Jackson 2.5.0. NullSerializer for MapProperty failing (no serializeWithType)

[Antibrumm]

I'm not sure if that's a bug our if it comes because of some configuration of mine.

In version 2.5.0 I had to fix the NullSerializer by implementing the serializeWithType method to call jgen.writeNull() as the default throws an UnsupportedOperationException.

I saw that the BeanPropertyWriter is calling serialize but MapProperty is calling serializeWithType.

Here's the stacktrace:

FixedNullSerializer.serializeWithType(Object, JsonGenerator, SerializerProvider, TypeSerializer) line: 26   
MapProperty.serializeAsField(Object, JsonGenerator, SerializerProvider) line: 102   
AntPathPropertyFilter.serializeAsField(Object, JsonGenerator, SerializerProvider, PropertyWriter) line: 157 
MapSerializer.serializeFilteredFields(Map<?,?>, JsonGenerator, SerializerProvider, PropertyFilter, Object) line: 731    
MapSerializer.serializeWithType(Map<?,?>, JsonGenerator, SerializerProvider, TypeSerializer) line: 493  
MapSerializer.serializeWithType(Object, JsonGenerator, SerializerProvider, TypeSerializer) line: 29 
TypeWrappedSerializer.serialize(Object, JsonGenerator, SerializerProvider) line: 32 
DefaultSerializerProvider$Impl(DefaultSerializerProvider).serializeValue(JsonGenerator, Object) line: 129   
ObjectMapper.writeValue(JsonGenerator, Object) line: 2238   
Jackson2Helper.writeFiltered(Object, String...) line: 51    

[cowtowncoder]

A unit test would help here, as the problem may be in multiple places -- off-hand, I wouldn't think that method should ever be called on that serializer. Which is why exception is thrown, as an assertion.
I would not want to mask the underlying problem for such case.

On the other hand it is possible that there could be a valid exception to this as well. A test could show either way. :)

[Antibrumm]

Here's a test case + a fixed test.

I just discovered that the NULL field is serialized twice once the exception throwing is fixed.

Output

NORMAL: {"id":"Test","NULL":null}
FIXED: {"@c":".HashMap","id":"Test","NULL":null,"NULL":null}

Tests

public class NullFailTest {

    @JsonTypeInfo(use = Id.MINIMAL_CLASS)
    public static class Mixin {

    }

    @Test(expected = JsonMappingException.class)
    public void testNullJsonMappingException() throws JsonProcessingException {
        Map<String, String> map = new HashMap<String, String>();
        map.put("id", "Test");
        map.put("NULL", null);

        ObjectMapper objectMapper = new ObjectMapper();
        System.out.println("NORMAL: " + objectMapper.writeValueAsString(map));

        objectMapper = new ObjectMapper();
        objectMapper.addMixIn(Object.class, Mixin.class);

        System.out.println("TYPED (throwing exception): " + objectMapper.writeValueAsString(map));

    }

    @Test
    public void testNullFixed() throws JsonProcessingException {

        Map<String, String> map = new HashMap<String, String>();
        map.put("id", "Test");
        map.put("NULL", null);

        ObjectMapper objectMapper = new ObjectMapper();
        objectMapper.addMixIn(Object.class, Mixin.class);
        objectMapper.getSerializerProvider().setNullValueSerializer(FixedNullSerializer.instance);
        System.out.println("FIXED (but serializing NULL field twice): " + objectMapper.writeValueAsString(map));
    }

    public static class FixedNullSerializer extends StdSerializer<Object> {
        /**
         * 
         */
        private static final long serialVersionUID = 1L;

        public final static FixedNullSerializer instance = new FixedNullSerializer();

        @Override
        public void serializeWithType(Object value, JsonGenerator jgen, SerializerProvider serializers,
            TypeSerializer typeSer) throws IOException {
            jgen.writeNull();
        }

        private FixedNullSerializer() {
            super(Object.class);
        }

        @Override
        public void serialize(Object value, JsonGenerator jgen, SerializerProvider provider) throws IOException {
            jgen.writeNull();
        }

        @Override
        public JsonNode getSchema(SerializerProvider provider, Type typeHint) throws JsonMappingException {
            return createSchemaNode("null");
        }

        @Override
        public void acceptJsonFormatVisitor(JsonFormatVisitorWrapper visitor, JavaType typeHint)
                throws JsonMappingException {
            visitor.expectNullFormat(typeHint);
        }
    }
}

[cowtowncoder]

This may well be a bug due to refactorings in 2.5.0, which try to ensure that all handlers are properly used.
Thank you for the test!

[cowtowncoder]

Unfortunately I am not able to reproduce the failure with above test case. Looking at the original stack trace, it seems like there is something additional, like JSON Filter in use? If so, could you show how it is added, to reproduce the exception?

[Antibrumm]

Strange. Did you run it against 2.5.0 or already a new snapshot?

I ran it through the antpath filter maven project (jackson 2.5.0). I thought the test case should be independant. As far as I remember the objectMapper together with the typeid generator raised the issue.
I can do another run.

[cowtowncoder]

Ah. Running the test exactly as shown and with 2.5.0 branch does reproduce it. So my simplified reproduction had some issue... Ok. Back to testing.

[cowtowncoder]

Ok I was able to follow the problem, and you were exactly right here regarding the root cause.

And the reason why my test failed to catch this was somewhat interesting: since I did not use a mix-in, @JsonTypeInfo when added to a Map type applies to values. However, when added to base java.lang.Object, type can not be detected to be a container type (Map or Collection), and thereby it does apply to type itself, not contents.

I did add serializeWithType(), as well as handle duplicate writing of null. I think this was a 2.5 regression.