diff --git a/core/core.services.yml b/core/core.services.yml
index 4634033..3bfea9a 100644
--- a/core/core.services.yml
+++ b/core/core.services.yml
@@ -1239,7 +1239,7 @@ services:
     lazy: true
   batch.processor:
     class: Drupal\Core\Batch\BatchProcessor
-    arguments: ['@batch.queue', '@date.formatter', '@form_submitter', '@batch.storage', '@request_stack']
+    arguments: ['@app.root', '@batch.queue', '@date.formatter', '@form_submitter', '@batch.storage', '@request_stack']
   batch.queue:
     class: Drupal\Core\Batch\BatchQueueHandler
   replica_database_ignore__subscriber:
diff --git a/core/lib/Drupal/Core/Batch/BatchProcessor.php b/core/lib/Drupal/Core/Batch/BatchProcessor.php
index 1b65da5..13b0b98 100644
--- a/core/lib/Drupal/Core/Batch/BatchProcessor.php
+++ b/core/lib/Drupal/Core/Batch/BatchProcessor.php
@@ -36,14 +36,14 @@ class BatchProcessor implements BatchProcessorInterface {
   protected $batchQueue;
 
   /**
-   * The date formatter.
+   * The date formatter used to calculate the needed time for the batch.
    *
    * @var \Drupal\Core\Datetime\DateFormatter
    */
   protected $dateFormatter;
 
   /**
-   * The form submitter.
+   * The form submitter used to redirect at the end of the batch.
    *
    * @var \Drupal\Core\Form\FormSubmitterInterface
    */
@@ -64,15 +64,36 @@ class BatchProcessor implements BatchProcessorInterface {
   protected $requestStack;
 
   /**
+   * The app root.
+   *
+   * @var string
+   */
+  protected $root;
+
+  /**
    * Creates a new BatchProcessor.
+   *
+   * @param string $root
+   *   The app root.
+   * @param \Drupal\Core\Batch\BatchQueueHandlerInterface $batch_queue
+   *   The batch queue.
+   * @param \Drupal\Core\Datetime\DateFormatter $date_formatter
+   *   The date formatter used to calculate the needed time for the batch.
+   * @param \Drupal\Core\Form\FormSubmitterInterface $form_submitter
+   *   The form submitter used to redirect at the end of the batch.
+   * @param \Drupal\Core\Batch\BatchStorageInterface $batch_storage
+   *   The batch storage.
+   * @param \Symfony\Component\HttpFoundation\RequestStack $request_stack
+   *   The request stack.
    */
-  public function __construct(BatchQueueHandlerInterface $batch_queue, DateFormatter $date_formatter, FormSubmitterInterface $form_submitter, BatchStorageInterface $batch_storage, RequestStack $request_stack) {
+  public function __construct($root, BatchQueueHandlerInterface $batch_queue, DateFormatter $date_formatter, FormSubmitterInterface $form_submitter, BatchStorageInterface $batch_storage, RequestStack $request_stack) {
+    $this->root = $root;
     $this->batchQueue = $batch_queue;
     $this->dateFormatter = $date_formatter;
     $this->formSubmitter = $form_submitter;
     $this->batchStorage = $batch_storage;
     $this->requestStack = $request_stack;
-    $this->batch = array('sets' => array());
+    $this->batch = ['sets' => []];
   }
 
   /**
@@ -114,13 +135,13 @@ public function process() {
         list($callback, $args) = $item->data;
 
         // Build the 'context' array and execute the function call.
-        $batch_context = array(
+        $batch_context = [
           'sandbox'  => &$current_set['sandbox'],
           'results'  => &$current_set['results'],
           'finished' => &$finished,
           'message'  => &$task_message,
-        );
-        call_user_func_array($callback, array_merge($args, array(&$batch_context)));
+        ];
+        call_user_func_array($callback, array_merge($args, [&$batch_context]));
 
         if ($finished >= 1) {
           // Make sure this step is not counted twice when computing $current.
@@ -128,7 +149,7 @@ public function process() {
           // Remove the processed operation and clear the sandbox.
           $queue->deleteItem($item);
           $current_set['count']--;
-          $current_set['sandbox'] = array();
+          $current_set['sandbox'] = [];
         }
       }
 
