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How to alter existing service in Drupal 9 ?

In this article, we will explore the concept of service providers in Drupal and show you how you can use them to alter services in your own Drupal projects.

A Service Provider is responsible for registering and altering services with the container. When a Service Provider is loaded, it registers its services with the container so that they can be used by other parts of the site.

They also implement a method called alter, which is responsible for altering the behavior of an existing service.

To create a Drupal service provider, we start by creating a new class that extends the ServiceProviderBase class. This base class provides a set of methods that we can override to define our own services. Here is an example of a simple service provider class:

namespace Drupal\my_module;

use Drupal\Core\DependencyInjection\ContainerBuilder;
use Drupal\Core\DependencyInjection\ServiceProviderBase;

class MyServiceProvider extends ServiceProviderBase {

  public function alter(ContainerBuilder $container) {
    // Get an existing service and modify it.
    if ($container->hasDefinition('drupa_service')) {
      $definition = $container->getDefinition('drupal_service');


In this example, we're modifying an existing service  called drupal_service. We've created a new class called MyNewService, which extends drupal_service and provides some additional functionality. We want to use our new class instead of the default drupal_service class provided by Drupal.

To do this, we first get the definition of the existing drupal_service  from the container. We then modify this definition by setting the class to our new MyNewService class. This tells Drupal to use our new class instead of the default one when the drupal_service is requested.

In conclusion, Drupal service providers are a powerful way to extend the functionality of your Drupal site. By implementing a class that extends ServiceProviderBase, you can define the services that your provider will offer and how those services will be provided to other modules and components.