Fix: Make metadata mutable in AzureVectorStore (Azure AI Search) (spring-projects#4131)

* fix(azure): parse metadata JSON into mutable Map before injecting distance

Avoid UnsupportedOperationException by ensuring metadata is always a LinkedHashMap.
Add `parseMetadataToMutable` helper and use it in similarity search path.

* test(azure): add unit tests for mutable metadata parsing and distance

Add `AzureVectorStoreMetadataParsingTests` to verify:
- valid JSON → mutable LinkedHashMap copy
- blank/invalid JSON → empty LinkedHashMap
- can inject `distance` without throwing UnsupportedOperationException

These tests fail on the previous implementation and pass with the fix,
guarding against regressions.

* refactor(azure): replace LinkedHashMap with HashMap in parseMetadataToMutable

We don’t rely on insertion order for metadata, so a regular HashMap is sufficient.
This change addresses reviewer feedback while still ensuring the returned map is
always mutable by creating a defensive copy or returning an empty instance.

Auto-cherry-pick to 1.0.x
Fixes spring-projects#4117

Signed-off-by: Jinwoo Lee <[email protected]>

Author: jinlee1703

vector-stores/spring-ai-azure-store/src/main/java/org/springframework/ai/vectorstore/azure/AzureVectorStore.java

@@ -18,6 +18,7 @@
 
 import java.util.ArrayList;
 import java.util.Collections;
+import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.Optional;
@@ -75,6 +76,7 @@
  * @author Josh Long
  * @author Thomas Vitale
  * @author Soby Chacko
+ * @author Jinwoo Lee
  */
 public class AzureVectorStore extends AbstractObservationVectorStore implements InitializingBean {
 
@@ -239,10 +241,7 @@ public List<Document> doSimilaritySearch(SearchRequest request) {
 
 				final AzureSearchDocument entry = result.getDocument(AzureSearchDocument.class);
 
-				Map<String, Object> metadata = (StringUtils.hasText(entry.metadata()))
-						? JSONObject.parseObject(entry.metadata(), new TypeReference<>() {
-
-						}) : Map.of();
+				Map<String, Object> metadata = parseMetadataToMutable(entry.metadata());
 
 				metadata.put(DocumentMetadata.DISTANCE.value(), 1.0 - result.getScore());
 
@@ -325,6 +324,21 @@ public <T> Optional<T> getNativeClient() {
 		return Optional.of(client);
 	}
 
+	static Map<String, Object> parseMetadataToMutable(@Nullable String metadataJson) {
+		if (!StringUtils.hasText(metadataJson)) {
+			return new HashMap<>();
+		}
+		try {
+			Map<String, Object> parsed = JSONObject.parseObject(metadataJson, new TypeReference<Map<String, Object>>() {
+			});
+			return (parsed == null) ? new HashMap<>() : new HashMap<>(parsed);
+		}
+		catch (Exception ex) {
+			logger.warn("Failed to parse metadata JSON. Using empty metadata. json={}", metadataJson, ex);
+			return new HashMap<>();
+		}
+	}
+
 	public record MetadataField(String name, SearchFieldDataType fieldType) {
 
 		public static MetadataField text(String name) {

vector-stores/spring-ai-azure-store/src/test/java/org/springframework/ai/vectorstore/azure/AzureVectorStoreMetadataTests.java

@@ -0,0 +1,55 @@
+/*
+ * Copyright 2023-2025 the original author or authors.
+ *
+ * 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
+ *
+ *      https://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 org.springframework.ai.vectorstore.azure;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.util.Map;
+
+import org.junit.jupiter.api.Test;
+
+/**
+ * Unit tests for {@link AzureVectorStore#parseMetadataToMutable(String)}.
+ *
+ * @author Jinwoo Lee
+ */
+class AzureVectorStoreMetadataTests {
+
+	@Test
+	void returnsMutableMapForBlankOrNull() {
+		Map<String, Object> m1 = AzureVectorStore.parseMetadataToMutable(null);
+		m1.put("distance", 0.1);
+		assertThat(m1).containsEntry("distance", 0.1);
+
+		Map<String, Object> m2 = AzureVectorStore.parseMetadataToMutable("");
+		m2.put("distance", 0.2);
+		assertThat(m2).containsEntry("distance", 0.2);
+
+		Map<String, Object> m3 = AzureVectorStore.parseMetadataToMutable("   ");
+		m3.put("distance", 0.3);
+		assertThat(m3).containsEntry("distance", 0.3);
+	}
+
+	@Test
+	void wrapsParsedJsonInLinkedHashMapSoItIsMutable() {
+		Map<String, Object> map = AzureVectorStore.parseMetadataToMutable("{\"k\":\"v\"}");
+		assertThat(map).containsEntry("k", "v");
+		map.put("distance", 0.4);
+		assertThat(map).containsEntry("distance", 0.4);
+	}
+
+}