How to Efficiently Run Tests in Parallel with TestNG and Selenium

Discover how to correctly set up your Selenium tests to run in parallel using TestNG, avoiding common pitfalls like `NullPointerException` issues.
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How to Efficiently Run Tests in Parallel with TestNG and Selenium

Running tests in parallel can drastically reduce the execution time of your test suite, particularly when working with Selenium and TestNG. However, as you've started experimenting with running tests in parallel, you may encounter some frustrating issues, such as NullPointerException. This article will walk you through tackling this problem effectively, ensuring that your tests can run simultaneously without clashing.

The Problem: NullPointerException in Parallel Runs

If you are using TestNG v7.6.0 and Selenium v4.2.2, you might find yourself in a situation where your tests are failing with an error similar to:

[[See Video to Reveal this Text or Code Snippet]]

The problem occurs when you run the same test method in parallel using invocationCount and threadPoolSize. Your setup is designed to use ThreadLocal<WebDriver>, which allows each thread to have its own instance of the WebDriver. However, if not implemented correctly, this can lead to issues where the driver variable becomes null.

Understanding the Issue

Before we explore the solution, let's dissect why you're encountering this problem:

When you define this.driver = new ThreadLocal<WebDriver>(); inside the setup method, this creates a new ThreadLocal variable each time a test method runs.

Therefore, when the second test method tries to retrieve the driver instance, it doesn't find the correct object and returns null, resulting in the NullPointerException.

A Proper Approach to Parallel Testing

To fix the issue, the ThreadLocal object needs to be initialized at the class level. Here's how you can implement this:

The Correct Implementation

Define the ThreadLocal variable as static: This makes it consistent across all instances of the LoginTest class.

Initialize it directly: While there’s no need to reinitialize it in the setup method, ensure the setup only sets the value for the current thread.

Here’s a refactored version of your test class:

[[See Video to Reveal this Text or Code Snippet]]

Key Takeaways

Static ThreadLocal: By making your ThreadLocal instance static, each thread can correctly reference its own WebDriver without overwriting.

Setup Method: The setup method is only responsible for setting the WebDriver for its respective thread, ensuring isolation between parallel executions.

Conclusion

Running your tests in parallel doesn't have to be a headache. By initializing ThreadLocal<WebDriver> at the class level and using it within your test methods, you can efficiently execute multiple instances of your tests without the risk of data interference. This setup ensures that your tests can run in isolation, optimizing your testing strategy effectively.

Now you're ready to maximize the performance of your Selenium tests with TestNG. Happy testing!