@@ -197,7 +218,7 @@ public function process() {
         $label = $task_message;
       }
 
-      return array($percentage, $message, $label);
+      return [$percentage, $message, $label];
     }
     else {
       // If we are not in progressive mode, the entire batch has been processed.
@@ -224,7 +245,7 @@ public function nextSet() {
         // We use our stored copies of $form and $form_state to account for
         // possible alterations by previous form submit handlers.
         $complete_form = &$this->batch['form_state']->getCompleteForm();
-        call_user_func_array($callback, array(&$complete_form, &$this->batch['form_state']));
+        call_user_func_array($callback, [&$complete_form, &$this->batch['form_state']]);
       }
       return TRUE;
     }
@@ -239,12 +260,18 @@ public function finished() {
       if (isset($batch_set['finished'])) {
         // Check if the set requires an additional file for function definitions.
         if (isset($batch_set['file']) && is_file($batch_set['file'])) {
-          include_once DRUPAL_ROOT . '/' . $batch_set['file'];
+          include_once $this->root . '/' . $batch_set['file'];
         }
         if (is_callable($batch_set['finished'])) {
           $queue = $this->batchQueue->getQueue($batch_set);
           $operations = $queue->getAllItems();
-          call_user_func_array($batch_set['finished'], array($batch_set['success'], $batch_set['results'], $operations, $this->dateFormatter->formatInterval($batch_set['elapsed'] / 1000)));
+          $batch_set_result = call_user_func_array($batch_set['finished'], array($batch_set['success'], $batch_set['results'], $operations, $this->dateFormatter->formatInterval($batch_set['elapsed'] / 1000)));
+          // If a batch 'finished' callback requested a redirect after the batch
+          // is complete, save that for later use. If more than one batch set
+          // returned a redirect, the last one is used.
+          if ($batch_set_result instanceof RedirectResponse) {
+            $batch_finished_redirect = $batch_set_result;
+          }
         }
       }
     }
@@ -281,12 +308,21 @@ public function finished() {
       }
       if ($_batch['form_state']->getRedirect() === NULL) {
         $redirect = $_batch['batch_redirect'] ?: $_batch['source_url'];
-        $options = UrlHelper::parse($redirect);
-        if (!UrlHelper::isExternal($options['path'])) {
-          $options['path'] = $GLOBALS['base_url'] . '/' . $options['path'];
+        // Any path with a scheme does not correspond to a route.
+        if (!$redirect instanceof Url) {
+          $options = UrlHelper::parse($redirect);
+          if (parse_url($options['path'], PHP_URL_SCHEME)) {
+            $redirect = Url::fromUri($options['path'], $options);
+          }
+          else {
+            $redirect = \Drupal::pathValidator()->getUrlIfValid($options['path']);
+            if (!$redirect) {
+              // Stay on the same page if the redirect was invalid.
+              $redirect = Url::fromRoute('<current>');
+            }
+            $redirect->setOptions($options);
+          }
         }
-        $redirect = Url::createFromPath($options['path']);
-        $redirect->setOptions($options);
         $_batch['form_state']->setRedirectUrl($redirect);
       }
 
@@ -304,16 +340,17 @@ public function finished() {
         $_SESSION['batch_form_state'] = $_batch['form_state'];
       }
       $callback = $_batch['redirect_callback'];
+      /** @var \Drupal\Core\Url $source_url */
+      $source_url = $_batch['source_url'];
       if (is_callable($callback)) {
-        $callback($_batch['source_url'], array('query' => array('op' => 'finish', 'id' => $_batch['id'])));
+        $callback($_batch['source_url'], ['query' => ['op' => 'finish', 'id' => $_batch['id']]]);
       }
       elseif ($callback === NULL) {
         // Default to RedirectResponse objects when nothing specified.
-        $url = url($_batch['source_url'], array(
-          'absolute' => TRUE,
-          'query' => array('op' => 'finish', 'id' => $_batch['id']),
-        ));
-        return new RedirectResponse($url);
+        $url = $source_url
+          ->setAbsolute()
+          ->setOption('query', ['op' => 'finish', 'id' => $_batch['id']]);
+        return new RedirectResponse($url->toString());
       }
     }
   }