Network interception (CDP) in Selenium Java

Introduction

Network interception lets you watch and change web traffic during tests. It helps check if your app talks to servers correctly.

To check if a web page loads the right data from the server.

To simulate slow or failed network responses during testing.

To block unwanted requests like ads or trackers in tests.

To modify server responses to test app behavior with different data.

To log all network calls for debugging test failures.

Syntax

Selenium Java

DevTools devTools = ((HasDevTools) driver).getDevTools();
devTools.createSession();
devTools.send(Network.enable(Optional.empty(), Optional.empty(), Optional.empty()));
devTools.addListener(Network.requestWillBeSent(), request -> {
    // handle request
});

This uses Chrome DevTools Protocol (CDP) via Selenium's DevTools interface.

You must create a DevTools session before intercepting network calls.

Examples

Prints every URL requested by the browser during the test.

Selenium Java

DevTools devTools = ((HasDevTools) driver).getDevTools();
devTools.createSession();
devTools.send(Network.enable(Optional.empty(), Optional.empty(), Optional.empty()));
devTools.addListener(Network.requestWillBeSent(), request -> {
    System.out.println("Request URL: " + request.getRequest().getUrl());
});

Logs the HTTP status code of each response received.

Selenium Java

devTools.addListener(Network.responseReceived(), response -> {
    System.out.println("Response status: " + response.getResponse().getStatus());
});

Blocks all requests to URLs matching the ads domain pattern.

Selenium Java

devTools.send(Network.setBlockedURLs(List.of("*://*.ads.com/*")));

Sample Program

This test opens a Chrome browser, starts network interception, blocks ad URLs, and prints all requested URLs while loading example.com.

Selenium Java

import org.openqa.selenium.chrome.ChromeDriver;
import org.openqa.selenium.devtools.DevTools;
import org.openqa.selenium.devtools.v114.network.Network;
import java.util.Optional;
import java.util.List;

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

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

        devTools.send(Network.enable(Optional.empty(), Optional.empty(), Optional.empty()));

        devTools.addListener(Network.requestWillBeSent(), request -> {
            System.out.println("Request URL: " + request.getRequest().getUrl());
        });

        devTools.send(Network.setBlockedURLs(List.of("*://*.ads.com/*")));

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

        driver.quit();
    }
}

Important Notes

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

Network interception works only with browsers supporting CDP, like Chrome.

Blocking URLs helps test app behavior without external content.

Summary

Network interception lets tests watch and control web traffic.

Use Selenium's DevTools to enable and listen to network events.

You can block, log, or modify requests and responses during tests.

Practice

1. What is the main purpose of using Network Interception with Selenium DevTools Protocol (CDP)?

easy

A. To replace Selenium WebDriver with a faster browser engine

B. To speed up test execution by skipping UI rendering

C. To monitor and control network requests and responses during tests

D. To automatically generate test data from network traffic

Solution

1. Step 1: Understand Network Interception

   Network interception allows tests to watch and manipulate network traffic between the browser and server.

2. Step 2: Identify the purpose in Selenium CDP context

   Selenium's DevTools Protocol integration enables listening to and controlling network events during tests.

3. Final Answer:

   To monitor and control network requests and responses during tests -> Option C

4. Quick Check:

   Network interception = monitor and control traffic [OK]

Hint: Network interception means watching and controlling web traffic [OK]

Common Mistakes:

• Thinking it speeds up tests by skipping UI
• Confusing it with test data generation
• Assuming it replaces WebDriver

2. Which Java code snippet correctly creates a DevTools session to enable network interception in Selenium 4+?

easy

A. DevTools devTools = driver.getDevTools(); devTools.startSession(); Network.enable();

B. DevTools devTools = new DevTools(); devTools.startSession(); devTools.enableNetwork();

C. DevTools devTools = driver.createDevTools(); devTools.start(); Network.enable();

D. DevTools devTools = driver.getDevTools(); devTools.createSession(); devTools.send(Network.enable(Optional.empty(), Optional.empty(), Optional.empty()));

Solution

1. Step 1: Check correct DevTools session creation

   In Selenium 4+, use driver.getDevTools() and then createSession() to start.

2. Step 2: Verify enabling network interception

   Use devTools.send(Network.enable(Optional.empty(), Optional.empty(), Optional.empty())) to enable network events.

