private function getExpressionLanguage()
{
if (null === $this->expressionLanguage) {
if (!class_exists('Symfony\Component\ExpressionLanguage\ExpressionLanguage')) {
throw new RuntimeException('Unable to use expressions as the Symfony ExpressionLanguage component is not installed.');
}
$this->expressionLanguage = new ExpressionLanguage(null, $this->expressionLanguageProviders);
}
return $this->expressionLanguage;
}