Performance metrics via DevTools in Selenium Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Understanding DevTools Performance Metrics Retrieval

What will be the output of the following Java Selenium code snippet that uses Chrome DevTools Protocol to get the page load time metric?

Assume the page fully loads and metrics are available.

Selenium Java

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

Map<String, Object> metrics = devTools.send(Page.getMetrics());
System.out.println(metrics.get("Timestamp"));

A Prints a double value representing the timestamp in seconds since navigation start

B Prints a string with the page title

C Throws a NullPointerException because getMetrics() returns null

D Prints an integer representing the number of metrics collected

💡 Hint

Think about what the 'Timestamp' metric represents in DevTools metrics.

The 'Timestamp' metric from DevTools is a double value representing the time in seconds since the navigation started. The code prints this value. Other options are incorrect because getMetrics() returns a map of metrics, not page title or count, and it does not return null.

❓ assertion

intermediate

Correct Assertion for Page Load Time Metric

Which assertion correctly verifies that the page load time metric 'DomContentLoaded' is less than 5 seconds using JUnit in Selenium with DevTools?

Selenium Java

DevTools devTools = driver.getDevTools();
devTools.createSession();
Map<String, Object> metrics = devTools.send(Page.getMetrics());
Double domContentLoaded = (Double) metrics.get("DomContentLoaded");

A assertFalse(domContentLoaded > 5);

B assertEquals(5, domContentLoaded, 0.1);

C assertTrue(domContentLoaded < 5, "DomContentLoaded should be less than 5 seconds");

D assertNotNull(domContentLoaded);

💡 Hint

You want to check if the value is less than 5 seconds.

Option C correctly asserts that the DomContentLoaded metric is less than 5 seconds with a clear failure message. Option C checks equality which is not the requirement. Option C asserts false on greater than 5 which is logically correct but less clear. Option C only checks for null, not the value.

🔧 Debug

advanced

Debugging DevTools Session Creation Failure

You run the following Selenium Java code to start a DevTools session but get an IllegalStateException: "No active session found". What is the most likely cause?

Selenium Java

DevTools devTools = driver.getDevTools();
// Missing devTools.createSession();
Map<String, Object> metrics = devTools.send(Page.getMetrics());

A The driver instance is null

B You forgot to call devTools.createSession() before sending commands

C Page.getMetrics() is deprecated and cannot be used

D You need to cast the driver to ChromeDriver first

💡 Hint

DevTools commands require an active session.

DevTools commands require an active session created by devTools.createSession(). Without it, sending commands throws IllegalStateException. The driver being null or casting is unrelated. Page.getMetrics() is valid.

🧠 Conceptual

advanced

Understanding Performance Metrics Collection Timing

When using Selenium with Chrome DevTools Protocol to collect performance metrics, at what point is it best to capture the metrics to get accurate page load performance data?

A After waiting for the 'load' event or a specific element to be visible

B Before calling driver.get(url)

C Immediately after driver.get(url) returns

D After closing the browser

💡 Hint

Think about when the page is fully loaded and metrics are stable.

Performance metrics reflect page load events. Capturing them after the 'load' event or after a key element is visible ensures the page has fully loaded. Capturing immediately after driver.get(url) may be too early. Before driver.get(url) or after closing browser is invalid.

❓ framework

expert

Integrating DevTools Performance Metrics in Test Framework

You want to integrate Chrome DevTools performance metrics collection into a JUnit 5 Selenium test framework. Which approach ensures metrics are collected for each test and reported properly?

A Collect metrics after driver.quit() in @AfterAll method

B Collect metrics once in a static block before all tests run

C Collect metrics only in the first test method and reuse for others

D Create a @BeforeEach method to start DevTools session and a @AfterEach method to collect and log metrics

💡 Hint

Think about test isolation and per-test reporting.

Starting and collecting metrics per test ensures isolation and accurate reporting. Collecting once or after all tests risks mixing data or missing metrics. Collecting after driver.quit() is too late.