diff --git a/core/modules/automatic_updates/automatic_updates.install b/core/modules/automatic_updates/automatic_updates.install
index f4059c8203..c16b2fb3c0 100644
--- a/core/modules/automatic_updates/automatic_updates.install
+++ b/core/modules/automatic_updates/automatic_updates.install
@@ -2,7 +2,7 @@
 
 /**
  * @file
- * Automatic updates install file.
+ * Contains install and update functions for Automatic Updates.
  */
 
 use Drupal\automatic_updates\ReadinessChecker\ReadinessCheckerManagerInterface;
diff --git a/core/modules/automatic_updates/automatic_updates.module b/core/modules/automatic_updates/automatic_updates.module
index 1b23b96d3e..12574bf49f 100644
--- a/core/modules/automatic_updates/automatic_updates.module
+++ b/core/modules/automatic_updates/automatic_updates.module
@@ -2,7 +2,7 @@
 
 /**
  * @file
- * Contains automatic_updates.module..
+ * Provides hook implementations for Automatic Updates.
  */
 
 use Drupal\automatic_updates\ReadinessChecker\ReadinessCheckerManagerInterface;
diff --git a/core/modules/automatic_updates/automatic_updates.services.yml b/core/modules/automatic_updates/automatic_updates.services.yml
index c2fe5a973e..0e902f8506 100644
--- a/core/modules/automatic_updates/automatic_updates.services.yml
+++ b/core/modules/automatic_updates/automatic_updates.services.yml
@@ -4,14 +4,11 @@ services:
     arguments: ['automatic_updates']
   automatic_updates.disk_space_checker:
     class: Drupal\automatic_updates\ReadinessChecker\DiskSpace
-    arguments:
-      - '@app.root'
+    arguments: ['%app.root%']
     tags:
       - { name: readiness_checker, category: error}
   automatic_updates.readiness_checker:
     class: Drupal\automatic_updates\ReadinessChecker\ReadinessCheckerManager
-    arguments:
-      - '@keyvalue'
-      - '@config.factory'
+    arguments: ['@keyvalue', '@config.factory']
     tags:
       - { name: service_collector, tag: readiness_checker, call: addChecker }
diff --git a/core/modules/automatic_updates/src/Controller/ReadinessCheckerController.php b/core/modules/automatic_updates/src/Controller/ReadinessCheckerController.php
index d49d0d4af8..0976e7bb80 100644
--- a/core/modules/automatic_updates/src/Controller/ReadinessCheckerController.php
+++ b/core/modules/automatic_updates/src/Controller/ReadinessCheckerController.php
@@ -8,7 +8,7 @@
 use Symfony\Component\DependencyInjection\ContainerInterface;
 
 /**
- * Class ReadinessCheckerController.
+ * A controller for running Readiness Checkers.
  */
 class ReadinessCheckerController extends ControllerBase {
 
@@ -23,7 +23,7 @@ class ReadinessCheckerController extends ControllerBase {
    * ReadinessCheckerController constructor.
    *
    * @param \Drupal\automatic_updates\ReadinessChecker\ReadinessCheckerManagerInterface $checker
-   *   The readiness checker.
+   *   The readiness checker manager.
    * @param \Drupal\Core\StringTranslation\TranslationInterface $string_translation
    *   The string translation service.
    */
@@ -46,7 +46,7 @@ public static function create(ContainerInterface $container) {
    * Run the readiness checkers.
    *
    * @return \Symfony\Component\HttpFoundation\RedirectResponse
-   *   A redirect
+   *   A redirect response object.
    */
   public function run() {
     $messages = [];
diff --git a/core/modules/automatic_updates/src/Form/SettingsForm.php b/core/modules/automatic_updates/src/Form/SettingsForm.php
index bef89782ae..dc3e8bab83 100644
--- a/core/modules/automatic_updates/src/Form/SettingsForm.php
+++ b/core/modules/automatic_updates/src/Form/SettingsForm.php
@@ -8,18 +8,18 @@
 use Symfony\Component\DependencyInjection\ContainerInterface;
 
 /**
- * Settings form for automatic updates.
+ * Settings form for Automatic Updates.
  */
 class SettingsForm extends ConfigFormBase {
   /**
-   * The readiness checker.
+   * The readiness checker manager.
    *
    * @var \Drupal\automatic_updates\ReadinessChecker\ReadinessCheckerManagerInterface
    */
   protected $checker;
 
