diff --git a/javatests/com/google/gerrit/plugins/codeowners/backend/CodeOwnerBackendIdTest.java b/javatests/com/google/gerrit/plugins/codeowners/backend/CodeOwnerBackendIdTest.java
new file mode 100644
index 0000000..3f5ee09
--- /dev/null
+++ b/javatests/com/google/gerrit/plugins/codeowners/backend/CodeOwnerBackendIdTest.java
@@ -0,0 +1,82 @@
+// Copyright (C) 2021 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.plugins.codeowners.backend;
+
+import static com.google.common.truth.Truth.assertThat;
+import static com.google.gerrit.testing.GerritJUnit.assertThrows;
+
+import com.google.gerrit.common.Nullable;
+import com.google.gerrit.entities.Project.NameKey;
+import com.google.gerrit.plugins.codeowners.acceptance.AbstractCodeOwnersTest;
+import com.google.gerrit.plugins.codeowners.backend.findowners.FindOwnersBackend;
+import com.google.gerrit.plugins.codeowners.backend.proto.ProtoBackend;
+import com.google.gerrit.server.IdentifiedUser;
+import java.nio.file.Path;
+import java.util.Optional;
+import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.revwalk.RevWalk;
+import org.junit.Test;
+
+/** Tests for {@link com.google.gerrit.plugins.codeowners.backend.CodeOwnerBackendId}. */
+public class CodeOwnerBackendIdTest extends AbstractCodeOwnersTest {
+  @Test
+  public void getBackendIdForCodeOwnersBackendClass() throws Exception {
+    assertThat(CodeOwnerBackendId.getBackendId(FindOwnersBackend.class))
+        .isEqualTo(FindOwnersBackend.ID);
+    assertThat(CodeOwnerBackendId.getBackendId(ProtoBackend.class)).isEqualTo(ProtoBackend.ID);
+
+    IllegalStateException exception =
+        assertThrows(
+            IllegalStateException.class,
+            () -> CodeOwnerBackendId.getBackendId(TestCodeOwnerBackend.class));
+    assertThat(exception)
+        .hasMessageThat()
+        .isEqualTo(
+            String.format("unknown code owner backend: %s", TestCodeOwnerBackend.class.getName()));
+  }
+
+  private static class TestCodeOwnerBackend implements CodeOwnerBackend {
+    @Override
+    public Optional<CodeOwnerConfig> getCodeOwnerConfig(
+        CodeOwnerConfig.Key codeOwnerConfigKey,
+        @Nullable RevWalk revWalk,
+        @Nullable ObjectId revision) {
+      throw new UnsupportedOperationException("not implemented");
+    }
+
+    @Override
+    public Optional<CodeOwnerConfig> upsertCodeOwnerConfig(
+        CodeOwnerConfig.Key codeOwnerConfigKey,
+        CodeOwnerConfigUpdate codeOwnerConfigUpdate,
+        @Nullable IdentifiedUser currentUser) {
+      throw new UnsupportedOperationException("not implemented");
+    }
+
+    @Override
+    public boolean isCodeOwnerConfigFile(NameKey project, String fileName) {
+      throw new UnsupportedOperationException("not implemented");
+    }
+
+    @Override
+    public Path getFilePath(CodeOwnerConfig.Key codeOwnerConfigKey) {
+      throw new UnsupportedOperationException("not implemented");
+    }
+
+    @Override
+    public Optional<PathExpressionMatcher> getPathExpressionMatcher() {
+      return Optional.empty();
+    }
+  }
+}