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.