   /**
-   * The data formatter.
+   * The date formatter.
    *
    * @var \Drupal\Core\Datetime\DateFormatterInterface
    */
diff --git a/core/modules/automatic_updates/src/ReadinessChecker/DiskSpace.php b/core/modules/automatic_updates/src/ReadinessChecker/DiskSpace.php
index 984031e030..82d86eef5e 100644
--- a/core/modules/automatic_updates/src/ReadinessChecker/DiskSpace.php
+++ b/core/modules/automatic_updates/src/ReadinessChecker/DiskSpace.php
@@ -5,7 +5,7 @@
 use Drupal\Component\FileSystem\FileSystem as FileSystemComponent;
 
 /**
- * Disk space checker.
+ * The disk space readiness checker.
  */
 class DiskSpace extends Filesystem {
 
@@ -29,36 +29,37 @@ protected function doCheck() {
   /**
    * Check if the filesystem has sufficient disk space.
    *
-   * @return array
+   * @return \Drupal\Core\StringTranslation\TranslatableMarkup[]
    *   An array of translatable strings if there is not sufficient space.
    */
   protected function diskSpaceCheck() {
     $messages = [];
+    $minimum_megabytes = static::MINIMUM_DISK_SPACE / static::MEGABYTE_DIVISOR;
     if (!$this->areSameLogicalDisk($this->getRootPath(), $this->getVendorPath())) {
       if (disk_free_space($this->getRootPath()) < static::MINIMUM_DISK_SPACE) {
         $messages[] = $this->t('Drupal root filesystem "@root" has insufficient space. There must be at least @space megabytes free.', [
           '@root' => $this->getRootPath(),
-          '@space' => static::MINIMUM_DISK_SPACE / static::MEGABYTE_DIVISOR,
+          '@space' => $minimum_megabytes,
         ]);
       }
       if (is_dir($this->getVendorPath()) && disk_free_space($this->getVendorPath()) < static::MINIMUM_DISK_SPACE) {
         $messages[] = $this->t('Vendor filesystem "@vendor" has insufficient space. There must be at least @space megabytes free.', [
           '@vendor' => $this->getVendorPath(),
-          '@space' => static::MINIMUM_DISK_SPACE / static::MEGABYTE_DIVISOR,
+          '@space' => $minimum_megabytes,
         ]);
       }
     }
     elseif (disk_free_space($this->getRootPath()) < static::MINIMUM_DISK_SPACE) {
       $messages[] = $this->t('Logical disk "@root" has insufficient space. There must be at least @space megabytes free.', [
         '@root' => $this->getRootPath(),
-        '@space' => static::MINIMUM_DISK_SPACE / static::MEGABYTE_DIVISOR,
+        '@space' => $minimum_megabytes,
       ]);
     }
     $temp = FileSystemComponent::getOsTemporaryDirectory();
     if (disk_free_space($temp) < static::MINIMUM_DISK_SPACE) {
       $messages[] = $this->t('Directory "@temp" has insufficient space. There must be at least @space megabytes free.', [
         '@temp' => $temp,
-        '@space' => static::MINIMUM_DISK_SPACE / static::MEGABYTE_DIVISOR,
+        '@space' => $minimum_megabytes,
       ]);
     }
     return $messages;
diff --git a/core/modules/automatic_updates/src/ReadinessChecker/Filesystem.php b/core/modules/automatic_updates/src/ReadinessChecker/Filesystem.php
index de0914ff2a..48eb4b87c8 100644
--- a/core/modules/automatic_updates/src/ReadinessChecker/Filesystem.php
+++ b/core/modules/automatic_updates/src/ReadinessChecker/Filesystem.php
@@ -30,8 +30,8 @@ abstract class Filesystem implements ReadinessCheckerInterface {
    * @param string $app_root
    *   The app root.
    */
-  public function __construct($app_root) {
-    $this->rootPath = (string) $app_root;
+  public function __construct(string $app_root) {
+    $this->rootPath = $app_root;
   }
 
