diff --git a/java/com/google/gerrit/json/BUILD b/java/com/google/gerrit/json/BUILD
index 7282dc46ed..030dddcb63 100644
--- a/java/com/google/gerrit/json/BUILD
+++ b/java/com/google/gerrit/json/BUILD
@@ -4,5 +4,6 @@ java_library(
     visibility = ["//visibility:public"],
     deps = [
         "//lib:gson",
+        "//lib/flogger:api",
     ],
 )
diff --git a/java/com/google/gerrit/json/EnumTypeAdapterFactory.java b/java/com/google/gerrit/json/EnumTypeAdapterFactory.java
new file mode 100644
index 0000000000..dc74f67dbf
--- /dev/null
+++ b/java/com/google/gerrit/json/EnumTypeAdapterFactory.java
@@ -0,0 +1,78 @@
+// Copyright (C) 2019 The Android Open Source Project
+//
+// 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 com.google.gerrit.json;
+
+import com.google.common.flogger.FluentLogger;
+import com.google.gson.Gson;
+import com.google.gson.TypeAdapter;
+import com.google.gson.TypeAdapterFactory;
+import com.google.gson.internal.bind.TypeAdapters;
+import com.google.gson.reflect.TypeToken;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonToken;
+import com.google.gson.stream.JsonWriter;
+import java.io.IOException;
+
+/**
+ * A {@code TypeAdapterFactory} for enums.
+ *
+ * <p>This factory introduces a wrapper around Gson's own default enum handler to add the following
+ * special behavior: log when input which doesn't match any existing enum value is encountered.
+ */
+public class EnumTypeAdapterFactory implements TypeAdapterFactory {
+  private static final FluentLogger logger = FluentLogger.forEnclosingClass();
+
+  @SuppressWarnings({"unchecked"})
+  @Override
+  public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
+    TypeAdapter<T> defaultEnumAdapter = TypeAdapters.ENUM_FACTORY.create(gson, typeToken);
+    if (defaultEnumAdapter == null) {
+      // Not an enum. -> Enum type adapter doesn't apply.
+      return null;
+    }
+
+    return (TypeAdapter<T>) new EnumTypeAdapter(defaultEnumAdapter, typeToken);
+  }
+
+  private static class EnumTypeAdapter<T extends Enum<T>> extends TypeAdapter<T> {
+
+    private final TypeAdapter<T> defaultEnumAdapter;
+    private final TypeToken<T> typeToken;
+
+    public EnumTypeAdapter(TypeAdapter<T> defaultEnumAdapter, TypeToken<T> typeToken) {
+      this.defaultEnumAdapter = defaultEnumAdapter;
+      this.typeToken = typeToken;
+    }
+
+    @Override
+    public T read(JsonReader in) throws IOException {
+      // Still handle null values. -> Check them first.
+      if (in.peek() == JsonToken.NULL) {
+        in.nextNull();
+        return null;
+      }
+      T enumValue = defaultEnumAdapter.read(in);
+      if (enumValue == null) {
+        logger.atWarning().log("Expected an existing value for enum %s.", typeToken);
+      }
+      return enumValue;
+    }
+
+    @Override
+    public void write(JsonWriter out, T value) throws IOException {
+      defaultEnumAdapter.write(out, value);
+    }
+  }
+}
diff --git a/java/com/google/gerrit/json/OutputFormat.java b/java/com/google/gerrit/json/OutputFormat.java
index a2d174f4cc..3e7c319aa7 100644
--- a/java/com/google/gerrit/json/OutputFormat.java
+++ b/java/com/google/gerrit/json/OutputFormat.java
@@ -55,7 +55,8 @@ public enum OutputFormat {
     GsonBuilder gb =
         new GsonBuilder()
             .setFieldNamingPolicy(FieldNamingPolicy.LOWER_CASE_WITH_UNDERSCORES)
-            .registerTypeAdapter(Timestamp.class, new SqlTimestampDeserializer());
+            .registerTypeAdapter(Timestamp.class, new SqlTimestampDeserializer())
+            .registerTypeAdapterFactory(new EnumTypeAdapterFactory());
     if (this == OutputFormat.JSON) {
       gb.setPrettyPrinting();
     }
diff --git a/javatests/com/google/gerrit/json/JsonEnumMappingTest.java b/javatests/com/google/gerrit/json/JsonEnumMappingTest.java
new file mode 100644
index 0000000000..6e57b011ef
--- /dev/null
+++ b/javatests/com/google/gerrit/json/JsonEnumMappingTest.java
@@ -0,0 +1,84 @@
+// Copyright (C) 2019 The Android Open Source Project
+//
+// 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 com.google.gerrit.json;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import com.google.gson.Gson;
+import org.junit.Test;
+
+public class JsonEnumMappingTest {
+
+  // Use the regular, pre-configured Gson object we use throughout the Gerrit server to ensure that
+  // the EnumTypeAdapterFactory is properly set up.
+  private final Gson gson = OutputFormat.JSON.newGson();
+
+  @Test
+  public void nullCanBeWrittenAndParsedBack() {
+    String resultingJson = gson.toJson(null, TestEnum.class);
+    TestEnum value = gson.fromJson(resultingJson, TestEnum.class);
+    assertThat(value).isNull();
+  }
+
+  @Test
+  public void enumValueCanBeWrittenAndParsedBack() {
+    String resultingJson = gson.toJson(TestEnum.ONE, TestEnum.class);
+    TestEnum value = gson.fromJson(resultingJson, TestEnum.class);
+    assertThat(value).isEqualTo(TestEnum.ONE);
+  }
+
+  @Test
+  public void enumValueCanBeParsed() {
+    TestData data = gson.fromJson("{\"value\":\"ONE\"}", TestData.class);
+    assertThat(data.value).isEqualTo(TestEnum.ONE);
+  }
+
+  @Test
+  public void mixedCaseEnumValueIsTreatedAsUnset() {
+    TestData data = gson.fromJson("{\"value\":\"oNe\"}", TestData.class);
+    assertThat(data.value).isNull();
+  }
+
+  @Test
+  public void lowerCaseEnumValueIsTreatedAsUnset() {
+    TestData data = gson.fromJson("{\"value\":\"one\"}", TestData.class);
+    assertThat(data.value).isNull();
+  }
+
+  @Test
+  public void notExistingEnumValueIsTreatedAsUnset() {
+    TestData data = gson.fromJson("{\"value\":\"FOUR\"}", TestData.class);
+    assertThat(data.value).isNull();
+  }
+
+  @Test
+  public void emptyEnumValueIsTreatedAsUnset() {
+    TestData data = gson.fromJson("{\"value\":\"\"}", TestData.class);
+    assertThat(data.value).isNull();
+  }
+
+  private static class TestData {
+    TestEnum value;
+
+    public TestData(TestEnum value) {
+      this.value = value;
+    }
+  }
+
+  private enum TestEnum {
+    ONE,
+    TWO
+  }
+}