use it when later steps depend on an earlier one succeeding. A SoftAssert records failures but keeps executing, then reports all of them together when assertAll() is called — use it to verify several independent fields (e.g., multiple values on a confirmation page) in one run so a single broken field doesn't hide the others. Forgetting to call assertAll() is the classic bug — failures get silently recorded but never surfacing.

Q: How does TestNG achieve parallel execution, and what's the biggest risk? A: Via the parallel attribute (methods, classes, tests, instances) on <suite>/<test> in testng.xml, combined with thread-count. The biggest risk is shared mutable state across threads — most commonly a non-thread-safe WebDriver instance field. The standard fix is ThreadLocal<WebDriver> so each thread gets an isolated driver instance.

Q: What does dependsOnMethods do, and why should it be used sparingly? A: It makes a test method run only after its declared dependency passes; if the dependency fails, the dependent test is marked SKIPPED, not run at all — which is important for accurate reporting. It should be used sparingly because dependent tests can't run in parallel with each other (TestNG serializes them to respect order), and it couples test cases together, making the suite more fragile to reorder or refactor.

Q: How do you pass a dynamic set of test data into a test method? A: @DataProvider — a method returning Object[][] (or an Iterator) referenced by name from @Test(dataProvider = "..."). Each row executes as an independently reported test case, so one failing row doesn't block the others. It can also be marked parallel = true to run data rows concurrently, and commonly pulls from external sources (CSV, Excel, a database) to decouple test data from test code.

Q: How would you retry a flaky test without retrying every test in the suite? A: Implement IRetryAnalyzer, cap the retry count (e.g., 2 attempts), and attach it via @Test(retryAnalyzer = RetryAnalyzer.class) on the specific flaky test — or apply it globally via an IAnnotationTransformer listener so individual tests don't need to be annotated one by one. Retries should be capped and tracked in reporting, not used to permanently mask a genuinely broken test.

Q: How does testng.xml help with CI pipeline design? A: It lets you define multiple <test> blocks with different group inclusions/exclusions (smoke vs. regression), different parallelism, and different parameters — all without touching Java code. A CI pipeline can run mvn test -Dgroups=smoke on every PR for fast feedback and a nightly job with the full regression suite, just by pointing at different groups or XML files.

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11. One-Line Summary

TestNG's edge over plain JUnit is operational: XML-driven suite composition, native parallel execution, data providers, and group/dependency modeling let you shape a large Selenium/API automation suite around real CI pipelines instead of bolting that structure on afterward.