   /**
@@ -90,7 +90,7 @@ protected function getVendorPath() {
    * @return bool
    *   TRUE if same file system, FALSE otherwise.
    */
-  protected function areSameLogicalDisk($root, $vendor) {
+  protected function areSameLogicalDisk(string $root, string $vendor) {
     $root_statistics = stat($root);
     $vendor_statistics = stat($vendor);
     return $root_statistics && $vendor_statistics && $root_statistics['dev'] === $vendor_statistics['dev'];
diff --git a/core/modules/automatic_updates/src/ReadinessChecker/ReadinessCheckerManager.php b/core/modules/automatic_updates/src/ReadinessChecker/ReadinessCheckerManager.php
index f6cfab74af..c69f841de6 100644
--- a/core/modules/automatic_updates/src/ReadinessChecker/ReadinessCheckerManager.php
+++ b/core/modules/automatic_updates/src/ReadinessChecker/ReadinessCheckerManager.php
@@ -50,7 +50,7 @@ public function __construct(KeyValueFactoryInterface $key_value, ConfigFactoryIn
   /**
    * {@inheritdoc}
    */
-  public function addChecker(ReadinessCheckerInterface $checker, $category = 'warning', $priority = 0) {
+  public function addChecker(ReadinessCheckerInterface $checker, string $category = 'warning', $priority = 0) {
     if (!in_array($category, $this->getCategories(), TRUE)) {
       throw new \InvalidArgumentException(sprintf('Readiness checker category "%s" is invalid. Use "%s" instead.', $category, implode('" or "', $this->getCategories())));
     }
@@ -61,11 +61,12 @@ public function addChecker(ReadinessCheckerInterface $checker, $category = 'warn
   /**
    * {@inheritdoc}
    */
-  public function run($category) {
-    $messages = [];
+  public function run(string $category) {
     if (!$this->isEnabled()) {
-      return $messages;
+      return [];
     }
+    $messages = [];
+
     if (!isset($this->getSortedCheckers()[$category])) {
       // @todd Adding only 1 checker so will error for "warning".
       // throw new \InvalidArgumentException(sprintf('No readiness checkers exist of category "%s"', $category));
diff --git a/core/modules/automatic_updates/src/ReadinessChecker/ReadinessCheckerManagerInterface.php b/core/modules/automatic_updates/src/ReadinessChecker/ReadinessCheckerManagerInterface.php
index 09523eafac..b656943f30 100644
--- a/core/modules/automatic_updates/src/ReadinessChecker/ReadinessCheckerManagerInterface.php
+++ b/core/modules/automatic_updates/src/ReadinessChecker/ReadinessCheckerManagerInterface.php
@@ -34,7 +34,7 @@ interface ReadinessCheckerManagerInterface {
    *
    * @return $this
    */
-  public function addChecker(ReadinessCheckerInterface $checker, $category = 'warning', $priority = 0);
+  public function addChecker(ReadinessCheckerInterface $checker, string $category = 'warning', $priority = 0);
 
   /**
    * Run checks.
@@ -45,7 +45,7 @@ public function addChecker(ReadinessCheckerInterface $checker, $category = 'warn
    * @return array
    *   An array of translatable strings.
    */
-  public function run($category);
+  public function run(string $category);
 
   /**
    * Get results of most recent run.
@@ -75,10 +75,10 @@ public function timestamp();
   public function isEnabled();
 
   /**
-   * Get the categories of checkers.
+   * Get the checker categories.
    *
-   * @return array
-   *   The categories of checkers.
+   * @return string[]
+   *   The checkers categories.
    */
   public function getCategories();
 
diff --git a/core/modules/automatic_updates/tests/src/Kernel/ReadinessChecker/DiskSpaceTest.php b/core/modules/automatic_updates/tests/src/Kernel/ReadinessChecker/DiskSpaceTest.php
index a3b499947f..7efc53affe 100644
--- a/core/modules/automatic_updates/tests/src/Kernel/ReadinessChecker/DiskSpaceTest.php
+++ b/core/modules/automatic_updates/tests/src/Kernel/ReadinessChecker/DiskSpaceTest.php
@@ -17,26 +17,24 @@ class DiskSpaceTest extends KernelTestBase {
   /**
    * {@inheritdoc}
    */
-  public static $modules = [
-    'automatic_updates',
-  ];
+  protected static $modules = ['automatic_updates'];
 
   /**
    * Tests the functionality of disk space readiness checks.
    */
   public function testDiskSpace() {
     // No disk space issues.
-    $disk_space = new DiskSpace($this->container->get('app.root'));
+    $disk_space = new DiskSpace($this->container->getParameter('app.root'));
     $messages = $disk_space->run();
     $this->assertEmpty($messages);
 
     // Out of space.
-    $disk_space = new TestDiskSpace($this->container->get('app.root'));
+    $disk_space = new TestDiskSpace($this->container->getParameter('app.root'));
     $messages = $disk_space->run();
     $this->assertCount(2, $messages);
 
     // Out of space not the same logical disk.
-    $disk_space = new TestDiskSpaceNonSameDisk($this->container->get('app.root'));
+    $disk_space = new TestDiskSpaceNonSameDisk($this->container->getParameter('app.root'));
     $messages = $disk_space->run();
     $this->assertCount(3, $messages);
   }
@@ -63,7 +61,7 @@ class TestDiskSpaceNonSameDisk extends TestDiskSpace {
   /**
    * {@inheritdoc}
    */
-  protected function areSameLogicalDisk($root, $vendor) {
+  protected function areSameLogicalDisk(string $root, string $vendor) {
     return FALSE;
   }