Docker execution environment in Selenium Java - Test Execution Trace

Test Overview

This test runs a Selenium WebDriver test inside a Docker container. It opens a browser, navigates to a website, clicks a button, and verifies the page title to ensure the test environment works correctly inside Docker.

Test Code - JUnit 5 with Selenium WebDriver

Selenium Java

import org.openqa.selenium.By;
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.WebElement;
import org.openqa.selenium.chrome.ChromeOptions;
import org.openqa.selenium.remote.RemoteWebDriver;
import org.junit.jupiter.api.AfterEach;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.assertEquals;
import java.net.URL;

public class DockerSeleniumTest {
    private WebDriver driver;

    @BeforeEach
    public void setUp() throws Exception {
        ChromeOptions options = new ChromeOptions();
        options.addArguments("--headless");
        options.addArguments("--no-sandbox");
        options.addArguments("--disable-dev-shm-usage");
        driver = new RemoteWebDriver(new URL("http://localhost:4444/wd/hub"), options);
    }

    @Test
    public void testButtonClickChangesTitle() {
        driver.get("https://example.com");
        WebElement moreInfoLink = driver.findElement(By.cssSelector("a[href='https://www.iana.org/domains/example']"));
        moreInfoLink.click();
        String expectedTitle = "IANA — IANA-managed Reserved Domains";
        assertEquals(expectedTitle, driver.getTitle());
    }

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

Execution Trace - 5 Steps

Step	Action	System State	Assertion	Result
1	Test starts and sets up RemoteWebDriver with ChromeOptions in headless mode connecting to Selenium Grid in Docker at http://localhost:4444/wd/hub	Docker container running Selenium Grid with Chrome browser ready	—	PASS
2	Driver navigates to https://example.com	Browser inside Docker container loads example.com homepage	—	PASS
3	Finds the link with href 'https://www.iana.org/domains/example' and clicks it	Browser navigates to IANA reserved domains page	—	PASS
4	Checks the page title equals 'IANA — IANA-managed Reserved Domains'	Browser page title is 'IANA — IANA-managed Reserved Domains'	assertEquals(expectedTitle, driver.getTitle())	PASS
5	Test finishes and quits the WebDriver session	Browser session closed, Docker container remains running	—	PASS

Failure Scenario

Failing Condition: Selenium Grid Docker container is not running or not reachable at http://localhost:4444/wd/hub

Execution Trace Quiz - 3 Questions

What does the test verify after clicking the link on example.com?

A The page title changes to 'IANA — IANA-managed Reserved Domains'

B The URL remains https://example.com

C The button becomes disabled

D An alert popup appears

Key Result

Always verify that your Docker Selenium Grid container is running and accessible before running tests. Use headless browser mode to run tests smoothly inside Docker without GUI.

Practice

1. What is the main benefit of using Docker for Selenium test execution?

easy

A. It provides a clean, consistent browser environment for tests.

B. It automatically writes test scripts for you.

C. It replaces the need for assertions in tests.

D. It runs tests only on local machines.

Solution

1. Step 1: Understand Docker's role in Selenium testing

   Docker creates isolated containers that have clean browser setups, avoiding environment conflicts.

2. Step 2: Compare options

   Only It provides a clean, consistent browser environment for tests. correctly states Docker provides a clean, consistent browser environment. Other options describe unrelated or incorrect features.

3. Final Answer:

   It provides a clean, consistent browser environment for tests. -> Option A

4. Quick Check:

   Docker environment = clean browser setup [OK]

Hint: Docker isolates browsers for consistent tests [OK]

Common Mistakes:

• Thinking Docker writes test scripts automatically
• Believing Docker removes need for assertions
• Assuming Docker only runs tests locally

2. Which Java class is used to connect Selenium tests to a browser running inside a Docker container?

easy

A. DockerClient

B. ChromeDriver

C. FirefoxDriver

D. RemoteWebDriver

Solution

1. Step 1: Identify the correct Selenium class for remote connections

   RemoteWebDriver is designed to connect to browsers running on remote servers or containers like Docker.

