Add NullKeyToEmptyStringSerializer - to allow jackson serialization of a map with a null key

qqq-backend-core/src/main/java/com/kingsrook/qqq/backend/core/utils/JsonUtils.java

@@ -30,11 +30,14 @@ import java.util.List;
 import java.util.Map;
 import java.util.function.Consumer;
 import com.fasterxml.jackson.annotation.JsonInclude;
+import com.fasterxml.jackson.core.JsonGenerator;
 import com.fasterxml.jackson.core.JsonProcessingException;
 import com.fasterxml.jackson.core.type.TypeReference;
 import com.fasterxml.jackson.databind.ObjectMapper;
 import com.fasterxml.jackson.databind.ObjectWriter;
 import com.fasterxml.jackson.databind.SerializationFeature;
+import com.fasterxml.jackson.databind.SerializerProvider;
+import com.fasterxml.jackson.databind.ser.std.StdSerializer;
 import com.fasterxml.jackson.datatype.jsr310.JavaTimeModule;
 import com.kingsrook.qqq.backend.core.logging.QLogger;
 import com.kingsrook.qqq.backend.core.model.data.QRecord;
@@ -54,6 +57,11 @@ public class JsonUtils
 {
    private static final QLogger LOG = QLogger.getLogger(JsonUtils.class);
 
+   //////////////////////////////////////////////////////////////////////
+   // see https://www.baeldung.com/jackson-map-null-values-or-null-key //
+   //////////////////////////////////////////////////////////////////////
+   public static NullKeyToEmptyStringSerializer nullKeyToEmptyStringSerializer = new NullKeyToEmptyStringSerializer();
+
 
 
    /*******************************************************************************
@@ -396,4 +404,41 @@ public class JsonUtils
       return (record);
    }
 
+
+
+   /***************************************************************************
+    **
+    ***************************************************************************/
+   public static class NullKeyToEmptyStringSerializer extends StdSerializer<Object>
+   {
+      /***************************************************************************
+       **
+       ***************************************************************************/
+      public NullKeyToEmptyStringSerializer()
+      {
+         this(null);
+      }
+
+
+
+      /***************************************************************************
+       **
+       ***************************************************************************/
+      public NullKeyToEmptyStringSerializer(Class<Object> t)
+      {
+         super(t);
+      }
+
+
+
+      /***************************************************************************
+       **
+       ***************************************************************************/
+      @Override
+      public void serialize(Object nullKey, JsonGenerator jsonGenerator, SerializerProvider unused) throws IOException
+      {
+         jsonGenerator.writeFieldName("");
+      }
+   }
+
 }

qqq-backend-core/src/test/java/com/kingsrook/qqq/backend/core/utils/JsonUtilsTest.java

@@ -35,11 +35,13 @@ import com.kingsrook.qqq.backend.core.model.actions.tables.query.QFilterCriteria
 import com.kingsrook.qqq.backend.core.model.actions.tables.query.QFilterOrderBy;
 import com.kingsrook.qqq.backend.core.model.actions.tables.query.QQueryFilter;
 import com.kingsrook.qqq.backend.core.model.data.QRecord;
+import com.kingsrook.qqq.backend.core.utils.collections.MapBuilder;
 import org.json.JSONArray;
 import org.json.JSONException;
 import org.json.JSONObject;
 import org.junit.jupiter.api.Test;
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
@@ -318,4 +320,30 @@ class JsonUtilsTest extends BaseTest
       assertEquals("age", qQueryFilter.getOrderBys().get(0).getFieldName());
       assertTrue(qQueryFilter.getOrderBys().get(0).getIsAscending());
    }
+
+
+
+   /*******************************************************************************
+    **
+    *******************************************************************************/
+   @Test
+   void testNullKeyInMap()
+   {
+      Map<Object, String> mapWithNullKey = MapBuilder.of(null, "foo");
+
+      //////////////////////////////////////////////////////
+      // assert default behavior throws with null map key //
+      //////////////////////////////////////////////////////
+      assertThatThrownBy(() -> JsonUtils.toJson(mapWithNullKey)).rootCause().hasMessageContaining("Null key for a Map not allowed in JSON");
+
+      ////////////////////////////////////////////////////////////////////////
+      // assert that the nullKeyToEmptyStringSerializer does what we expect //
+      ////////////////////////////////////////////////////////////////////////
+      assertEquals("""
+         {"":"foo"}""", JsonUtils.toJson(mapWithNullKey, mapper ->
+      {
+         mapper.getSerializerProvider().setNullKeySerializer(JsonUtils.nullKeyToEmptyStringSerializer);
+      }));
+   }
+
 }