Getting Started with Headless Chrome

seu景庄
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原文链接： developers.google.com

TL;DR

Headless Chrome is shipping in Chrome 59. It's a way to run the Chrome browser in a headless environment. Essentially, running Chrome without chrome! It brings all modern web platform features provided by Chromium and the Blink rendering engine to the command line.

Why is that useful?

A headless browser is a great tool for automated testing and server environments where you don't need a visible UI shell. For example, you may want to run some tests against a real web page, create a PDF of it, or just inspect how the browser renders an URL.

Caution: Headless mode is available on Mac and Linux in Chrome 59. Windows support is coming in Chrome 60. To check what version of Chrome you have, open chrome://version.

Starting Headless (CLI)

The easiest way to get started with headless mode is to open the Chrome binary from the command line. If you've got Chrome 59+ installed, start Chrome with the --headless flag:

chrome \
  --headless \                   # Runs Chrome in headless mode.
  --disable-gpu \                # Temporarily needed for now.
  --remote-debugging-port=9222 \
  https://www.chromestatus.com   # URL to open. Defaults to about:blank.
Note: Right now, you'll also want to include the --disable-gpu flag. That will eventually go away.

chrome should point to your installation of Chrome. The exact location will vary from platform to platform. Since I'm on Mac, I created convenient aliases for each version of Chrome that I have installed.

If you're on the stable channel of Chrome and cannot get the Beta, I recommend using chrome-canary:

alias chrome="/Applications/Google\ Chrome.app/Contents/MacOS/Google\ Chrome"
alias chrome-canary="/Applications/Google\ Chrome\ Canary.app/Contents/MacOS/Google\ Chrome\ Canary"
alias chromium="/Applications/Chromium.app/Contents/MacOS/Chromium"

Download Chrome Canary here.

Command line features

In some cases, you may not need to programmatically script Headless Chrome. There are some useful command line flags to perform common tasks.

Printing the DOM

The --dump-dom flag prints document.body.innerHTML to stdout:

chrome --headless --disable-gpu --dump-dom https://www.chromestatus.com/

Create a PDF

The --print-to-pdf flag creates a PDF of the page:

chrome --headless --disable-gpu --print-to-pdf https://www.chromestatus.com/

Taking screenshots

To capture a screenshot of a page, use the --screenshot flag:

chrome --headless --disable-gpu --screenshot https://www.chromestatus.com/

# Size of a standard letterhead.
chrome --headless --disable-gpu --screenshot --window-size=1280,1696 https://www.chromestatus.com/

# Nexus 5x
chrome --headless --disable-gpu --screenshot --window-size=412,732 https://www.chromestatus.com/

Running with --screenshot will produce a file named screenshot.png in the current working directory. If you're looking for full page screenshots, things are a tad more involved. There's a great blog post from David Schnurr that has you covered. Check out Using headless Chrome as an automated screenshot tool .

REPL mode (read-eval-print loop)

The --repl flag runs Headless in a mode where you can evaluate JS expressions in the browser, right from the command line:

$ chrome --headless --disable-gpu --repl https://www.chromestatus.com/
[0608/112805.245285:INFO:headless_shell.cc(278)] Type a Javascript expression to evaluate or "quit" to exit.
>>> location.href
{"result":{"type":"string","value":"https://www.chromestatus.com/features"}}
>>> quit
$

Debugging Chrome without a browser UI?

When you run Chrome with --remote-debugging-port=9222, it starts an instance with the DevTools Protocol enabled. The protocol is used to communicate with Chrome and drive the headless browser instance. It's also what tools like Sublime, VS Code, and Node use for remote debugging an application. #synergy

Since you don't have browser UI to see the page, navigate to http://localhost:9222 in another browser to check that everything is working. You'll see a list of inspectable pages where you can click through and see what Headless is rendering:

DevTools Remote
DevTools remote debugging UI

From here, you can use the familiar DevTools features to inspect, debug, and tweak the page as you normally would. If you're using Headless programmatically, this page is also a powerful debugging tool for seeing all the raw DevTools protocol commands going across the wire, communicating with the browser.

Using programmatically (Node)

Launching Chrome

In the previous section, we started Chrome manually using --headless --remote-debugging-port=9222. However, to fully automate tests, you'll probably want to spawn Chrome from your application.

One way is to use child_process:

const execFile = require('child_process').execFile;

function launchHeadlessChrome(url, callback) {
  // Assuming MacOSx.
  const CHROME = '/Applications/Google\ Chrome.app/Contents/MacOS/Google\ Chrome';
  execFile(CHROME, ['--headless', '--disable-gpu', '--remote-debugging-port=9222', url], callback);
}

launchHeadlessChrome('https://www.chromestatus.com', (err, stdout, stderr) => {
  ...
});

But things get tricky if you want a portable solution that works across multiple platforms. Just look at that hard-coded path to Chrome :(

Using ChromeLauncher

Lighthouse is a marvelous tool for testing the quality of your web apps. One thing people don't realize is that it ships with some really nice helper modules for working with Chrome. One of those modules is ChromeLauncher. ChromeLauncher will find where Chrome is installed, set up a debug instance, launch the browser, and kill it when your program is done. Best part is that it works cross-platform thanks to Node!

Note: The Lighthouse team is exploring a standalone package for ChromeLauncher with an improved API. Let us know if you have feedback.

