Performance metrics via DevTools in Selenium Java - Test Execution Trace

Test Overview

This test opens a web page using Selenium with Chrome DevTools Protocol (CDP) integration. It collects performance metrics like First Contentful Paint and DOMContentLoaded event time, then verifies these metrics are present and valid numbers.

Test Code - JUnit

Selenium Java

import org.openqa.selenium.chrome.ChromeDriver;
import org.openqa.selenium.chrome.ChromeOptions;
import org.openqa.selenium.devtools.DevTools;
import org.openqa.selenium.devtools.v114.performance.Performance;
import org.openqa.selenium.devtools.v114.performance.model.Metric;
import org.junit.jupiter.api.AfterEach;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;
import java.util.List;
import static org.junit.jupiter.api.Assertions.*;

public class PerformanceMetricsTest {
    private ChromeDriver driver;
    private DevTools devTools;

    @BeforeEach
    public void setUp() {
        ChromeOptions options = new ChromeOptions();
        driver = new ChromeDriver(options);
        devTools = driver.getDevTools();
        devTools.createSession();
        devTools.send(Performance.enable(java.util.Optional.empty()));
    }

    @Test
    public void testPerformanceMetrics() {
        driver.get("https://www.example.com");

        // Wait briefly to allow metrics collection
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }

        List<Metric> metrics = devTools.send(Performance.getMetrics()).getMetrics();

        // Check that key metrics exist and have valid values
        boolean hasFCP = metrics.stream().anyMatch(m -> m.getName().equals("FirstContentfulPaint") && m.getValue() > 0);
        boolean hasDOMLoaded = metrics.stream().anyMatch(m -> m.getName().equals("DomContentLoaded") && m.getValue() > 0);

        assertTrue(hasFCP, "FirstContentfulPaint metric should be present and greater than 0");
        assertTrue(hasDOMLoaded, "DomContentLoaded metric should be present and greater than 0");
    }

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

Execution Trace - 6 Steps

Step	Action	System State	Assertion	Result
1	Test starts and ChromeDriver with DevTools session is created	Chrome browser is launched with DevTools session enabled for performance monitoring	—	PASS
2	Navigates to https://www.example.com	Browser loads the example.com homepage	—	PASS
3	Waits 2 seconds to allow performance metrics to be collected	Page is fully loaded and metrics are being gathered by DevTools	—	PASS
4	Retrieves performance metrics via DevTools Performance.getMetrics()	Performance metrics data is available in the test	Check that FirstContentfulPaint metric exists and value > 0	PASS
5	Checks that DomContentLoaded metric exists and value > 0	Metrics list contains DomContentLoaded with valid value	DomContentLoaded metric is present and greater than 0	PASS
6	Test ends and browser is closed, DevTools session disabled	Browser closed, resources cleaned up	—	PASS

Failure Scenario

Failing Condition: Performance metrics are missing or have zero/invalid values

Execution Trace Quiz - 3 Questions

What is the purpose of enabling Performance domain in DevTools before navigation?

A To disable browser caching

B To take screenshots of the page

C To start collecting performance metrics during page load

D To block network requests

Key Result

Always enable the DevTools Performance domain before navigation to capture accurate performance metrics, and verify key metrics have valid values to ensure meaningful test results.

Practice

1. What is the main purpose of using DevTools performance metrics in Selenium Java tests?

easy

A. To handle browser alerts automatically

B. To locate elements on the web page

C. To take screenshots during test execution

D. To measure page load speed and browser performance

Solution

1. Step 1: Understand DevTools role in Selenium

   DevTools integration allows Selenium to collect browser performance data like load times.

2. Step 2: Identify the purpose of performance metrics

   Performance metrics help measure page speed and resource usage to improve user experience.

3. Final Answer:

   To measure page load speed and browser performance -> Option D

4. Quick Check:

   Performance metrics = page speed measurement [OK]

Hint: Performance metrics measure speed, not element location [OK]

Common Mistakes:

• Confusing performance metrics with element locators
• Thinking DevTools handles alerts
• Assuming screenshots are related to performance

2. Which Java code snippet correctly enables performance monitoring using DevTools in Selenium?

easy

A. devTools.send(Performance.enable());

B. driver.get("https://example.com");

C. driver.manage().timeouts().pageLoadTimeout(Duration.ofSeconds(10));

D. devTools.send(Network.enable(Optional.empty(), Optional.empty(), Optional.empty()));

Solution

1. Step 1: Identify the correct DevTools command for performance

   The Performance domain in DevTools is enabled by calling Performance.enable().

