How do you access Doctrine DBAL in a Symfony2 service class?-Collection of common programming errors

First off you should add a constructor to your class and pass in the @doctrine.dbal.connection service

namespace Acme\TestBundle\Toolbox;
use Doctrine\DBAL\Connection;

class StringToolbox
{
    /**
    *
    * @var Connection
    */
    private $connection;

    public function __construct(Connection $dbalConnection)  {
        $this->connection = $dbalConnection;    
    }

    public function lookupSomething($foo)
    {

    $sql = "SELECT bar FROM bar_list WHERE foo = :foo";
    $stmt = $this->connection->prepare($sql);
    $stmt->bindValue("foo", $foo);
    $stmt->execute();


    return $bar;
    }


}

Your service configuration should now look like this:

services:
 toolbox:
   class:        Acme\TestBundle\Toolbox\StringToolbox
    arguments:   [@doctrine.dbal.connection]

What you are saying with this configuration is “make me a service named toolbox that will receive the doctrine.dbal.connection service as the first constructor argument”

There are other injection methods besides Constructor injection and you should read the http://symfony.com/doc/current/book/service_container.html documentation to get a grasp of all possibilities (Setter injection, Factory injection, etc) and to better understand how Dependency Injection works