By default, ChromeLauncher will try to launch Chrome Canary (if it's installed), but you can change that to manually select which Chrome to use. To use it, first install Lighthouse from npm:

yarn add lighthouse

Example - using ChromeLauncher to launch Headless

const chromeLauncher = require('lighthouse/chrome-launcher/chrome-launcher');

/**
 * Launches a debugging instance of Chrome.
 * @param {boolean=} headless True (default) launches Chrome in headless mode.
 *     False launches a full version of Chrome.
 * @return {Promise<ChromeLauncher>}
 */
async function launchChrome(headless = true) {
  return await chromeLauncher.launch({
    // port: 9222, // Uncomment to force a specific port of your choice.
    chromeFlags: [
      '--window-size=412,732',
      '--disable-gpu',
      headless ? '--headless' : ''
    ]
  });
}

launchChrome(true).then(chrome => {
  ...
});

Running this script doesn't do much, but you should see an instance of Chrome fire up in the task manager that loaded about:blank. Remember, there won't be any browser UI. We're headless.

To control the browser, we need the DevTools protocol!

Retrieving information about the page

chrome-remote-interface is a great Node package that provides usable APIs for the DevTools Protocol. You can use it to orchestrate Headless Chrome, navigate to pages, and fetch information about those pages.

Warning: The DevTools protocol can do a ton of interesting stuff, but it can be a bit daunting at first. I recommend spending a bit of time browsing the DevTools Protocol Viewer, first. Then, move on to the chrome-remote-interface API docs to see how it wraps the raw protocol.

Let's install the library:

yarn add chrome-remote-interface

Examples

Example - print the user agent

const CDP = require('chrome-remote-interface');

...

launchChrome().then(async chrome => {
  const version = await CDP.Version({port: chrome.port});
  console.log(version['User-Agent']));
});

Results in something like: HeadlessChrome/60.0.3082.0

Example - check if the site has a web app manifest

const CDP = require('chrome-remote-interface');

...

(async function() {

const chrome = await launchChrome();
const protocol = await CDP({port: chrome.port});

// Extract the DevTools protocol domains we need and enable them.
// See API docs: https://chromedevtools.github.io/devtools-protocol/
const {Page} = protocol;
await Page.enable();

Page.navigate({url: 'https://www.chromestatus.com/'});

// Wait for window.onload before doing stuff.
Page.loadEventFired(async () => {
  const manifest = await Page.getAppManifest();

  if (manifest.url) {
    console.log('Manifest: ' + manifest.url);
    console.log(manifest.data);
  } else {
    console.log('Site has no app manifest');
  }

  protocol.close();
  chrome.kill(); // Kill Chrome.
});

})();

Example - extract the <title> of the page using DOM APIs.

const CDP = require('chrome-remote-interface');

...

(async function() {

const chrome = await launchChrome();
const protocol = await CDP({port: chrome.port});

// Extract the DevTools protocol domains we need and enable them.
// See API docs: https://chromedevtools.github.io/devtools-protocol/
const {Page, Runtime} = protocol;
await Promise.all([Page.enable(), Runtime.enable()]);

Page.navigate({url: 'https://www.chromestatus.com/'});

// Wait for window.onload before doing stuff.
Page.loadEventFired(async () => {
  const js = "document.querySelector('title').textContent";
  // Evaluate the JS expression in the page.
  const result = await Runtime.evaluate({expression: js});

  console.log('Title of page: ' + result.result.value);

  protocol.close();
  chrome.kill(); // Kill Chrome.
});

})();

Further resources

Here are some useful resources to get you started:

Docs

Tools

Demos

  • "The Headless Web" - Paul Kinlan's great blog post on using Headless with api.ai.

FAQ

Do I need the --disable-gpu flag?

Yes, for now. The --disable-gpu flag is a temporary requirement to work around a few bugs. You won't need this flag in future versions of Chrome. See crbug.com/546953#c152 and crbug.com/695212 for more information.

So I still need Xvfb?

No. Headless Chrome doesn't use a window so a display server like Xvfb is no longer needed. You can happily run your automated tests without it.

What is Xvfb? Xvfb is an in-memory display server for Unix-like systems that enables you to run graphical applications (like Chrome) without an attached physical display. Many people use Xvfb to run earlier versions of Chrome to do "headless" testing.

How do I create a Docker container that runs Headless Chrome?

Check out lighthouse-ci. It has an example Dockerfile that uses Ubuntu as a base image, and installs + runs Lighthouse in an App Engine Flexible container.

Can I use this with Selenium / WebDriver / ChromeDriver?

Right now, Selenium opens a full instance of Chrome. In other words, it's an automated solution but not completely headless. However, Selenium could use --headless in the future.

If you want to bleed on the edge, I recommend Running Selenium with Headless Chrome to set things up yourself.

Note: you may encounter bugs using ChromeDriver. At the time of writing, the latest release (2.29) only supports Chrome 58. Headless Chrome requires Chrome 59 or later.

How is this related to PhantomJS?

Headless Chrome is similar to tools like PhantomJS. Both can be used for automated testing in a headless environment. The main difference between the two is that Phantom uses an older version of WebKit as its rendering engine while Headless Chrome uses the latest version of Blink.

At the moment, Phantom also provides a higher level API than the DevTools Protocol.

Where do I report bugs?

For bugs against Headless Chrome, file them on crbug.com.

For bugs in the DevTools protocol, file them at github.com/ChromeDevTo….

Except as otherwise noted, the content of this page is licensed under the Creative Commons Attribution 3.0 License, and code samples are licensed under the Apache 2.0 License. For details, see our Site Policies. Java is a registered trademark of Oracle and/or its affiliates.

Last updated June 9, 2017.

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