Map serialization, null values and @JsonValue

[cowtowncoder]

(note: moved from FasterXML/jackson-core#50)

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Jackson allows skip null values when serializing maps. This doesn't work, when map value is generated by the method annotated with @JsonValue. For example:

@JsonProperty
private Map<String, SomeClass> fields;

and:

class SomeClass {
  @JsonValue
  public String getValue() { ... }
}

result:

{"id1":"a","id2":null}

[cowtowncoder]

This will be tricky one to resolve because code that handles inclusion rules will not see null but the POJO, and translation to null occurs later... when property name was already been written. This means that it is unlikely to be fixed for 2.1, as it will require bigger change.

[cowtowncoder]

Come to think of this, I do not think this is supportable as defined.
Since actual values are not nulls, they will not be blocked by NON_NULL. Jackson 2.9 will support custom inclusion rules (see @JsonInclude and specifically content and new contentFilter).
Without this, an alternative would be custom serializer for SomeClass, which may define isEmpty() method; and if properly defined could then used NON_EMPTY to remove such entries.

But the trick is that determination MUST be based on Java value -- instance of SomeClass -- and not on whatever @JsonValue accessor may convert it to.