Fix #4963

Author: cowtowncoder

release-notes/VERSION-2.x

@@ -49,6 +49,8 @@ Project: jackson-databind
 #4953: Allow clearing all caches to avoid classloader leaks
  (contributed by Joren I)
 #4959: Add explicit deserializer for `ThreadGroup`
+#4963: Serializing `Map.Entry` as Bean with `@JsonFormat.shape = Shape.OBJECT`
+  fails on JDK 17+
 
 2.18.3 (not yet released)
 

src/main/java/com/fasterxml/jackson/databind/ser/BeanSerializerFactory.java

@@ -18,6 +18,7 @@
 import com.fasterxml.jackson.databind.jsontype.TypeResolverBuilder;
 import com.fasterxml.jackson.databind.jsontype.TypeSerializer;
 import com.fasterxml.jackson.databind.ser.impl.FilteredBeanPropertyWriter;
+import com.fasterxml.jackson.databind.ser.impl.MapEntryAsPOJOSerializer;
 import com.fasterxml.jackson.databind.ser.impl.ObjectIdWriter;
 import com.fasterxml.jackson.databind.ser.impl.PropertyBasedObjectIdGenerator;
 import com.fasterxml.jackson.databind.ser.impl.UnsupportedTypeSerializer;
@@ -394,6 +395,12 @@ protected JsonSerializer<Object> constructBeanOrAddOnSerializer(SerializerProvid
         if (_isUnserializableJacksonType(prov, type)) {
             return new ToEmptyObjectSerializer(type);
         }
+        // 08-Feb-2025, tatu: [databind#4963] Need to have explicit serializer for
+        //   Map.Entry type that are from JDK (for others just use regular introspection)
+        if (type.isTypeOrSubTypeOf(Map.Entry.class)
+                && ClassUtil.isJDKClass(type.getRawClass())) {
+            return (JsonSerializer<Object>)(JsonSerializer<?>) MapEntryAsPOJOSerializer.create(prov, type);
+        }
         final SerializationConfig config = prov.getConfig();
         BeanSerializerBuilder builder = constructBeanSerializerBuilder(beanDesc);
         builder.setConfig(config);

src/main/java/com/fasterxml/jackson/databind/ser/impl/MapEntryAsPOJOSerializer.java

@@ -0,0 +1,46 @@
+package com.fasterxml.jackson.databind.ser.impl;
+
+import java.io.IOException;
+import java.util.Map;
+import java.util.Map.Entry;
+
+import com.fasterxml.jackson.core.JsonGenerator;
+import com.fasterxml.jackson.databind.JavaType;
+import com.fasterxml.jackson.databind.SerializerProvider;
+import com.fasterxml.jackson.databind.ser.std.StdSerializer;
+
+/**
+ * Serializer used to serialize Map.Entry as POJOs: that is, as if
+ * introspected as POJOs so that there's intermediate "key" and "value"
+ * properties.
+ *<p>
+ * TODO: does not fully handle contextualization, type resolution and so on.
+ *
+ * @since 2.19
+ */
+public class MapEntryAsPOJOSerializer extends StdSerializer<Map.Entry<?,?>>
+{
+    private static final long serialVersionUID = 1L;
+
+    protected MapEntryAsPOJOSerializer(JavaType type) {
+        super(type);
+    }
+
+    public static MapEntryAsPOJOSerializer create(SerializerProvider ctxt,
+            JavaType type)
+    {
+        return new MapEntryAsPOJOSerializer(type);
+    }
+
+    @Override
+    public void serialize(Entry<?, ?> value, JsonGenerator gen, SerializerProvider ctxt)
+            throws IOException
+    {
+        gen.writeStartObject(value);
+        gen.writeFieldName("key");
+        ctxt.defaultSerializeValue(value.getKey(), gen);
+        gen.writeFieldName("value");
+        ctxt.defaultSerializeValue(value.getValue(), gen);
+        gen.writeEndObject();
+    }
+}

src/main/java/com/fasterxml/jackson/databind/ser/impl/MapEntrySerializer.java

@@ -394,7 +394,8 @@ protected void serializeDynamic(Map.Entry<?, ?> value, JsonGenerator gen,
                     if (valueSer.isEmpty(provider, valueElem)) {
                         return;
                     }
-                } if (_suppressableValue.equals(valueElem)) {
+                }
+                else if (_suppressableValue.equals(valueElem)) {
                     return;
                 }
             }

src/test/java/com/fasterxml/jackson/databind/util/ClassUtilTest.java

@@ -222,7 +222,7 @@ public void testFindEnumTypeNonJdk()
         assertEquals(TestEnum.class, ClassUtil.findEnumType(TestEnum.B));
     }
 
-    // Some trouble with JDK 16+
+    // Some trouble with JDK 17+ (unless --add-opens to force access added)
     @Test
     public void testFindEnumSetTypeJDK()
     {
@@ -231,7 +231,7 @@ public void testFindEnumSetTypeJDK()
         assertEquals(TestEnum.class, ClassUtil.findEnumType(EnumSet.noneOf(TestEnum.class)));
     }
 
-    // Some trouble with JDK 16+
+    // Some trouble with JDK 17+ (unless --add-opens to force access added)
     @Test
     public void testFindEnumMapTypeJDK()
     {