ThreadGuard

This class is only available in the Java Binding

ThreadGuard checks that a driver is called only from the same thread that created it. Threading issues especially when running tests in Parallel may have mysterious and hard to diagnose errors. Using this wrapper prevents this category of errors and will raise an exception when it happens.

The following example simulate a clash of threads:

public class DriverClash {
  //thread main (id 1) created this driver
  private WebDriver protectedDriver = ThreadGuard.protect(new ChromeDriver()); 

  static {
    System.setProperty("webdriver.chrome.driver", "<Set path to your Chromedriver>");
  }
  
  //Thread-1 (id 24) is calling the same driver causing the clash to happen
  Runnable r1 = () -> {protectedDriver.get("https://selenium.dev");};
  Thread thr1 = new Thread(r1);
   
  void runThreads(){
    thr1.start();
  }

  public static void main(String[] args) {
    new DriverClash().runThreads();
  }
}

The result shown below:

Exception in thread "Thread-1" org.openqa.selenium.WebDriverException:
Thread safety error; this instance of WebDriver was constructed
on thread main (id 1)and is being accessed by thread Thread-1 (id 24)
This is not permitted and *will* cause undefined behaviour

As seen in the example:

  • protectedDriver Will be created in Main thread
  • We use Java Runnable to spin up a new process and a new Thread to run the process
  • Both Thread will clash because the Main Thread does not have protectedDriver in it’s memory.
  • ThreadGuard.protect will throw an exception.

Note:

This does not replace the need for using ThreadLocal to manage drivers when running parallel.

Last modified September 23, 2022: rename additional features section back to support features to re-emphasize the distinct purpose of support classes (c70272e7f8b)