/Drupal 8

function hook_file_url_alter


Alter the URL to a file.

This hook is called from file_create_url(), and is called fairly frequently (10+ times per page), depending on how many files there are in a given page. If CSS and JS aggregation are disabled, this can become very frequently (50+ times per page) so performance is critical.

This function should alter the URI, if it wants to rewrite the file URL.


$uri: The URI to a file for which we need an external URL, or the path to a shipped file.

Related topics

Define functions that alter the behavior of Drupal core.


core/lib/Drupal/Core/File/file.api.php, line 57
Hooks related to the File management system.


function hook_file_url_alter(&$uri) {
  $user = \Drupal::currentUser();

  // User 1 will always see the local file in this example.
  if ($user->id() == 1) {

  $cdn1 = 'http://cdn1.example.com';
  $cdn2 = 'http://cdn2.example.com';
  $cdn_extensions = array('css', 'js', 'gif', 'jpg', 'jpeg', 'png');

  // Most CDNs don't support private file transfers without a lot of hassle,
  // so don't support this in the common case.
  $schemes = array('public');

  $scheme = file_uri_scheme($uri);

  // Only serve shipped files and public created files from the CDN.
  if (!$scheme || in_array($scheme, $schemes)) {
    // Shipped files.
    if (!$scheme) {
      $path = $uri;
    // Public created files.
    else {
      $wrapper = \Drupal::service('stream_wrapper_manager')->getViaScheme($scheme);
      $path = $wrapper->getDirectoryPath() . '/' . file_uri_target($uri);

    // Clean up Windows paths.
    $path = str_replace('\\', '/', $path);

    // Serve files with one of the CDN extensions from CDN 1, all others from
    // CDN 2.
    $pathinfo = pathinfo($path);
    if (isset($pathinfo['extension']) && in_array($pathinfo['extension'], $cdn_extensions)) {
      $uri = $cdn1 . '/' . $path;
    else {
      $uri = $cdn2 . '/' . $path;

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