3. Final Answer:

   DevTools devTools = driver.getDevTools(); devTools.createSession(); devTools.send(Network.enable(Optional.empty(), Optional.empty(), Optional.empty())); -> Option D

4. Quick Check:

   Use getDevTools(), createSession(), then send Network.enable [OK]

Hint: Use driver.getDevTools() then createSession() before enabling network [OK]

Common Mistakes:

• Trying to instantiate DevTools with new keyword
• Calling Network.enable() without devTools.send()
• Missing createSession() call

3. Given the following code snippet, what will be printed when a network request is intercepted?

DevTools devTools = driver.getDevTools();
devTools.createSession();
devTools.send(Network.enable(Optional.empty(), Optional.empty(), Optional.empty()));
devTools.addListener(Network.requestWillBeSent(), request -> {
    System.out.println("Request URL: " + request.getRequest().getUrl());
});
// Browser navigates to https://example.com

medium

A. Request URL: https://example.com/

B. Request URL: null

C. No output because listener is not added correctly

D. Compilation error due to missing imports

Solution

1. Step 1: Understand listener setup

   The listener is added to Network.requestWillBeSent event, printing the URL of each request.

2. Step 2: Predict output on navigating to example.com

   When browser navigates, a request is sent to https://example.com/, triggering the listener and printing the URL.

3. Final Answer:

   Request URL: https://example.com/ -> Option A

4. Quick Check:

   Listener prints request URL on navigation [OK]

Hint: Listener prints URL of each request sent during navigation [OK]

Common Mistakes:

• Assuming listener won't trigger without explicit start
• Expecting null URL in request
• Thinking listener setup is incorrect

4. Identify the error in this Selenium Java code snippet for blocking image requests using CDP:

DevTools devTools = driver.getDevTools();
devTools.createSession();
devTools.send(Network.enable(Optional.empty(), Optional.empty(), Optional.empty()));
devTools.addListener(Network.requestWillBeSent(), request -> {
    if(request.getRequest().getUrl().endsWith(".png")) {
        devTools.send(Network.setBlockedURLs(List.of(request.getRequest().getUrl())));
    }
});

medium

A. Calling setBlockedURLs inside the listener causes repeated blocking calls

B. Missing call to devTools.createSession() before enabling network

C. Using endsWith(".png") is invalid for URL filtering

D. Network.enable() should not be called with Optional.empty()

Solution

1. Step 1: Analyze listener behavior

   The listener calls setBlockedURLs every time a request ends with .png, causing repeated calls.

2. Step 2: Understand correct usage of setBlockedURLs

   setBlockedURLs should be called once with all URLs to block, not repeatedly inside the listener.

3. Final Answer:

   Calling setBlockedURLs inside the listener causes repeated blocking calls -> Option A

4. Quick Check:

   setBlockedURLs called repeatedly inside listener is wrong [OK]

Hint: Call setBlockedURLs once, not inside each request event [OK]

Common Mistakes:

• Calling setBlockedURLs repeatedly inside listener
• Forgetting to create session before enabling network
• Misusing string methods for URL filtering

5. You want to modify the response body of a JSON API call during a Selenium test using CDP. Which approach correctly achieves this?

hard

A. Use Network.requestWillBeSent to change the request URL to a local JSON file

B. Enable Network.requestIntercepted event, fetch response body, modify it, then use Network.continueInterceptedRequest with modified body

C. Call Network.setBlockedURLs to block the API call and serve a local file instead

D. Modify the DOM after page load to replace JSON data

Solution

1. Step 1: Understand response modification with CDP

   To modify a response, intercept it, get the body, change it, and continue with the new content.

2. Step 2: Identify correct event and method usage

   Use Network.requestIntercepted to detect response, fetch body, then Network.continueInterceptedRequest to send modified response.

3. Final Answer:

   Enable Network.requestIntercepted event, fetch response body, modify it, then use Network.continueInterceptedRequest with modified body -> Option B

4. Quick Check:

   Modify response by intercepting and continuing with new body [OK]

Hint: Intercept response, modify body, then continue intercepted request [OK]

Common Mistakes:

• Trying to modify request instead of response
• Blocking URL instead of modifying response
• Changing DOM instead of network response