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/test/java/google/registry/persistence/EnumSetUserTypeTest.java

@@ -0,0 +1,79 @@
+// 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 static com.google.common.truth.Truth.assertThat;
+import static google.registry.model.transaction.TransactionManagerFactory.jpaTm;
+
+import com.google.common.collect.ImmutableSet;
+import google.registry.model.transaction.JpaTestRules;
+import google.registry.model.transaction.JpaTestRules.JpaUnitTestRule;
+import java.util.Set;
+import javax.persistence.Entity;
+import javax.persistence.Id;
+import org.hibernate.annotations.Type;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/** Unit tests for {@link EnumSetUserType}. */
+@RunWith(JUnit4.class)
+public class EnumSetUserTypeTest {
+  @Rule
+  public final JpaUnitTestRule jpaRule =
+      new JpaTestRules.Builder().withEntityClass(TestEntity.class).buildUnitTestRule();
+
+  public EnumSetUserTypeTest() {}
+
+  @Test
+  public void testRoundTrip() {
+    Set<TestEnum> enums = ImmutableSet.of(TestEnum.BAR, TestEnum.FOO);
+    TestEntity obj = new TestEntity("foo", enums);
+
+    jpaTm().transact(() -> jpaTm().getEntityManager().persist(obj));
+    TestEntity persisted =
+        jpaTm().transact(() -> jpaTm().getEntityManager().find(TestEntity.class, "foo"));
+    assertThat(persisted.data).isEqualTo(enums);
+  }
+
+  enum TestEnum {
+    FOO,
+    BAR,
+    BAZ;
+
+    public static class TestEnumType extends EnumSetUserType<TestEnum> {
+      @Override
+      protected Object convertToElem(Object value) {
+        return TestEnum.valueOf((String) value);
+      }
+    }
+  }
+
+  @Entity(name = "TestEntity")
+  static class TestEntity {
+    @Id String name;
+
+    @Type(type = "google.registry.persistence.EnumSetUserTypeTest$TestEnum$TestEnumType")
+    Set<TestEnum> data;
+
+    TestEntity() {}
+
+    TestEntity(String name, Set<TestEnum> data) {
+      this.name = name;
+      this.data = data;
+    }
+  }
+}