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.

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