diff --git a/autoload/functions_oxDIC.php b/autoload/functions_oxDIC.php
index 13c5d86..3d3025b 100644
--- a/autoload/functions_oxDIC.php
+++ b/autoload/functions_oxDIC.php
@@ -17,7 +17,8 @@ use D3\DIContainerHandler\d3DicHandler;
 use Symfony\Component\DependencyInjection\Container;
 
 /**
- * @return   Container
+ * @return Container
+ * @throws Exception
  */
 function d3GetOxidDIC(): Container
 {
diff --git a/composer.json b/composer.json
index f882e2f..fbee068 100644
--- a/composer.json
+++ b/composer.json
@@ -22,7 +22,11 @@
     "MIT"
   ],
   "require": {
-    "symfony/dependency-injection": "^3.1|^5.4.11|^6.0"
+    "symfony/dependency-injection": "^3.1|^5.4.11|^6.0",
+    "beberlei/assert": "^3.3"
+  },
+  "require-dev": {
+    "phpunit/phpunit": "^9.6"
   },
   "autoload": {
     "psr-4": {
@@ -31,5 +35,8 @@
     "files": [
       "autoload/functions_oxDIC.php"
     ]
+  },
+  "scripts": {
+    "phpunit": "XDEBUG_MODE=coverage ./vendor/bin/phpunit --bootstrap=source/bootstrap.php --config=vendor/d3/oxid-dic-handler/tests/"
   }
 }
diff --git a/d3DicHandler.php b/d3DicHandler.php
index 4c741bb..2107029 100644
--- a/d3DicHandler.php
+++ b/d3DicHandler.php
@@ -28,13 +28,11 @@ use Symfony\Component\DependencyInjection\Loader\YamlFileLoader;
 
 class d3DicHandler implements d3DicHandlerInterface
 {
-    /**
-     * instance
-     * @var null|Container
-     */
-    protected static ?Container $_instance = null;
+    protected static Container|null $_instance = null;
 
-    public static array $circularReferenceMethodNames = ['getViewConfig'];
+    public static array $circularReferenceMethodNames = [
+        'getViewConfig'
+    ];
 
     /**
      * get instance
@@ -48,14 +46,12 @@ class d3DicHandler implements d3DicHandlerInterface
         $caller = $trace[1];
         $functionName = $caller['function'];
 
-        if (in_array(strtolower($functionName), array_map('strtolower', self::$circularReferenceMethodNames))) {
-            throw oxNew(
-                Exception::class,
-                'method '.$functionName." can't use DIC due the danger of circular reference"
-            );
+        if (in_array(strtolower($functionName), array_map('strtolower', self::$circularReferenceMethodNames)))
+        {
+            throw oxNew(Exception::class, 'method '.$functionName." can't use DIC due the danger of circular reference");
         }
 
-        if (null === self::$_instance) {
+        if (null == self::$_instance) {
             $oDicHandler = oxNew(d3DicHandler::class);
             self::$_instance = $oDicHandler->buildContainer();
         }
@@ -75,11 +71,9 @@ class d3DicHandler implements d3DicHandlerInterface
         $caller = $trace[1];
         $functionName = $caller['function'];
 
-        if (in_array(strtolower($functionName), array_map('strtolower', self::$circularReferenceMethodNames))) {
-            throw oxNew(
-                Exception::class,
-                'method '.$functionName." can't use DIC due the danger of circular reference"
-            );
+        if (in_array(strtolower($functionName), array_map('strtolower', self::$circularReferenceMethodNames)))
+        {
+            throw oxNew(Exception::class, 'method '.$functionName." can't use DIC due the danger of circular reference");
         }
 
         $oDicHandler = oxNew(d3DicHandler::class);
@@ -103,8 +97,7 @@ class d3DicHandler implements d3DicHandlerInterface
 
     public function d3GetCacheFilePath(): string
     {
-        return $this->d3GetConfig()->getConfigParam('sCompileDir').'/d3DicContainer_'.
-               Registry::getConfig()->getShopId().'.php';
+        return $this->d3GetConfig()->getConfigParam('sCompileDir').'/d3DicContainer_'.Registry::getConfig()->getShopId().'.php';
     }
 
