Performance metrics via DevTools in Selenium Java

Introduction

Performance metrics help us see how fast a web page loads and works. Using DevTools lets us check these numbers automatically in tests.

You want to check if your website loads quickly for users.

You need to find slow parts of a page to improve them.

You want to compare performance before and after changes.

You want to make sure your site meets speed goals.

You want to catch performance problems early in testing.

Syntax

Selenium Java

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

// Enable performance monitoring
devTools.send(Performance.enable(Optional.empty()));

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

You need Selenium 4+ and a browser that supports DevTools (like Chrome).

Performance metrics include things like page load time and CPU usage.

Examples

This example starts a DevTools session, enables performance monitoring, and prints all metrics.

Selenium Java

DevTools devTools = ((HasDevTools) driver).getDevTools();
devTools.createSession();
devTools.send(Performance.enable(Optional.empty()));
List<Metric> metrics = devTools.send(Performance.getMetrics());
for (Metric metric : metrics) {
    System.out.println(metric.getName() + ": " + metric.getValue());
}

This example filters and prints only the TaskDuration metric, which shows the total time spent executing tasks.

Selenium Java

DevTools devTools = ((HasDevTools) driver).getDevTools();
devTools.createSession();
devTools.send(Performance.enable(Optional.empty()));
List<Metric> metrics = devTools.send(Performance.getMetrics());

// Find and print only the "TaskDuration" metric
metrics.stream()
    .filter(m -> m.getName().equals("TaskDuration"))
    .forEach(m -> System.out.println("TaskDuration: " + m.getValue()));

Sample Program

This test opens a Chrome browser, starts DevTools performance monitoring, loads example.com, then prints performance metrics like page load times.

Selenium Java

import org.openqa.selenium.WebDriver;
import org.openqa.selenium.chrome.ChromeDriver;
import org.openqa.selenium.devtools.DevTools;
import org.openqa.selenium.devtools.v114.performance.Performance;
import org.openqa.selenium.devtools.v114.performance.model.Metric;

import java.util.List;
import java.util.Optional;

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

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

        // Enable performance monitoring
        devTools.send(Performance.enable(Optional.empty()));

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

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

        // Print all metrics
        for (Metric metric : metrics) {
            System.out.println(metric.getName() + ": " + metric.getValue());
        }

        driver.quit();
    }
}

Important Notes

Make sure to use the correct ChromeDriver version matching your Chrome browser.

Performance metrics names and values may vary by browser version.

Always quit the driver to close the browser and free resources.

Summary

DevTools lets Selenium get performance data from the browser.

Enable performance monitoring before loading the page.

Use metrics to understand and improve page speed.

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