diff --git a/Readme.rst b/Readme.rst
index 86f6a6e..6dce9d2 100644
--- a/Readme.rst
+++ b/Readme.rst
@@ -136,3 +136,19 @@ Typo3Update.LegacyClassnames.DocComment: ``allowedTags``
            <property name="allowedTags" type="array" value="@param,@return,@var,@see,@throws"/>
        </properties>
    </rule>
+
+``mappingFile``
+    Configure where the `LegacyClassnames.php` is located, through ``ruleset.xml`` or using
+    ``--runtime-set``. Default is `LegacyClassnames.php` in the project root.
+
+    Example:
+
+.. code:: xml
+
+    <config name="mappingFile" value="/projects/typo3_installation/vendor/composer/autoload_classaliasmap.php"/>
+
+Example:
+
+.. code:: bash
+
+    --runtime-set mappingFile /projects/typo3_installation/vendor/composer/autoload_classaliasmap.php
diff --git a/src/Standards/Typo3Update/Sniffs/LegacyClassnames/ClassnameCheckerTrait.php b/src/Standards/Typo3Update/Sniffs/LegacyClassnames/ClassnameCheckerTrait.php
index 4297ae7..a59f8de 100644
--- a/src/Standards/Typo3Update/Sniffs/LegacyClassnames/ClassnameCheckerTrait.php
+++ b/src/Standards/Typo3Update/Sniffs/LegacyClassnames/ClassnameCheckerTrait.php
@@ -20,6 +20,7 @@ namespace Typo3Update\Sniffs\LegacyClassnames;
  * 02110-1301, USA.
  */
 
+use PHP_CodeSniffer as PhpCs;
 use PHP_CodeSniffer_File as PhpCsFile;
 
 /**
@@ -44,10 +45,14 @@ trait ClassnameCheckerTrait
     public $legacyExtensions = ['Extbase', 'Fluid'];
 
     /**
-     * @param string $mappingFile File containing php array for mapping.
+     * Initialize, used internally, to not initialize if not needed inside __construct.
      */
-    private function initialize($mappingFile = __DIR__ . '/../../../../../LegacyClassnames.php')
+    private function initialize()
     {
+        $mappingFile = PhpCs::getConfigData('mappingFile');
+        if (!$mappingFile) {
+            $mappingFile = __DIR__ . '/../../../../../LegacyClassnames.php';
+        }
         if ($this->legacyClassnames !== []) {
             return;
         }