2. Step 2: Eliminate other options

   ChromeDriver and FirefoxDriver are for local browsers. DockerClient is unrelated to Selenium.

3. Final Answer:

   RemoteWebDriver -> Option D

4. Quick Check:

   RemoteWebDriver connects to remote browsers [OK]

Hint: Use RemoteWebDriver for Docker browser connections [OK]

Common Mistakes:

• Using ChromeDriver for Docker browsers
• Confusing DockerClient with Selenium classes
• Using FirefoxDriver for remote Docker sessions

3. Given this Java code snippet connecting to a Docker Selenium container:

URL url = new URL("http://localhost:4444/wd/hub");
ChromeOptions options = new ChromeOptions();
RemoteWebDriver driver = new RemoteWebDriver(url, options);
driver.get("https://example.com");
String title = driver.getTitle();
System.out.println(title);

What will be printed if the page title is "Example Domain"?

medium

A. http://localhost:4444/wd/hub

B. Example Domain

C. ChromeOptions

D. RemoteWebDriver

Solution

1. Step 1: Understand the code flow

   The code opens the URL "https://example.com" in a remote Chrome browser inside Docker and retrieves the page title.

2. Step 2: Identify the output of getTitle()

   The getTitle() method returns the page title, which is "Example Domain" for that URL.

3. Final Answer:

   Example Domain -> Option B

4. Quick Check:

   driver.getTitle() = "Example Domain" [OK]

Hint: getTitle() returns the page title string [OK]

Common Mistakes:

• Printing the URL instead of the title
• Confusing options object with title
• Printing class names instead of values

4. You run Selenium tests inside Docker but get a connection refused error at "http://localhost:4444/wd/hub". What is the most likely cause?

medium

A. The URL "http://localhost:4444/wd/hub" is misspelled.

B. The test code uses ChromeDriver instead of RemoteWebDriver.

C. Docker container is not running or Selenium Grid is not started.

D. The test script lacks assertions.

Solution

1. Step 1: Analyze connection refused error

   This error means the test code cannot reach the Selenium server at the given URL.

2. Step 2: Identify common Docker causes

   If the Docker container running Selenium Grid is stopped or not started, the connection will fail.

3. Final Answer:

   Docker container is not running or Selenium Grid is not started. -> Option C

4. Quick Check:

   Connection refused = server not running [OK]

Hint: Check Docker container status if connection refused [OK]

Common Mistakes:

• Blaming ChromeDriver usage for connection errors
• Ignoring container running status
• Assuming URL typos without verification

5. You want to run parallel Selenium tests in Docker using RemoteWebDriver with ChromeOptions. Which approach best ensures tests run independently without interference?

hard

A. Start multiple Docker containers each running a Selenium node and connect each test to a different container URL.

B. Run all tests sequentially in a single Docker container using the same RemoteWebDriver instance.

C. Use ChromeDriver locally and ignore Docker for parallel tests.

D. Run tests in one container but share the same RemoteWebDriver instance across threads.

Solution

1. Step 1: Understand parallel test isolation needs

   Parallel tests require isolated browser sessions to avoid state conflicts.

2. Step 2: Evaluate Docker container usage

   Starting multiple containers with separate Selenium nodes allows independent browser sessions for each test.

3. Step 3: Eliminate unsafe options

   Sharing RemoteWebDriver instances or running sequentially defeats parallelism. Using local ChromeDriver ignores Docker benefits.

4. Final Answer:

   Start multiple Docker containers each running a Selenium node and connect each test to a different container URL. -> Option A

5. Quick Check:

   Parallel tests need isolated containers [OK]

Hint: Use separate containers for parallel Selenium tests [OK]

Common Mistakes:

• Sharing RemoteWebDriver instance across threads
• Running tests sequentially to avoid conflicts
• Ignoring Docker and using local drivers