code refactoring

Signed-off-by: Kuanysh <kuanysh4646@gmail.com>

server/src/main/java/org/opensearch/common/settings/AbstractScopedSettings.java

@@ -238,17 +238,21 @@ public synchronized Settings applySettings(Settings newSettings) {
 
     /**
      * Adds a settings consumer with a predicate that is only evaluated at update time.
-     * <p>
-     * Note: Only settings registered in {@link SettingsModule} can be changed dynamically.
-     * </p>
-     * @param validator an additional validator that is only applied to updates of this setting.
-     *                  This is useful to add additional validation to settings at runtime compared to at startup time.
+     * This method allows registering an additional validator that is only applied to updates of a specific setting.
+     * It is useful to add additional validation to settings at runtime compared to at startup time.
+     * Please note that only settings registered in the {@link SettingsModule} can be changed dynamically.
+     *
+     * @param setting The setting for which the consumer is registered.
+     * @param consumer The consumer to be invoked when the setting is updated.
+     * @param validator An additional validator that is only applied to updates of this setting.
+     * @throws SettingsException if the setting is not registered for the given key.
+     * @throws NullPointerException if the setting is not registered.
      */
     public synchronized <T> void addSettingsUpdateConsumer(Setting<T> setting, Consumer<T> consumer, Consumer<T> validator) {
-        if (!setting.equals(get(setting.getKey()))) {
+        if (setting.getKey() != null && !setting.equals(get(setting.getKey()))) {
             throw new SettingsException("Setting is not registered for key [" + setting.getKey() + "]");
         } else if (setting.getKey() == null) {
-            logger.error("not registered setting key");
+            throw new NullPointerException("Setting is not registered");
         } else {
             addSettingsUpdater(setting.newUpdater(consumer, logger, validator));
         }

server/src/test/java/org/opensearch/common/settings/ScopedSettingsTests.java

@@ -1472,8 +1472,12 @@ public void testAddSettingsUpdateConsumer() {
         service.addSettingsUpdateConsumer(testSetting2, consumer2::set, (s) -> assertTrue(s > 0));
         Setting<Integer> wrongKeySetting = Setting.intSetting("foo.bar.wrong", 1, Property.Dynamic, Property.NodeScope);
 
-        expectThrows(SettingsException.class, () -> {
-            service.addSettingsUpdateConsumer(wrongKeySetting, consumer2::set, (i) -> { if (i == 42) throw new AssertionError("boom"); });
-        });
+        expectThrows(SettingsException.class, () -> service.addSettingsUpdateConsumer(wrongKeySetting, consumer2::set, (i) -> {
+            if (i == 42) throw new AssertionError("wrong key");
+        }));
+
+        expectThrows(NullPointerException.class, () -> service.addSettingsUpdateConsumer(null, consumer2::set, (i) -> {
+            if (i == 42) throw new AssertionError("empty key");
+        }));
     }
 }