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Parallel Execution

Parallel execution allows TestNG to run multiple tests at the same time instead of sequentially.
This is one of the most powerful features of TestNG and is crucial for reducing execution time in large automation suites.

What is Parallel Execution?​

In parallel execution, TestNG uses multiple threads to execute tests concurrently.

This is especially useful for:

  • Large regression suites
  • Cross-browser testing
  • CI/CD pipelines with time constraints

Where Parallel Execution is Configured​

Parallel execution is controlled mainly through testng.xml, not in test code.

<suite name="ParallelSuite" parallel="tests" thread-count="3">

Parallel Execution Levels​

TestNG supports parallel execution at four levels:

LevelDescription
testsRuns each <test> block in parallel
classesRuns test classes in parallel
methodsRuns test methods in parallel
instancesRuns class instances in parallel

Parallel = "tests" (Most Recommended)​

<suite parallel="tests" thread-count="2">

    <test name="ChromeTests">
        <parameter name="browser" value="chrome"/>
        <classes>
            <class name="tests.LoginTest"/>
        </classes>
    </test>

    <test name="EdgeTests">
        <parameter name="browser" value="edge"/>
        <classes>
            <class name="tests.LoginTest"/>
        </classes>
    </test>

</suite>

Execution Diagram​

Thread 1 → ChromeTests
Thread 2 → EdgeTests

✔️ Safest
✔️ Ideal for cross-browser execution

Parallel = "classes"​

<suite parallel="classes" thread-count="2">
Thread 1 → LoginTest
Thread 2 → PaymentTest

⚠️ Shared resources must be handled carefully.

Parallel = "methods" (Advanced)​

<suite parallel="methods" thread-count="3">
Thread 1 → testLogin
Thread 2 → testLogout
Thread 3 → testPayment

⚠️ Order is ignored
⚠️ High risk of race conditions
⚠️ Avoid static/shared variables

Parallel = "instances"​

Used when the same class is instantiated multiple times (usually with @Factory).

Thread 1 → Instance 1
Thread 2 → Instance 2

Advanced use case, rarely needed for beginners.

Thread Count Behavior​

  • thread-count is the maximum number of threads
  • It is global, not per test or class
  • TestNG schedules tasks dynamically within this limit

Interaction with Annotations​

  • @BeforeMethod and @AfterMethod run per thread
  • @BeforeClass runs once per class instance
  • @BeforeSuite always runs once

Parallel execution does not change annotation order, only concurrency.

Parallel DataProviders​

@DataProvider(parallel = true)
  • Each dataset runs in parallel
  • Combined with parallel="methods" increases speed
  • Requires thread-safe test design

Common Issues in Parallel Execution​

  • Using static WebDriver
  • Sharing mutable data between tests
  • Relying on execution order
  • Improper cleanup in @AfterMethod

Best Practices​

  • Prefer parallel="tests"
  • Use ThreadLocal for WebDriver
  • Avoid static/shared state
  • Design tests to be independent
  • Start with low thread count

Key Takeaways​

  • Parallel execution improves speed
  • XML controls parallel behavior
  • tests level is safest
  • Thread safety is critical