Serializing boxed value with type information

[hoomanv]

Hi,

I need to serialize a value with type information and be able to deserialize it with ease. The value is either a String or BigDecimal.

Expected XML output is like:

<data><text>foo</text></data>
<data><decimal>123</decimal></data>

Here is the Box class I use to wrap the value.

@JacksonXmlRootElement(localName = "data")
class Box
{
    @JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.WRAPPER_OBJECT)
    @JsonSubTypes({
        @Type(value = String.class, name = "text"),
        @Type(value = BigDecimal.class, name = "decimal")
    })
    @JacksonXmlText
    Object value;
}

Here is the result when serializing string "foo" and decimal "123":

<data>foo</data>
<data>
  <value>123</value>
</data>

As you see, type info is missing for string value and is wrong for decimal value.

Removing JacksonXmlText annotation will result in:

<data>
  <value>foo</value>
</data>
<data>
  <value>
    <decimal>123</decimal>
  </value>
</data>

This time type info for decimal value was included, however there is a redundant wrapper element around it.

There is yet another acceptable output format that looks like:

<data type="text">foo</data>
<data type="decimal">123</data>

But so far I had no luck getting it to work either.

Tested with jackson 2.9.5

[cowtowncoder]

In addition to showing how you are tackling the problem, it would be good to know bit about context maybe.
Since types to use (String and number, ignoring for a moment it should ideally be BigDecimal) are "natural" types, meaning that they map to/from java.lang.Object, you would probably be better off NOT trying to use @JsonTypeInfo, even though usually it would make sense for polymorphic values.

However... doing so would not let you get extra wrapper level.

So, it would be good to know expected use case: what are you trying to achieve in Java, regardless of what XML should be produced.

[hoomanv]

This is the protocol we use store customField values in a XML database column. As mentioned above, the format is:

<data><text>foo</text></data>
<data><decimal>123</decimal></data>

XML mapping was powered by SimpleXML framework so far. Until recently that we decided to migrate to Jackson XML. So to retain backward compatibility, it is necessary to deserialize existing records as is. I am currently doing low level serialization/deserialization with STAX as a workaround, however it'd be greater if Jackson databind could handle it.

[cowtowncoder]

Will hope to investigate if something could be done for this in 2.12.

[cowtowncoder]

Couple of notes:

• Use of @JacksonXmlText is probably not a good idea here (unlikely to end up working)
• String is a so-called "natural" type (due to JSON heritage) so exclusion of type id is expected. However, since XML is not JSON, this may end up causing issues.
• XML module in general does not deal well with Object as polymorphic base type, for various reasons.
• Jackson's polymorphic type id in XML is more verbose as it can not combine "property name" part (value) from type id wrapper element decimal. Somehow replacing latter with former ("flattening") has been requested, but I do not see a way to implement that easily -- databind requires property name, and WRAPPER_OBJECT adds type id as wrapping element.

[cowtowncoder]

At this point not sure what to do here; will leave open, but may combine with other issues. 2 basic things:

1. Since XML can not reliably detect "natural" types (int, String, boolean, double are all just text segments, should XML backend be able to add Type Id even for such types (ideally yes, but how to make databind do that)
2. Type Id inclusion with WRAPPER_ELEMENT is verbose, and there is no way to simply replace "property name" with "type id". It would be good to somehow allow it, but this may be trick (however! Not insurmountable, necessarily...) -- serialization might be easyish, but deserialization the problem

Will try to see if there is an issue already for (2), to link to.