Implement a hibernate type for enums (#446)

* Implement a hibernate type for enums

Leverage GenericCollectionUserType to store enum fields as an array of
strings.

* Changes requested in review

* Remove extraneous import

Curiously, this causes checkstyle to fail now that the import was /removed
from a comment./

core/src/main/java/google/registry/persistence/EnumSetUserType.java

@@ -0,0 +1,37 @@
+// Copyright 2020 The Nomulus Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package google.registry.persistence;
+
+import java.util.HashSet;
+import java.util.Set;
+
+/** Abstract Hibernate user type for storing/retrieving {@link Set<Enum<E>>}. */
+public class EnumSetUserType<E extends Enum<E>> extends GenericCollectionUserType<Set<Enum<E>>> {
+
+  @Override
+  Set<Enum<E>> getNewCollection() {
+    return new HashSet<Enum<E>>();
+  }
+
+  @Override
+  ArrayColumnType getColumnType() {
+    return ArrayColumnType.STRING;
+  }
+
+  @Override
+  public Class returnedClass() {
+    return Set.class;
+  }
+}

core/src/main/java/google/registry/persistence/GenericCollectionUserType.java

@@ -66,7 +66,7 @@ public abstract class GenericCollectionUserType<T extends Collection> extends Mu
     if (rs.getArray(names[0]) != null) {
       T result = getNewCollection();
       for (Object element : (Object[]) rs.getArray(names[0]).getArray()) {
-        result.add(element);
+        result.add(convertToElem(element));
       }
       return result;
     }
@@ -85,4 +85,14 @@ public abstract class GenericCollectionUserType<T extends Collection> extends Mu
     Array arr = st.getConnection().createArrayOf(getColumnType().getTypeName(), list.toArray());
     st.setArray(index, arr);
   }
+
+  /**
+   * Override this to convert an element value retrieved from the database to a different type.
+   *
+   * <p>This method is useful when encoding a java type to one of the types that can be used as an
+   * array element.
+   */
+  protected Object convertToElem(Object columnValue) {
+    return columnValue;
+  }
 }