Support repeated fields in MessageOneofEvaluator

src/main/java/build/buf/protovalidate/FieldEvaluator.java

@@ -99,12 +99,7 @@ public List<RuleViolation.Builder> evaluate(Value val, boolean failFast)
     if (message == null) {
       return RuleViolation.NO_VIOLATIONS;
     }
-    boolean hasField;
-    if (descriptor.isRepeated()) {
-      hasField = message.getRepeatedFieldCount(descriptor) != 0;
-    } else {
-      hasField = message.hasField(descriptor);
-    }
+    boolean hasField = isFieldSet(message, descriptor);
     if (required && !hasField) {
       return Collections.singletonList(
           RuleViolation.newBuilder()
@@ -121,4 +116,15 @@ public List<RuleViolation.Builder> evaluate(Value val, boolean failFast)
     return valueEvaluator.evaluate(
         new ObjectValue(descriptor, message.getField(descriptor)), failFast);
   }
+
+  /**
+   * Returns whether the given field is set on the message. Handles repeated and map fields, which
+   * are not supported by {@link Message#hasField}.
+   */
+  static boolean isFieldSet(Message message, FieldDescriptor field) {
+    if (field.isRepeated()) {
+      return message.getRepeatedFieldCount(field) != 0;
+    }
+    return message.hasField(field);
+  }
 }

src/main/java/build/buf/protovalidate/MessageOneofEvaluator.java

@@ -51,7 +51,7 @@ public List<RuleViolation.Builder> evaluate(Value val, boolean failFast)
     }
     int hasCount = 0;
     for (FieldDescriptor field : fields) {
-      if (msg.hasField(field)) {
+      if (FieldEvaluator.isFieldSet(msg, field)) {
         hasCount++;
       }
     }

src/test/java/build/buf/protovalidate/Issue427Test.java

@@ -0,0 +1,59 @@
+// Copyright 2023-2025 Buf Technologies, Inc.
+//
+// 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 build.buf.protovalidate;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import build.buf.validate.Violation;
+import org.junit.jupiter.api.Test;
+
+public class Issue427Test {
+  @Test
+  public void testMessageOneofWithNameOnly() throws Exception {
+    com.example.imports.validationtest.Issue427 msg =
+        com.example.imports.validationtest.Issue427.newBuilder().setName("foo").build();
+    Validator validator = ValidatorFactory.newBuilder().build();
+    assertThat(validator.validate(msg).toProto().getViolationsList()).isEmpty();
+  }
+
+  @Test
+  public void testMessageOneofWithTagsOnly() throws Exception {
+    com.example.imports.validationtest.Issue427 msg =
+        com.example.imports.validationtest.Issue427.newBuilder().addTags("a").addTags("b").build();
+    Validator validator = ValidatorFactory.newBuilder().build();
+    assertThat(validator.validate(msg).toProto().getViolationsList()).isEmpty();
+  }
+
+  @Test
+  public void testMessageOneofWithMappingsOnly() throws Exception {
+    com.example.imports.validationtest.Issue427 msg =
+        com.example.imports.validationtest.Issue427.newBuilder().putMappings("k", "v").build();
+    Validator validator = ValidatorFactory.newBuilder().build();
+    assertThat(validator.validate(msg).toProto().getViolationsList()).isEmpty();
+  }
+
+  @Test
+  public void testMessageOneofNoneSet() throws Exception {
+    com.example.imports.validationtest.Issue427 msg =
+        com.example.imports.validationtest.Issue427.getDefaultInstance();
+    Validator validator = ValidatorFactory.newBuilder().build();
+    assertThat(validator.validate(msg).toProto().getViolationsList())
+        .containsExactly(
+            Violation.newBuilder()
+                .setRuleId("message.oneof")
+                .setMessage("one of name, tags, mappings must be set")
+                .build());
+  }
+}

src/test/resources/proto/validationtest/issue427.proto

@@ -0,0 +1,29 @@
+// Copyright 2023-2025 Buf Technologies, Inc.
+//
+// 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.
+
+syntax = "proto3";
+
+package validationtest;
+
+import "buf/validate/validate.proto";
+
+message Issue427 {
+  option (buf.validate.message).oneof = {
+    fields: ["name", "tags", "mappings"]
+    required: true
+  };
+  string name = 1;
+  repeated string tags = 2;
+  map<string, string> mappings = 3;
+}