diff -u a/core/lib/Drupal/Core/DrupalKernel.php a/core/lib/Drupal/Core/DrupalKernel.php
--- a/core/lib/Drupal/Core/DrupalKernel.php
+++ a/core/lib/Drupal/Core/DrupalKernel.php
@@ -174,7 +174,7 @@
     }
 
     $this->configStorage = BootstrapConfigStorageFactory::get();
-    $this->moduleHandler = ModuleHandlerFactory::create($this->configStorage);
+    $this->moduleHandler = ModuleHandlerFactory::get($this->configStorage);
     $this->initializeContainer();
     $this->booted = TRUE;
     if ($this->containerNeedsDumping && !$this->dumpDrupalContainer($this->container, static::CONTAINER_BASE_CLASS)) {
@@ -280,10 +280,11 @@
    * changes.
    */
   public function rebuildContainer() {
-    // If we haven't yet booted, we don't need to do anything. If we have already
-    // booted, then reboot in order to refresh the bundle list and container.
+    // If we haven't yet booted, we don't need to do anything. If we have
+    // already booted, then reboot in order to refresh the bundle list and
+    // container.
     if ($this->booted) {
-      $this->moduleHandler = ModuleHandlerFactory::create($this->configStorage);
+      $this->moduleHandler = ModuleHandlerFactory::get($this->configStorage);
       $this->initializeContainer(TRUE);
     }
   }
@@ -575,6 +576,9 @@
 
   /**
    * Registers a list of namespaces.
+   *
+   * @param array $namespaces
+   *   An array of namespaces.to register.
    */
   protected function registerNamespaces(array $namespaces = array()) {
     foreach ($namespaces as $prefix => $path) {
@@ -583,8 +587,14 @@
   }
 
   /**
-   * @param $class
-   * @param $persist
+   * Initializes the cached container.
+   *
+   * @param string $class
+   *   The classname of the cached container.
+   * @param bool $persist
+   *   Whether or not to persist the cached container.
+   * @return ContainerInterface
+   *   The cached container.
    */
   protected function initializeCachedContainer($class, $persist) {
     $container = NULL;
diff -u a/core/lib/Drupal/Core/Extension/ModuleHandler.php a/core/lib/Drupal/Core/Extension/ModuleHandler.php
--- a/core/lib/Drupal/Core/Extension/ModuleHandler.php
+++ a/core/lib/Drupal/Core/Extension/ModuleHandler.php
@@ -64,8 +64,6 @@
    */
   protected $alterFunctions;
 
-  protected $moduleData;
-
   /**
    * Constructs a ModuleHandler object.
    *
@@ -142,40 +140,51 @@
     $this->resetImplementations();
   }
 
+  /**
+   * Returns the file name for each enabled module.
+   *
+   * @return array
+   *   An array of module names.
+   */
   public function getModuleNames() {
     return array_keys($this->moduleList);
   }
 
   /**
    * Returns the file name for each enabled module.
+   *
+   * @return array
+   *   An array of module filenames.
    */
   public function getModuleFileNames() {
     return array_values($this->moduleList);
   }
 
+  /**
+   * Returns the file name for a given enabled module.
+   *
+   * @return string
+   *   The module's filename.
+   */
   public function getModuleFileName($module) {
     return $this->moduleList[$module];
   }
 
-  public function getModuleDirectory($module) {
-    return dirname($this->moduleList[$module]);
-  }
-
   /**
-   * Returns the file name for each enabled module.
+   * Returns the directory name for a given enabled module.
+   *
+   * @return string
+   *   The module's directory name.
    */
-  public function findModuleFileNames($module_list) {
-    $filenames = array();
-    foreach ($module_list as $module => $weight) {
-      if ($data = $this->moduleData($module, $module_list)) {
-        $filenames[$module] = $data->uri;
-      }
-    }
-    return $filenames;
+  public function getModuleDirectory($module) {
+    return dirname($this->moduleList[$module]);
   }
 
