I have model generated by xjc based on the dtd. The model looks as follows:

@XmlAccessorType(XmlAccessType.FIELD)
@XmlType(name = "", propOrder = {
        "a",
        "b"
})
@XmlRootElement(name = "parent")
public class Parent {

    @XmlAttribute(name = "id")
    @XmlJavaTypeAdapter(CollapsedStringAdapter.class)
    @XmlID
    protected String id;
    @XmlElement(required = true)
    protected A a;
    @XmlElement(required = true)
    protected B b;
//getters and setters
}

@XmlAccessorType(XmlAccessType.FIELD)
@XmlType(name = "", propOrder = {
    "value"
})
@XmlRootElement(name = "a")
public class A {

    @XmlAttribute(name = "id")
    @XmlJavaTypeAdapter(CollapsedStringAdapter.class)
    @XmlID
    protected String id;
    @XmlValue
    protected String value;
//getters and setters
}

@XmlAccessorType(XmlAccessType.FIELD)
@XmlType(name = "", propOrder = {
    "value"
})
@XmlRootElement(name = "b")
public class B {

    @XmlAttribute(name = "id")
    @XmlJavaTypeAdapter(CollapsedStringAdapter.class)
    @XmlID
    protected String id;


    @XmlValue
    protected String value;
//getters and setters
}

All classes contains id which is annotated with @XmlID. To give whole context my parser looks as follows:

    XMLInputFactory inputFactory = new WstxInputFactory();
    inputFactory.setProperty(XMLInputFactory.IS_NAMESPACE_AWARE, Boolean.FALSE);
    inputFactory.setProperty(XMLInputFactory.SUPPORT_DTD, false);

    XmlFactory xmlFactory = new XmlFactory(inputFactory, new WstxOutputFactory());
    JacksonXmlModule jacksonXmlModule = new JacksonXmlModule();
    jacksonXmlModule.setDefaultUseWrapper(false);

    XmlMapper xmlMapper = new XmlMapper(xmlFactory, jacksonXmlModule);
    xmlMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);

    xmlMapper.registerModule(new JakartaXmlBindAnnotationModule());

The problem occurs when the xml does not contain the id attribute for tag a or b. Example xml:

<?xml version="1.0" encoding="UTF-8" ?>
<parent>
    <a id="ex">some content</a>
    <b>another content</b>
</parent>

I get the following error during mapping:

Exception in thread “main”
com.fasterxml.jackson.databind.deser.UnresolvedForwardReference:
Unresolved forward references for: at [Source: (StringReader); line:
5, column: 1]Object id [another content] (for mapping.B) at [Source:
(StringReader); line: 4, column: 27]. at
com.fasterxml.jackson.databind.deser.UnresolvedForwardReference.withStackTrace(UnresolvedForwardReference.java:104)
at
com.fasterxml.jackson.databind.deser.DefaultDeserializationContext.checkUnresolvedObjectId(DefaultDeserializationContext.java:184)
at
com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4907)
at
com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3848)
at
com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3816)
at mapping.Parser.main(Parser.java:36)

However, when the parent tag does not have the id attribute, everything maps correctly. So this xml will be mappe correctly:

<?xml version="1.0" encoding="UTF-8" ?>
<parent>
    <a id="ex">some content</a>
    <b id="bl">another content</b>
</parent>

I know that @XmlID helps to identify object across object graph during serialization and deserialization. And it allows to reference given object by id with the usage of @XmlIDREF.

I’m a little bit confused, so I would like to understand, how does this @XmlID annotation work? Why does the error only appear when the missing attribute is at the lowest level of the hierarchy? Can I configure Jackson to not throw this exception?