Performance metrics via DevTools in Selenium Java - Build an Automation Script

Measure page load performance metrics using Chrome DevTools Protocol in Selenium

Preconditions (3)

Step 1: Launch Chrome browser with Selenium WebDriver

Step 2: Enable Chrome DevTools Protocol (CDP) session

Step 3: Navigate to the URL 'https://example.com'

Step 4: Use CDP to collect performance metrics after page load

Step 5: Extract metrics such as First Contentful Paint, DOMContentLoaded, and Load event

Step 6: Verify that these metrics are present and have reasonable numeric values

✅ Expected Result: Performance metrics are successfully collected via DevTools and contain valid numeric values indicating page load timings.

Automation Requirements - Selenium WebDriver with Chrome DevTools Protocol (CDP) support in Java

Assertions Needed:

Assert that performance metrics map contains keys for 'FirstContentfulPaint', 'DomContentLoaded', and 'LoadEvent'

Assert that the metric values are greater than zero

Best Practices:

Use Chrome DevTools Protocol commands via Selenium's DevTools interface

Use explicit waits to ensure page load is complete before collecting metrics

Use meaningful assertions to validate metric presence and values

Close browser session properly after test

Automated Solution

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.v115.performance.Performance;
import org.openqa.selenium.devtools.v115.performance.model.Metric;
import org.openqa.selenium.devtools.v115.page.Page;
import org.openqa.selenium.devtools.v115.page.model.LifecycleEvent;
import org.openqa.selenium.support.ui.WebDriverWait;
import org.openqa.selenium.support.ui.ExpectedCondition;
import java.time.Duration;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class PerformanceMetricsTest {
    public static void main(String[] args) {
        System.setProperty("webdriver.chrome.driver", "path/to/chromedriver");

        ChromeOptions options = new ChromeOptions();
        ChromeDriver driver = new ChromeDriver(options);

        DevTools devTools = driver.getDevTools();
        devTools.createSession();

        // Enable performance domain
        devTools.send(Performance.enable());

        // Navigate to the page
        driver.get("https://example.com");

        // Wait for load event fired
        WebDriverWait wait = new WebDriverWait(driver, Duration.ofSeconds(10));
        wait.until(webDriver -> {
            List<LifecycleEvent> lifecycleEvents = devTools.send(Page.getLifecycleEvents());
            return lifecycleEvents.stream().anyMatch(e -> e.getName().equals("load"));
        });

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

        // Convert list to map for easy lookup
        Map<String, Double> metricsMap = metrics.stream()
                .collect(Collectors.toMap(Metric::getName, Metric::getValue));

        // Assertions
        if (!metricsMap.containsKey("FirstContentfulPaint")) {
            throw new AssertionError("Missing FirstContentfulPaint metric");
        }
        if (!metricsMap.containsKey("DomContentLoaded")) {
            throw new AssertionError("Missing DomContentLoaded metric");
        }
        if (!metricsMap.containsKey("LoadEvent")) {
            throw new AssertionError("Missing LoadEvent metric");
        }

        if (metricsMap.get("FirstContentfulPaint") <= 0) {
            throw new AssertionError("FirstContentfulPaint metric value is not greater than zero");
        }
        if (metricsMap.get("DomContentLoaded") <= 0) {
            throw new AssertionError("DomContentLoaded metric value is not greater than zero");
        }
        if (metricsMap.get("LoadEvent") <= 0) {
            throw new AssertionError("LoadEvent metric value is not greater than zero");
        }

        System.out.println("Performance metrics collected successfully:");
        System.out.println(metricsMap);

        driver.quit();
    }
}

This Java Selenium test uses ChromeDriver and the Chrome DevTools Protocol (CDP) to collect performance metrics.

First, it sets up ChromeDriver and creates a DevTools session.

It enables the Performance domain to access performance metrics.

Then it navigates to the target URL and waits explicitly for the page load event to ensure the page is fully loaded.

After that, it collects the performance metrics and converts them into a map for easy access.

The test asserts that key metrics like FirstContentfulPaint, DomContentLoaded, and LoadEvent exist and have values greater than zero, indicating valid timings.

Finally, it prints the metrics and closes the browser.

This approach uses explicit waits and CDP commands properly, following best practices for reliable and maintainable test automation.

Common Mistakes - 4 Pitfalls

Not enabling the Performance domain before requesting metrics

Using Thread.sleep() instead of explicit waits for page load

Not checking if metrics keys exist before accessing values

Hardcoding chromedriver path without flexibility

Bonus Challenge

Now add data-driven testing to collect performance metrics for three different URLs: 'https://example.com', 'https://wikipedia.org', and 'https://github.com'.

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