     /**
@@ -113,10 +106,7 @@ class d3DicHandler implements d3DicHandlerInterface
     public function d3GetCacheContainer(): Container
     {
         require_once $this->d3GetCacheFilePath();
-
-        /** @var Container $container */
-        $container = oxNew(d3DIContainerCache::class); /** @phpstan-ignore-line */
-        return $container;
+        return oxNew(d3DIContainerCache::class);
     }
 
     /**
@@ -127,8 +117,7 @@ class d3DicHandler implements d3DicHandlerInterface
     public function d3GetFileLoader(ContainerBuilder $container): YamlFileLoader
     {
         /** @var YamlFileLoader $fileLoader */
-        $fileLoader = oxNew(
-            YamlFileLoader::class,
+        $fileLoader = oxNew(YamlFileLoader::class,
             $container,
             oxNew(FileLocator::class, d3DicUtilities::getVendorDir())
         );
@@ -154,6 +143,22 @@ class d3DicHandler implements d3DicHandlerInterface
         }
     }
 
+    /**
+     * @return bool
+     */
+    protected function isNotInTest(): bool
+    {
+        return false == defined('OXID_PHP_UNIT') || true == defined('D3_MODCFG_TEST');
+    }
+
+    /**
+     * @return bool
+     */
+    protected function cacheFileExists(): bool
+    {
+        return file_exists($this->d3GetCacheFilePath());
+    }
+
     /**
      * @param bool $compileAndDump
      *
@@ -162,16 +167,14 @@ class d3DicHandler implements d3DicHandlerInterface
      */
     public function buildContainer(bool $compileAndDump = true): Container
     {
-        if (Registry::get(ConfigFile::class)->getVar('iDebug')) {
-            startProfile(__METHOD__);
-        }
+        if ((bool) Registry::get( ConfigFile::class)->getVar( 'iDebug')) startProfile(__METHOD__);
 
         $config = $this->d3GetConfig();
 
-        if ($config->isProductiveMode()
-             && ! $config->getConfigParam('iDebug')
-            && (! defined('OXID_PHP_UNIT') || defined('D3_MODCFG_TEST'))
-            && file_exists($this->d3GetCacheFilePath())
+        if ( $config->isProductiveMode()
+             && ! $config->getConfigParam( 'iDebug' )
+            && $this->isNotInTest()
+            && $this->cacheFileExists()
         ) {
             $container = $this->d3GetCacheContainer();
         } else {
@@ -181,16 +184,14 @@ class d3DicHandler implements d3DicHandlerInterface
             if ($compileAndDump) {
                 $container->compile();
 
-                if (! defined('OXID_PHP_UNIT')) {
+                if ($this->isNotInTest()) {
                     $dumper = new PhpDumper($container);
-                    file_put_contents($this->d3GetCacheFilePath(), $dumper->dump(['class' => 'd3DIContainerCache']));
+                    file_put_contents($this->d3GetCacheFilePath(), $dumper->dump(array('class' => 'd3DIContainerCache')));
                 }
             }
         }
 
-        if (Registry::get(ConfigFile::class)->getVar('iDebug')) {
-            stopProfile(__METHOD__);
-        }
+        if ((bool) Registry::get( ConfigFile::class)->getVar( 'iDebug')) stopProfile(__METHOD__);
 
         return $container;
     }
@@ -203,14 +204,10 @@ class d3DicHandler implements d3DicHandlerInterface
     /**
      * clone
      */
-    public function __clone()
-    {
-    }
+    public function __clone() {}
 
     /**
      * constructor
      */
-    public function __construct()
-    {
-    }
+    public function __construct() {}
 }
diff --git a/d3DicHandlerInterface.php b/d3DicHandlerInterface.php
index 5f08359..25b5931 100644
--- a/d3DicHandlerInterface.php
+++ b/d3DicHandlerInterface.php
@@ -17,10 +17,7 @@ namespace D3\DIContainerHandler;
 
 use Symfony\Component\DependencyInjection\Container;
 
