Support mapping xsi:nil marked elements as nulls (JsonToken.VALUE_NULL)

[cowtowncoder]

Although XML specification does not specify semantics for xsi:nil (with appropriate namespace), XML Schema specification does, and so some Java XML libraries (including JAXB) support mapping it into nulls.
(there is the related question of generation/writing, but this issue covers only read side).

It would be good if Jackson did same mapping automatically.

For background, see

• Handling XMLElement nillable like JAXB #89 (nil like JAXB)
• Exception on deserializing empty element with an xsi attribute on non String type. #184 (for BigDecimal etc values)
• Only self-closing tag with 'xsi:nil="true"' can be mapped #293 (nil for Collections)
• Xsi nil in collection fails to deserialize #353 (nil in Collections)

[cowtowncoder]

Note: one follow-up task, #359 -- but closed for 2.x

[henrik242]

@cowtowncoder I have a test that's failing after upgrading to 2.10.x. It worked fine with 2.9.x. Any idea if this issue is related, and how I can configure the mapper to fix it?

The test (kotlintest):

data class Silly(val hey: String? = null, val ho: String? = null)

"should encode inner tag with unexpected nil attribute" {
    val xml = """
        <Silly xmlns:a="http://www.w3.org/2001/XMLSchema-instance" xmlns:i="http://www.w3.org/2001/XMLSchema-instance">
            <a:Hey i:nil="true"/>
            <a:Ho>to</a:Ho>
        </Silly>"""

    val res = xml.parseXml<Silly>()

    res shouldBe Silly("", "to")
}

The failure:

org.opentest4j.AssertionFailedError: expected: Silly(hey=, ho=to) but was: Silly(hey=null, ho=null)
Expected :Silly(hey=, ho=to)
Actual   :Silly(hey=null, ho=null)

I'm fine with hey being null, but it's especially surprising that ho is null as well.

The parseXml() implementation:

import com.fasterxml.jackson.databind.DeserializationFeature.*
import com.fasterxml.jackson.databind.MapperFeature.*

private fun xmlFactory() = XmlFactory(newInstance().apply {
        setProperty(IS_NAMESPACE_AWARE, true)
})

private val jacksonXmlModule = JacksonXmlModule().apply {
        setDefaultUseWrapper(false)
        setXMLTextElementName("textContent")
}

private fun createMapper() = XmlMapper(xmlFactory(), jacksonXmlModule)
        .registerKotlinModule()
        .configure(ACCEPT_CASE_INSENSITIVE_PROPERTIES, true)
        .configure(FAIL_ON_UNKNOWN_PROPERTIES, false)
        .configure(ACCEPT_SINGLE_VALUE_AS_ARRAY, true)
        .configure(ACCEPT_EMPTY_STRING_AS_NULL_OBJECT, true)

inline fun <reified T : Any> String.parseXml(): T = createMapper().readValue(this, T::class.java)

[henrik242]

It works if I replace the http://www.w3.org/2001/XMLSchema-instance namespace with something else. Is there a way of disabling this feature? I guess this is related to #358 somehow.

[cowtowncoder]

It is probably related to the way xsi:nil is detected, yes, although things previously working may just be due to luck (i.e. not necessarily due to explicit support).
I think it's worth creating separate issue; it would be nice if reproduction is in pure Java (since tests can not depend on Kotlin, outside of Kotlin module), but I can probably convert them if need be.

[henrik242]

@cowtowncoder Created separate issue with test project in #378