[Backport 2.x] Accept null in arrays

CHANGELOG.md

@@ -7,6 +7,7 @@ Inspired from [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 ### Dependencies
 
 ### Changed
+Allow null values in arrays ([#687](https://github.com/opensearch-project/opensearch-java/pull/687))
 
 ### Deprecated
 

java-client/src/main/java/org/opensearch/client/json/JsonpDeserializerBase.java

@@ -335,8 +335,13 @@ public List<T> deserialize(JsonParser parser, JsonpMapper mapper, Event event) {
             if (event == Event.START_ARRAY) {
                 List<T> result = new ArrayList<>();
                 while ((event = parser.next()) != Event.END_ARRAY) {
-                    JsonpUtils.ensureAccepts(itemDeserializer, parser, event);
-                    result.add(itemDeserializer.deserialize(parser, mapper, event));
+                    // JSON null: add null unless the deserializer can handle it
+                    if (event == Event.VALUE_NULL && !itemDeserializer.accepts(event)) {
+                        result.add(null);
+                    } else {
+                        JsonpUtils.ensureAccepts(itemDeserializer, parser, event);
+                        result.add(itemDeserializer.deserialize(parser, mapper, event));
+                    }
                 }
                 return result;
             } else {

java-client/src/test/java/org/opensearch/client/opensearch/json/JsonpDeserializerBaseTest.java

@@ -0,0 +1,43 @@
+package org.opensearch.client.opensearch.json;
+
+import jakarta.json.stream.JsonParser;
+import java.io.StringReader;
+import java.util.List;
+import org.junit.Test;
+import org.opensearch.client.json.JsonpDeserializer;
+import org.opensearch.client.opensearch._types.FieldValue;
+import org.opensearch.client.opensearch.model.ModelTestCase;
+
+public class JsonpDeserializerBaseTest extends ModelTestCase {
+
+    @Test
+    public void testNullArrayItem() {
+
+        String json = "[\"a\", null, \"c\"]";
+
+        // Types that don't accept null events should end up as null values in the list
+        {
+            JsonpDeserializer<String> stringDeser = JsonpDeserializer.stringDeserializer();
+            assertFalse(stringDeser.accepts(JsonParser.Event.VALUE_NULL));
+
+            JsonParser parser = mapper.jsonProvider().createParser(new StringReader(json));
+
+            List<String> stringList = JsonpDeserializer.arrayDeserializer(stringDeser).deserialize(parser, mapper);
+            assertEquals("a", stringList.get(0));
+            assertNull(stringList.get(1));
+            assertEquals("c", stringList.get(2));
+        }
+
+        // Types that do accept null events should end up as their null representation
+        {
+            assertTrue(FieldValue._DESERIALIZER.accepts(JsonParser.Event.VALUE_NULL));
+
+            JsonParser parser = mapper.jsonProvider().createParser(new StringReader(json));
+            List<FieldValue> valueList = JsonpDeserializer.arrayDeserializer(FieldValue._DESERIALIZER).deserialize(parser, mapper);
+
+            assertEquals("a", valueList.get(0)._get());
+            assertTrue(valueList.get(1).isNull());
+            assertEquals("c", valueList.get(2)._get());
+        }
+    }
+}