🎉 First try

src/main/java/net/micedre/keycloak/registration/RegistrationProfileWithMailDomainCheck.java

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+package net.micedre.keycloak.registration;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+
+import javax.ws.rs.core.MultivaluedMap;
+
+import org.keycloak.authentication.FormAction;
+import org.keycloak.authentication.ValidationContext;
+import org.keycloak.authentication.forms.RegistrationPage;
+import org.keycloak.authentication.forms.RegistrationProfile;
+import org.keycloak.events.Details;
+import org.keycloak.events.Errors;
+import org.keycloak.models.AuthenticatorConfigModel;
+import org.keycloak.models.utils.FormMessage;
+import org.keycloak.provider.ProviderConfigProperty;
+import org.keycloak.services.messages.Messages;
+import org.keycloak.services.validation.Validation;
+
+public class RegistrationProfileWithMailDomainCheck extends RegistrationProfile implements FormAction {
+
+   public static final String PROVIDER_ID = "registration-mail-check-action";
+
+   @Override
+    public String getDisplayType() {
+        return "Profile Validation with email domain check";
+   }
+
+
+   @Override
+   public String getId() {
+      return PROVIDER_ID;
+   }
+
+   @Override
+    public boolean isConfigurable() {
+        return true;
+   }
+
+
+   @Override
+   public String getHelpText() {
+      return "Adds validation of domain emails for registration";
+   }
+
+   private static final List<ProviderConfigProperty> CONFIG_PROPERTIES = new ArrayList<ProviderConfigProperty>();
+
+   static {
+      ProviderConfigProperty property;
+      property = new ProviderConfigProperty();
+      property.setName("validDomains");
+      property.setLabel("Valid domain for emails");
+      property.setType(ProviderConfigProperty.MULTIVALUED_STRING_TYPE);
+      property.setHelpText("List mail domains authorized to register");
+      CONFIG_PROPERTIES.add(property);
+   }
+
+   @Override
+   public List<ProviderConfigProperty> getConfigProperties() {
+      return CONFIG_PROPERTIES;
+   }
+
+   @Override
+   public void validate(ValidationContext context) {
+      MultivaluedMap<String, String> formData = context.getHttpRequest().getDecodedFormParameters();
+
+      List<FormMessage> errors = new ArrayList<>();
+      String email = formData.getFirst(Validation.FIELD_EMAIL);
+
+      boolean emailDomainValid = false;
+      AuthenticatorConfigModel mailDomainConfig = context.getAuthenticatorConfig();
+      String eventError = Errors.INVALID_REGISTRATION;
+
+      String[] domains = mailDomainConfig.getConfig().getOrDefault("validDomains","exemple.org").split("##");
+      for (String domain : domains) {
+         if (email.endsWith(domain)) {
+            emailDomainValid = true;
+            break;
+         }
+      }
+      if (!emailDomainValid) {
+         System.out.println("here");
+         context.getEvent().detail(Details.EMAIL, email);
+         errors.add(new FormMessage(RegistrationPage.FIELD_EMAIL, Messages.INVALID_EMAIL));
+      }
+      if (errors.size() > 0) {
+         context.error(eventError);
+         context.validationError(formData, errors);
+         return;
+
+      } else {
+         context.success();
+      }
+
+   }
+
+}

src/main/resources/META-INF/jboss-deployment-structure.xml

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+<jboss-deployment-structure>
+  <deployment>
+    <dependencies>
+      <module
+        name="org.keycloak.keycloak-core"
+        export="true" />
+      <module
+        name="org.keycloak.keycloak-server-spi"
+        export="true" />
+      <module
+        name="org.keycloak.keycloak-server-spi-private"
+        export="true" />
+      <module
+        name="org.keycloak.keycloak-services"
+        export="true" />
+    </dependencies>
+  </deployment>
+</jboss-deployment-structure>

src/main/resources/META-INF/services/org.keycloak.authentication.FormActionFactory

@@ -0,0 +1 @@
+net.micedre.keycloak.registration.RegistrationProfileWithMailDomainCheck