Magento 2 - Redirect user to a specific page if not logged in

If we want to catch controller_action_predispatch, we can follow:

app/code/Vendor/Module/etc/events.xml

     <event name="controller_action_predispatch">
            <observer name="check_login_persistent" instance="Vendor\Module\Observer\CheckLoginPersistentObserver" />
     </event>

app/code/Vendor/Module/Observer/CheckLoginPersistentObserver.php

namespace Vendor\Module\Observer;
use Magento\Framework\Event\ObserverInterface;


class CheckLoginPersistentObserver implements ObserverInterface
{
         /**
         * @var \Magento\Framework\App\Response\RedirectInterface
         */
        protected $redirect;

        /**
         * Customer session
         *
         * @var \Magento\Customer\Model\Session
         */
        protected $_customerSession;

        public function __construct(
            \Magento\Customer\Model\Session $customerSession,
            \Magento\Framework\App\Response\RedirectInterface $redirect

        ) {

            $this->_customerSession = $customerSession;
            $this->redirect = $redirect;

        }

        public function execute(\Magento\Framework\Event\Observer $observer)
        {
            $actionName = $observer->getEvent()->getRequest()->getFullActionName();
            $controller = $observer->getControllerAction();

            $openActions = array(
                'create',
                'createpost',
                'login',
                'loginpost',
                'logoutsuccess',
                'forgotpassword',
                'forgotpasswordpost',
                'resetpassword',
                'resetpasswordpost',
                'confirm',
                'confirmation'
            );
            if ($controller == 'account' && in_array($actionName, $openActions)) {
                return $this; //if in allowed actions do nothing.
            }
            if(!$this->_customerSession->isLoggedIn()) {
                $this->redirect->redirect($controller->getResponse(), 'customer/account/login');
            }

        }

}

There is a much easier solution. Look at this file:

src/vendor/magento/module-sales/etc/di.xml

<type name="Magento\Sales\Controller\Order\History">
    <plugin name="authentication" type="Magento\Sales\Controller\Order\Plugin\Authentication"/>
</type>

So you just need to use the authentication plugin in your module di.xml

For more optimize and working code you can follow the below steps.

1. create event file @ app\code\Vendor\Module\etc\frontend\events.xml

   <?xml version='1.0'?>
   <config xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance' xsi:noNamespaceSchemaLocation='../../../../../lib/internal/Magento/Framework/Event/etc/events.xsd'>
       <event name='controller_action_predispatch'>
           <observer
                   name='checklogin'
                   instance='Vendor\Module\Model\Observer'
           />
       </event>
   </config>
   
   

2. Create Observer file app\code\Vendor\Module\Model\Observer.php

   namespace Vendor\Module\Model;
   
   class Observer implements \Magento\Framework\Event\ObserverInterface
   {
   
   public function execute(\Magento\Framework\Event\Observer $observer)
   {
       # check if user is logged in
       $objectManager = \Magento\Framework\App\ObjectManager::getInstance();
       $customerSession = $objectManager->get('Magento\Customer\Model\Session');
       if(!$customerSession->isLoggedIn())
       {
           $request = $objectManager->get('Magento\Framework\App\Request\Http');
           //get instance for URL interface
           /** @var \Magento\Framework\UrlInterface $urlInterface */
           $urlInterface = $objectManager->get('Magento\Framework\UrlInterface');
           // URL to redirect to
           $url = $urlInterface->getUrl('customer/account/login');
           if(strpos($request->getPathInfo(), '/customer/account/') !== 0)
           {
               # redirect to /customer/account/login
               $observer->getControllerAction()
                   ->getResponse()
                   ->setRedirect($url);
           }
       }
   }