Fix #4922: @JsonMerge with custom map (#4925)

Author: cowtowncoder

release-notes/VERSION-2.x

@@ -21,6 +21,8 @@ Project: jackson-databind
 #4908: Deserialization behavior change with @JsonCreator and
   @ConstructorProperties between 2.17 and 2.18
  (reported by Gustavo B)
+#4922: Failing `@JsonMerge` with a custom Map
+ (reported by @nlisker)
 
 2.18.2 (27-Nov-2024)
 

src/main/java/com/fasterxml/jackson/databind/deser/BasicDeserializerFactory.java

@@ -979,12 +979,6 @@ public JsonDeserializer<?> createMapDeserializer(DeserializationContext ctxt,
                         mapClass = type.getRawClass();
                         // But if so, also need to re-check creators...
                         beanDesc = config.introspectForCreation(type);
-                    } else {
-                        // [databind#292]: Actually, may be fine, but only if polymorphic deser enabled
-                        if (type.getTypeHandler() == null) {
-                            throw new IllegalArgumentException("Cannot find a deserializer for non-concrete Map type "+type);
-                        }
-                        deser = AbstractDeserializer.constructForNonPOJO(beanDesc);
                     }
                 } else {
                     // 10-Jan-2017, tatu: `java.util.Collections` types need help:

src/test/java/com/fasterxml/jackson/databind/deser/merge/CustomMapMerge4922Test.java

@@ -0,0 +1,40 @@
+package com.fasterxml.jackson.databind.deser.merge;
+
+import java.util.*;
+
+import org.junit.jupiter.api.Test;
+
+import com.fasterxml.jackson.annotation.JsonMerge;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.databind.testutil.DatabindTestUtil;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+@SuppressWarnings("serial")
+public class CustomMapMerge4922Test
+    extends DatabindTestUtil
+{
+    // [databind#4922]
+    interface MyMap4922<K, V> extends Map<K, V> {}
+
+    static class MapImpl<K, V> extends HashMap<K, V> implements MyMap4922<K, V> {}
+
+    static class MergeMap4922 {
+        @JsonMerge // either here
+        public MyMap4922<Integer, String> map = new MapImpl<>();
+    }
+
+    private final ObjectMapper MAPPER = newJsonMapper();
+
+    // [databind#4922]: Merge for custom maps fails
+    @Test
+    void testJDKMapperReading() throws Exception {
+        MergeMap4922 input = new MergeMap4922();
+        input.map.put(3, "ADS");
+
+        String json = MAPPER.writerWithDefaultPrettyPrinter().writeValueAsString(input);
+        MergeMap4922 merge2 = MAPPER.readValue(json, MergeMap4922.class);
+        assertNotNull(merge2);
+    }
+
+}