2. Step 2: Check other options for correctness

   devTools.send(Network.enable(Optional.empty(), Optional.empty(), Optional.empty())); enables Network, not Performance. driver.manage().timeouts().pageLoadTimeout(Duration.ofSeconds(10)); sets timeout, and B loads a page.

3. Final Answer:

   devTools.send(Performance.enable()); -> Option A

4. Quick Check:

   Enable performance with Performance.enable() [OK]

Hint: Use Performance.enable() to start performance monitoring [OK]

Common Mistakes:

• Using Network.enable() instead of Performance.enable()
• Confusing timeout settings with performance enabling
• Calling driver.get() instead of enabling metrics

3. Given the following code snippet, what will be the output type of metrics.getMetrics()?

DevTools devTools = driver.getDevTools();
devTools.createSession();
devTools.send(Performance.enable());
PerformanceMetrics metrics = devTools.send(Performance.getMetrics());
System.out.println(metrics.getMetrics().get(0).getName());

medium

A. A boolean indicating if metrics are enabled

B. An integer value of the first metric

C. A String representing the name of the first performance metric

D. A list of WebElement objects

Solution

1. Step 1: Understand Performance.getMetrics() return type

   It returns a PerformanceMetrics object containing a list of Metric objects, each with a name and value.

2. Step 2: Analyze metrics.getMetrics().get(0).getName()

   This accesses the first Metric's name, which is a String describing the metric.

3. Final Answer:

   A String representing the name of the first performance metric -> Option C

4. Quick Check:

   getName() returns String metric name [OK]

Hint: getName() returns metric name as String, not value [OK]

Common Mistakes:

• Assuming getName() returns a number
• Confusing metrics with WebElements
• Expecting a boolean from getMetrics()

4. Identify the error in this Selenium Java code snippet that tries to enable performance metrics:

DevTools devTools = driver.getDevTools();
devTools.send(Performance.enable());
devTools.createSession();

medium

A. Performance.enable() requires parameters

B. createSession() must be called before sending commands

C. DevTools cannot be used with ChromeDriver

D. No error, code is correct

Solution

1. Step 1: Check the order of DevTools commands

   Session must be created before sending any DevTools commands like Performance.enable().

2. Step 2: Verify method usage

   Calling Performance.enable() before createSession() causes an error because no session exists yet.

3. Final Answer:

   createSession() must be called before sending commands -> Option B

4. Quick Check:

   Call createSession() first, then enable performance [OK]

Hint: Always createSession() before sending DevTools commands [OK]

Common Mistakes:

• Calling enable() before createSession()
• Thinking Performance.enable() needs parameters
• Assuming DevTools is unsupported in ChromeDriver

5. You want to measure the page load time using Selenium Java and DevTools. Which approach correctly captures the 'DomContentLoaded' metric after page load?

hard

A. Enable performance, load page, then get metrics and filter for 'DomContentLoaded'

B. Load page first, then enable performance and get metrics immediately

C. Enable performance, get metrics before loading page, then load page

D. Only use driver.manage().timeouts().pageLoadTimeout() to measure load time

Solution

1. Step 1: Enable performance monitoring before page load

   Performance must be enabled before loading the page to capture all metrics during load.

2. Step 2: Load the page and then retrieve metrics filtering for 'DomContentLoaded'

   After page load, get metrics and select the 'DomContentLoaded' metric to measure load time.

3. Final Answer:

   Enable performance, load page, then get metrics and filter for 'DomContentLoaded' -> Option A

4. Quick Check:

   Enable first, load page, then get metrics [OK]

Hint: Enable before load, get metrics after load for accurate timing [OK]

Common Mistakes:

• Enabling performance after page load misses metrics
• Getting metrics before loading page
• Relying only on timeout settings for load time