How to get container instance in sonata Admin class?

probably already resolved, because its an old question, but just for reference, the container is already available in admin class using the configuration pool...

$this->getConfigurationPool()->getContainer();

Of course, it is better practice to inject services in the admin class, but, like in the controllers. why would someone, take the time to configure setter injection if already has the container available?

Add in your Admin class

/** @var \Symfony\Component\DependencyInjection\ContainerInterface */
private $container;

public function setContainer (\Symfony\Component\DependencyInjection\ContainerInterface $container) {
    $this->container = $container;
}

And add calls in services configuration(configuration can be specified in YAML, XML or PHP):

YAML

calls:
    - [ setContainer, [ @service_container ] ]

XML

 <call method="setContainer">
     <argument type="service" id="service_container" />
 </call>

Now you can using the normal symfony method: $this->container->get()

For more information see Service Container documentation