-/**
- * Interface d3DicHandlerInterface
- */
 interface d3DicHandlerInterface
 {
-    public static function getInstance(): ?Container;
-}
+    public static function getInstance(): Container;
+}
\ No newline at end of file
diff --git a/d3DicUtilities.php b/d3DicUtilities.php
index b3c7155..96330dd 100644
--- a/d3DicUtilities.php
+++ b/d3DicUtilities.php
@@ -23,7 +23,7 @@ class d3DicUtilities
      *
      * @return string
      */
-    public static function getServiceId(string $classNameSpace, ?string $additional = null): string
+    public static function getServiceId(string $classNameSpace, string $additional = null): string
     {
         return strtolower(
             ($additional ? $additional.'.' : '').
@@ -51,6 +51,6 @@ class d3DicUtilities
      */
     public static function getVendorDir(): string
     {
-        return rtrim(dirname(dirname(dirname(__FILE__))), '/').'/';
+        return rtrim( dirname( __FILE__, 3 ), '/') . '/';
     }
 }
diff --git a/definitionFileContainer.php b/definitionFileContainer.php
index ecdb1bf..201f5f6 100644
--- a/definitionFileContainer.php
+++ b/definitionFileContainer.php
@@ -15,60 +15,54 @@ declare(strict_types=1);
 
 namespace D3\DIContainerHandler;
 
-use InvalidArgumentException;
+use Assert\Assert;
+use Assert\InvalidArgumentException;
 
 class definitionFileContainer
 {
     public const TYPE_YAML = 'yml';
 
     protected array $definitionFiles = [
-        self::TYPE_YAML => [],
+        self::TYPE_YAML => []
     ];
 
     protected array $allowedTypes = [
-        self::TYPE_YAML,
+        self::TYPE_YAML
     ];
 
     public function __construct()
     {
-        $this->addYamlDefinitions('d3/modcfg/Config/services.yaml');
     }
 
     /**
-     * @param string $definitionFile
-     * @param string $type
+     * @param $definitionFile
+     * @param $type
+     * @throws InvalidArgumentException
      *
      * @return void
      */
-    public function addDefinitions(string $definitionFile, string $type): void
+    public function addDefinitions($definitionFile, $type): void
     {
-        if (!in_array($type, $this->allowedTypes)) {
-            throw new InvalidArgumentException('invalid definition file type');
-        }
+        Assert::that($type)->inArray($this->allowedTypes, 'invalid definition file type');
+        Assert::that(rtrim(dirname(__FILE__, 3).'/').$definitionFile)->file('invalid definition file');
 
         $this->definitionFiles[$type][md5($definitionFile)] = $definitionFile;
     }
 
-    /**
-     * @param string $definitionFile
-     *
-     * @return void
-     */
-    public function addYamlDefinitions(string $definitionFile): void
+    public function addYamlDefinitions($definitionFile): void
     {
         $this->addDefinitions($definitionFile, self::TYPE_YAML);
     }
 
     /**
-     * @param string $type
+     * @param $type
      *
      * @return array
+     * @throws InvalidArgumentException
      */
-    public function getDefinitions(string $type): array
+    public function getDefinitions($type): array
     {
-        if (!in_array($type, $this->allowedTypes)) {
-            throw new InvalidArgumentException('invalid definition file type');
-        }
+        Assert::that($type)->inArray($this->allowedTypes, 'invalid definition file type');
 
         return $this->definitionFiles[$type];
     }
@@ -82,14 +76,8 @@ class definitionFileContainer
     }
 
     /**
-     * @param string $definitionFile
-     * @return bool
+     * @return array[]
      */
-    public function has(string $definitionFile): bool
-    {
-        return isset($this->definitionFiles[md5($definitionFile)]);
-    }
-
     public function getAll(): array
     {
         return $this->definitionFiles;
@@ -99,4 +87,4 @@ class definitionFileContainer
     {
         $this->definitionFiles = [];
     }
-}
+}
\ No newline at end of file