diff --git a/src/App.php b/src/App.php
index 3fe0fa8..fd23105 100644
--- a/src/App.php
+++ b/src/App.php
@@ -203,9 +203,19 @@ class App {
    *
    * @param string $key The configuration variable key
    * @param mixed $value The configuration variable default value
+   * @param boolean $override Set to true to override default value if already set (optional,
+   *                          default: false)
    * @return boolean
    **/
-  public static function set_default($key, $value) {
+  public static function set_default($key, $value, $override=false) {
+    if (is_array($value)) {
+      $result = true;
+      foreach($value as $subkey => $subvalue)
+        $result = $result && self :: set_default("$key.$subkey", $subvalue, $override);
+      return $result;
+    }
+    if (self :: isset("default.$key") && !$override)
+      return true;
     return Config::set("default.$key", $value, self :: $options);
   }
 
@@ -214,12 +224,14 @@ class App {
    *
    * @param array<string,mixed> $values Associative array of configuration variables name and
    *                                    default values
+   * @param boolean $override Set to true to override default value if already set (optional,
+   *                          default: false)
    * @return boolean
    **/
-  public static function set_defaults($values) {
+  public static function set_defaults($values, $override=false) {
     $error = false;
     foreach($values as $key => $value)
-      if (!self :: set_default($key, $value))
+      if (!self :: set_default($key, $value, $override))
         $error = true;
     return !$error;
   }