Making those classes final that only have one private ctor, which means you can not inherit from them anyways

subsystem/src/main/java/org/keycloak/subsystem/extension/CredentialRemoveHandler.java

@@ -27,7 +27,7 @@ import org.jboss.dmr.ModelNode;
  *
  * @author Stan Silvert ssilvert@redhat.com (C) 2014 Red Hat Inc.
  */
-public class CredentialRemoveHandler extends AbstractRemoveStepHandler {
+public final class CredentialRemoveHandler extends AbstractRemoveStepHandler {
 
     public static CredentialRemoveHandler INSTANCE = new CredentialRemoveHandler();
 

subsystem/src/main/java/org/keycloak/subsystem/extension/KeycloakAdapterConfigService.java

@@ -38,7 +38,7 @@ import static org.jboss.as.controller.descriptions.ModelDescriptionConstants.ADD
  *
  * @author Stan Silvert ssilvert@redhat.com (C) 2013 Red Hat Inc.
  */
-public class KeycloakAdapterConfigService implements Service<KeycloakAdapterConfigService> {
+public final class KeycloakAdapterConfigService implements Service<KeycloakAdapterConfigService> {
     private static final String CREDENTIALS_JSON_NAME = "credentials";
 
     // Right now this is used as a service, but I'm not sure it really needs to be implemented that way.

subsystem/src/main/java/org/keycloak/subsystem/extension/RealmAddHandler.java

@@ -34,7 +34,7 @@ import org.jboss.msc.service.ServiceController;
  *
  * @author Stan Silvert ssilvert@redhat.com (C) 2013 Red Hat Inc.
  */
-public class RealmAddHandler extends AbstractAddStepHandler {
+public final class RealmAddHandler extends AbstractAddStepHandler {
 
     public static RealmAddHandler INSTANCE = new RealmAddHandler();
 

subsystem/src/main/java/org/keycloak/subsystem/extension/RealmRemoveHandler.java

@@ -27,7 +27,7 @@ import org.jboss.dmr.ModelNode;
  *
  * @author Stan Silvert ssilvert@redhat.com (C) 2013 Red Hat Inc.
  */
-public class RealmRemoveHandler extends AbstractRemoveStepHandler {
+public final class RealmRemoveHandler extends AbstractRemoveStepHandler {
 
     public static RealmRemoveHandler INSTANCE = new RealmRemoveHandler();
 

subsystem/src/main/java/org/keycloak/subsystem/extension/SecureDeploymentAddHandler.java

@@ -34,7 +34,7 @@ import org.jboss.msc.service.ServiceController;
  *
  * @author Stan Silvert ssilvert@redhat.com (C) 2013 Red Hat Inc.
  */
-public class SecureDeploymentAddHandler extends AbstractAddStepHandler {
+public final class SecureDeploymentAddHandler extends AbstractAddStepHandler {
 
     public static SecureDeploymentAddHandler INSTANCE = new SecureDeploymentAddHandler();
 

subsystem/src/main/java/org/keycloak/subsystem/extension/SecureDeploymentRemoveHandler.java

@@ -27,7 +27,7 @@ import org.jboss.dmr.ModelNode;
  *
  * @author Stan Silvert ssilvert@redhat.com (C) 2013 Red Hat Inc.
  */
-public class SecureDeploymentRemoveHandler extends AbstractRemoveStepHandler {
+public final class SecureDeploymentRemoveHandler extends AbstractRemoveStepHandler {
 
     public static SecureDeploymentRemoveHandler INSTANCE = new SecureDeploymentRemoveHandler();