Parsing empty tags without default no-arguments constructor fails in 2.14

As in https://github.com/FasterXML/jackson-dataformat-xml/issues/491, I’ve found an additional issue where parsing empty tags without default constructor fails. This worked in 2.11.3, but has failed since.

Check should_parse_empty_tag_without_default_constructor() in https://github.com/henrik242/jackson-xml-problem/blob/no-string-argument/src/test/java/jackson/xml/NoStringArgumentTest.java#L37

Test (java):

    @Test
    void  should_parse_empty_tag_without_default_constructor() {
        String xml = "<outer><inner></inner></outer>";
        Outer res = XmlTool.parseOuter(xml);
        assertEquals(res, new Outer(null)); // SUCCESS in Jackson 2.11.x, but FAIL in 2.14.x
    }

Code (kotlin):

object XmlTool {
    val xmlIn = XMLInputFactory.newInstance()
    val factory = XmlFactory(xmlIn)
    val xmlModule = JacksonXmlModule()
    val mapper = XmlMapper(factory, xmlModule).registerKotlinModule()

    @JvmStatic
    fun parseOuter(xml: String?): Outer = mapper.readValue(xml, Outer::class.java)

    data class Inner(val code: String?)
    data class Outer(val inner: Inner?)
}

Fails with:

com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot construct instance of `jackson.xml.XmlTool$Inner` (although at least one Creator exists): no default no-arguments constructor found
 at [Source: (StringReader); line: 1, column: 15] (through reference chain: jackson.xml.XmlTool$Outer["inner"])
	at app//com.fasterxml.jackson.databind.exc.MismatchedInputException.from(MismatchedInputException.java:63)
	at app//com.fasterxml.jackson.databind.DeserializationContext.reportInputMismatch(DeserializationContext.java:1733)
	at app//com.fasterxml.jackson.databind.DeserializationContext.handleMissingInstantiator(DeserializationContext.java:1358)
	at app//com.fasterxml.jackson.databind.deser.ValueInstantiator.createUsingDefault(ValueInstantiator.java:248)
	at app//com.fasterxml.jackson.databind.deser.std.StdValueInstantiator.createUsingDefault(StdValueInstantiator.java:275)
	at app//com.fasterxml.jackson.databind.deser.BeanDeserializerBase.getEmptyValue(BeanDeserializerBase.java:1042)
	at app//com.fasterxml.jackson.databind.deser.std.StdDeserializer._deserializeFromEmptyString(StdDeserializer.java:322)
	at app//com.fasterxml.jackson.databind.deser.std.StdDeserializer._deserializeFromString(StdDeserializer.java:270)
	at app//com.fasterxml.jackson.databind.deser.BeanDeserializerBase.deserializeFromString(BeanDeserializerBase.java:1500)
	at app//com.fasterxml.jackson.databind.deser.BeanDeserializer._deserializeOther(BeanDeserializer.java:197)
	at app//com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:187)
	at app//com.fasterxml.jackson.databind.deser.SettableBeanProperty.deserialize(SettableBeanProperty.java:542)
	at app//com.fasterxml.jackson.databind.deser.BeanDeserializer._deserializeWithErrorWrapping(BeanDeserializer.java:564)
	at app//com.fasterxml.jackson.databind.deser.BeanDeserializer._deserializeUsingPropertyBased(BeanDeserializer.java:439)
	at app//com.fasterxml.jackson.databind.deser.BeanDeserializerBase.deserializeFromObjectUsingNonDefault(BeanDeserializerBase.java:1405)
	at app//com.fasterxml.jackson.databind.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:352)
	at app//com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:185)
	at app//com.fasterxml.jackson.dataformat.xml.deser.XmlDeserializationContext.readRootValue(XmlDeserializationContext.java:91)
	at app//com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4697)
	at app//com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3652)
	at app//com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3620)
	at app//jackson.xml.XmlTool.parseOuter(XmlTool.kt:22)
	at app//jackson.xml.NoStringArgumentTest.should_parse_empty_tag_without_default_constructor(NoStringArgumentTest.java:39)
	at java.base@11.0.16.1/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base@11.0.16.1/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at java.base@11.0.16.1/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base@11.0.16.1/java.lang.reflect.Method.invoke(Method.java:566)
	at app//org.junit.platform.commons.util.ReflectionUtils.invokeMethod(ReflectionUtils.java:675)
	at app//org.junit.jupiter.engine.execution.MethodInvocation.proceed(MethodInvocation.java:60)
	at ...

This is the exact same issue as mentioned in https://github.com/FasterXML/jackson-module-kotlin/issues/396, and is already covered a couple of places with existing failing tests:

• https://github.com/FasterXML/jackson-integration-tests/blob/master/src/test/kotlin/com/fasterxml/jackson/failing/Jackson212MissingConstructorTest.kt (see https://github.com/FasterXML/jackson-integration-tests/pull/7)
• https://github.com/FasterXML/jackson-module-kotlin/blob/master/src/test/kotlin/com/fasterxml/jackson/module/kotlin/test/github/failing/Github396.kt

[cowtowncoder]

@henrik242 Ok, I think that what you can do is to add @JsonCreator(mode = JsonCreator.Mode.DELEGATING) for constructor of Inner. In this case String "" will be passed (not null).

I will try to see if the alternative (Inner declared to take PROPERTIES) could possibly be made to also work but thought I’ll add a note of work-around; and you’d probably want to indicate mode in either since behavior does generally differ between two modes.

EDIT: there is actually another work-around that does work with PROPERTIES approach – just add no-argument constructor. That will make deserializer happy.

[cowtowncoder]

I can’t think of any based on specific fix, but that does not mean someone could not figure out a way around it.

However… I suspect that you could use AnnotationIntrospector to essentially force mode = Mode.DELEGATING for specific inner classes; detect cases where it is needed, return. I don’t remember off-hand which method in AnnotationIntrospector but shouldn’t be too hard to find.