   /**
    * Gets the namespaces of each enabled module.
+   *
+   * @return array
+   *   An array of module namespaces.
    */
   public function getModuleNamespaces() {
     $namespaces = array();
diff -u a/core/lib/Drupal/Core/Extension/ModuleHandlerFactory.php a/core/lib/Drupal/Core/Extension/ModuleHandlerFactory.php
--- a/core/lib/Drupal/Core/Extension/ModuleHandlerFactory.php
+++ a/core/lib/Drupal/Core/Extension/ModuleHandlerFactory.php
@@ -10,19 +10,51 @@
 use Drupal\Core\Config\StorageInterface;
 use Drupal\Core\SystemListing;
 
+/**
+ * A factory for ModuleHandler that is responsible for finding both available
+ * and enabled modules.
+ */
 class ModuleHandlerFactory {
+
+  /**
+   * A cache to hold module data once it's been built.
+   *
+   * @var array
+   */
   protected $moduleData;
 
+  /**
+   * An constructor.
+   *
+   * @param StorageInterface
+   *   A config storage object.
+   */
   public function __construct(StorageInterface $config_storage) {
     $this->configStorage = $config_storage;
   }
 
-  public static function create(StorageInterface $config_storage) {
+  /**
+   * A static shortcut to encapsulate the instantiation of the factory as well
+   * as creating the ModuleHandler.
+   *
+   * @param StorageInterface
+   *   A config storage object.
+   * @return ModuleHandler
+   *   A module handler.
+   */
+  public static function get(StorageInterface $config_storage) {
     $factory = new static($config_storage);
-    return $factory->get();
+    return $factory->create();
   }
 
-  public function get() {
+  /**
+   * Factory method that finds available/enabled modules and instantiates a
+   * new ModuleHandler.
+   *
+   * @return ModuleHandler
+   *   An instance of ModuleHandler.
+   */
+  public function create() {
     $module_info = $this->configStorage->read('system.module');
     $enabled_modules = isset($module_info['enabled']) ? $module_info['enabled'] : array();
     $module_list = $this->findFileNames($enabled_modules);
@@ -31,6 +63,12 @@
 
   /**
    * Returns the file name for each enabled module.
+   *
+   * @param array $module_list
+   *   An associative array of modules where keys are module names and values
+   *   are weight.
+   * @return array
+   *   An array of module filenames.
    */
   public function findFileNames($module_list) {
     $filenames = array();
@@ -47,7 +85,9 @@
    *
    * @param $module
    *   The name of the module.
-   *
+   * @param array $module_list
+   *   An associative array of modules where keys are module names and values
+   *   are weight.
    * @return \stdClass|bool
    *   Returns a stdClass object if the module data is found containing at
    *   least an uri property with the module path, for example
@@ -60,6 +100,13 @@
     return isset($this->moduleData[$module]) ? $this->moduleData[$module] : FALSE;
   }
 
+  /**
+   * Builds moduleData cache by scanning the filesystem.
+   *
+   * @param array $module_list
+   *   An associative array of modules where keys are module names and values
+   *   are weight.
+   */
   protected function buildModuleData($module_list) {
     // First, find profiles.
     $profiles_scanner = new SystemListing();
diff -u a/core/modules/simpletest/lib/Drupal/simpletest/ConfigMemoryStorage.php a/core/modules/simpletest/lib/Drupal/simpletest/ConfigMemoryStorage.php
--- a/core/modules/simpletest/lib/Drupal/simpletest/ConfigMemoryStorage.php
+++ a/core/modules/simpletest/lib/Drupal/simpletest/ConfigMemoryStorage.php
@@ -6,6 +6,11 @@
 
 class ConfigMemoryStorage implements StorageInterface {
 
+  /**
+   * The module list that will be returned to the module handler.
+   *
+   * @var array
+   */
   protected $moduleList;
 
   /**
diff -u a/core/modules/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php a/core/modules/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php
--- a/core/modules/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php
+++ a/core/modules/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php
@@ -64,6 +64,11 @@
    */
   protected $keyValueFactory;
 
+  /**
+   * An instance of ConfigMemoryStorage.
+   *
+   * @var ConfigMemoryStorage
+   */
   protected $bootstrapConfig;
 
   /**
@@ -133,6 +138,12 @@
     \Drupal::config('system.theme')->set('default', 'stark');
   }
 
+  /**
+   * Getter for $bootstrapConfig.
+   *
+   * @return ConfigMemoryStorage
+   *   An instance of ConfigMemoryStorage.
+   */
   public function getBootstrapConfig() {
     return $this->bootstrapConfig;
   }