When `Include.NON_DEFAULT` setting is used on POJO, empty values are not included in json if default is `null`

[ragnhov]

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Describe the bug

After upgrading to 2.18.0, empty Strings/objects are no longer included in json output if NON_DEFAULT annotation is used on the class and the default value is null. This is possibly related to fixing #4464
As far as I can understand, the documentation states that the test below should pass, as it does in 2.17.1.

Version Information

2.18.0

Reproduction

@Test
void testSerialization() throws JsonProcessingException {
    MyString input = new MyString();
    input.setValue("");
    ObjectMapper objectMapper = new ObjectMapper();
    String json = objectMapper.writeValueAsString(input);
    MyString output = objectMapper.readValue(json, MyString.class);
    assertEquals(input.getValue(), output.getValue());
}

@JsonInclude(JsonInclude.Include.NON_DEFAULT)
@Data
public static class MyString {
    private String value = null;
}

Expected behavior

No response

Additional context

No response

[pjfanning]

That annotation has this in the javadoc:

When used for a POJO, definition is that only values that differ from the default values of POJO properties are included.

I don't think this annotation value is about nulls - there are other annotation values you can use. If you look at the test cases in jackson-databind, you will see that none of the tests related to Include.NON_DEFAULT are about null handling.

Have a look at https://www.javadoc.io/doc/com.fasterxml.jackson.core/jackson-annotations/latest/index.html

You could try @JsonInclude(JsonInclude.Include.ALWAYS). This seems to be closer to what you want.

You say that the behaviour of Include.NON_DEFAULT has changed in Jackson 2.18 and that needs investigation.

[pjfanning]

I created #4743 which does demo your issue. I have verified that 2.18 behaves differently.

[pjfanning]

Reasonably good chance that this relates to #4464

[rPraml]

Hello, we have the same problem. We have lots of pojos, where the default value is null and may be set to empty string "". This makes a semantic difference. We used also @JsonInclude(Include.NON_DEFAULT) not to serialize the default values (mainly nulls or empty lists etc.)

That annotation has this in the javadoc:

When used for a POJO, definition is that only values that differ from the default values of POJO properties are included.

I don't think this annotation value is about nulls

Sorry, I do think so, as this is clearly mentioned in the javadoc

... This is done by creating an instance of POJO using zero-argument constructor, and accessing property values:
value is used as the default value by using equals() method, except for the case where property has null value in which case straight null check is used.

Roland

[JooHyukKim]

Tough call... seems like everyone has their own interpretation of the JavaDoc.
@teodord Could you help us out here? This issue here is sort of contracting to what we agreed to.
Your opinion would matter, be welcome as well

[ragnhov]

The documentation states that the behavior depends on context. I believe that the context is different in this case and @teodord 's case. The POJO case should include empty strings, the global case should not.

[JooHyukKim]

I believe that the context is different in this case and @teodord 's case. The POJO case should include empty strings, the global case should not.

I concur. I might as well re-read the documentation one more time and see what we can do to fix the regression later today. Thank you for explanting this @ragnhov @rPraml @pjfanning !

[JooHyukKim]

Okay, re-reading through the documentation I am leaning toward towards keeping both behavior, thus #4744.

I would like to ask @cowtowncoder for direction we should head, while I am going back to re-examine the documentation-behavior sync one more time.

EDIT : Yup, it seems like both this issue and #4464 are meant to be suuported. This issue about per-POJO cases and latter about global setting.

[cowtowncoder]

On this particular case: in this example if POJO has null as value for String, then null AND ONLY null should be excluded, not empty String. So I think this is valid bug report indeed.