When `Include.NON_DEFAULT` setting is used, `isEmpty()` method is not called on the serializer

[Teodor Danciu]

Search before asking

• I searched in the issues and found nothing similar.

Describe the bug

Documentation says that when Include.NON_DEFAULT is used, null, empty and default values of a property are skipped on serialization. It is not clear how the empty check is done because the isEmpty method of the serializer is called only when Include.NON_EMPTY is used explicitly.

I have a case where I set Include.NON_DEFAULT globally on the ObjectMapper, and I have implemented a custom serializer to override the isEmpty method and thus let Jackson know when my object is empty. But my custom implementation of the isEmpty method is not called.
The only way to have it called by Jackson is to explicitly override the property inclusion configuration and use NON_EMPTY instead of the global NON_DEFAULT.

Version Information

2.15.3

Reproduction

<-- Any of the following

1. Brief code sample/snippet: include here in preformatted/code section
2. Longer example stored somewhere else (diff repo, snippet), add a link
3. Textual explanation: include here
   -->

// Your code here

Expected behavior

No response

Additional context

No response

[Kim, Joo Hyuk]

Could you provide simple reproduction, with Mapper configuration, class declarations and such?

I tried writing a reproduction as below, but it seems to behave differently than the decription above.

    public static class Foo {
        public String getFoo() {
            return "";
        }
    }

    @Test
    public void test86() throws IOException {
        ObjectMapper mapper = JsonMapper.builder()
                .serializationInclusion(JsonInclude.Include.NON_DEFAULT)
                .build();
        String json = mapper.writeValueAsString(new Foo());
        assertEquals("{}", json);
    }
    ```

[Teodor Danciu]

I probably did not make myself clear, but my case is about a custom serializer overriding the isEmpty method, which does not get called when NON_DEFAULT is used. It gets called only when NON_EMPTY is used, although documentation says NON_DEFAULT also excludes empty values.

I attached a small Maven project demonstrating the issue. It works if we uncomment the NON_EMPTY annotation in Foo.java at line 9.

jackson-test.zip

[Kim, Joo Hyuk]

Documentation says that when Include.NON_DEFAULT is used, null, empty and default values of a property are skipped on serialization.

Hmmm, the documentation is somewhat context-driven, meaning the behavior varies depending on context, POJO vs non-POJO, etc....

Could you check again, if it really matches your case? I attached the documenation.

[Kim, Joo Hyuk]

From what I read the documenation, one thing for sure is that NON_DEFAULT is not a superset of NON_EMPTY or vice versa.~~ ~~How about using NON_EMPTY` if possible? It seems more straight forward 🤔?

[Teodor Danciu]

I am not sure why you say NON_DEFAULT is not a superset of NON_EMPTY. In the documentation snippet you posted above, I am clearly in this case:

I have set the NON_DEFAULT globally, on the ObjectMapper and so I qualify for this case.
First bullet says: "All Values considerer empty as per NON_EMPTY are excluded". So it thus confirms NON_DEFAULT is a superset of NON_EMPTY.
What do I get wrong here?
Furthermore, your own test showed that when NON_DEFAULT is used globally, empty strings are excluded.
What does not work is when custom serializer is used.

[Kim, Joo Hyuk]

Furthermore, your own test showed that when NON_DEFAULT is used globally, empty strings are excluded.
What does not work is when custom serializer is used.

Thank you for pointing out. I sort of drifted to a wrong point.

You are right that isEmpty() is not called with custom serializer.

[Teodor Danciu]

Thank you for confirming, but is this a bug or it was meant to work this way and documentation is faulty or incomplete?

[Kim, Joo Hyuk]

Bug most likely.
But not sure when the fix will be, or which version.

[Kim, Joo Hyuk]

Here is the missing reproduction

public class Test4464Test {
    public static class BarSerializer extends StdScalarSerializer<Bar> {
        
        public BarSerializer() {
            super((Class<Bar>) null);
        }

        @Override
        public void serialize(Bar value, JsonGenerator gen, SerializerProvider provider) throws IOException {
            gen.writeObject(value);
        }

        @Override
        public boolean isEmpty(SerializerProvider provider, Bar value) {
            return Bar.I_AM_EMPTY.equals(value.getName());
        }
    }

    public static class Bar {
        public static final String I_AM_EMPTY = "I_AM_EMPTY";

        public String getName() {
            return I_AM_EMPTY;
        }
    }

    public static class Foo {
        //@JsonInclude(Include.NON_EMPTY)
        @JsonSerialize(using = BarSerializer.class)
        public Bar getBar() {
            return new Bar();
        }
    }

    @Test
    public void test86() throws IOException {
        ObjectMapper mapper = JsonMapper.builder().serializationInclusion(JsonInclude.Include.NON_DEFAULT).build();
        String json = mapper.writeValueAsString(new Foo());
        assertEquals("{}", json);
    }
}

[Tatu Saloranta]

Thank you @JooHyukKim -- merged for inclusion in 2.18.0.