protected $customerSession;
protected $urlInterface;

public function __construct(
    MagentoCustomerModelSession $customerSession,
    MagentoFrameworkUrlInterface $urlInterface,
    ...
)
{
    $this->urlInterface = $urlInterface;
    $this->customerSession = $customerSession;
    ...
}

public function execute()
{
    //Redirect to login page if customer is not logged in
    if(!$this->customerSession->isLoggedIn()) {
        $this->customerSession->setAfterAuthUrl($this->urlInterface->getCurrentUrl());
        $this->customerSession->authenticate();
    }
    ...
}