java, xsd & marshalling: jre bug, my fault or xsd issues?

Question

I am running into a problem while marshalling a JAXB model tree to a xml file.

I created those model classes using xjc. I c

Answer 1

TL;DR

java, xsd & marshalling: jre bug, my fault or xsd issues?

It's your fault :). Continue reading for the reason why.

---

Java Model

Below is a Java model that is based on the one hinted at in your question that can be used to produce the same exception that you are seeing.

Root

Below is a root object that has one property. The type of that property has two subclasses. One of the subclasses (B) corresponds to a named type, and the other (A) to an anonymous type.

package com.mypackage;

import javax.xml.bind.annotation.XmlRootElement;
@XmlRootElement
public class Root {

    private BaseType baseType;

    public BaseType getBaseType() {
        return baseType;
    }

    public void setBaseType(BaseType baseType) {
        this.baseType = baseType;
    }

}

BaseType

This is the super class. It has two subclasses: A and B.

package com.mypackage;

import javax.xml.bind.annotation.XmlSeeAlso;

@XmlSeeAlso({A.class, B.class})
public class BaseType {

}

A

The A class corresponds to an anonymous complex type. An anonymous type is represented with the @XmlType(name="") annotation.

package com.mypackage;

import javax.xml.bind.annotation.XmlType;

@XmlType(name="")
public class A extends BaseType {

}

B

The B class corresponds to the named complex type b. The default is the same as having @XmlType(name="b").

package com.mypackage;

public class B extends BaseType {

}

Demo Code - That Reproduces Error

Demo

When the baseType property on the Root class is one of the subclasses then the xsi:type attribute is added to qualify the element (similar to a cast in Java). The instance is required to correspond to a named complex type. In this case A does not.

package com.mypackage;

import javax.xml.bind.*;

public class Demo {

    public static void main(String[] args) throws Exception {
        JAXBContext jc = JAXBContext.newInstance(Root.class);

        Root root = new Root();
        BaseType baseType = new A(); // Invalid Value
        root.setBaseType(baseType);

        Marshaller marshaller = jc.createMarshaller();
        marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
        marshaller.marshal(root, System.out);
    }

}

Exception

This results in the following exception. As stated by the exception A is not a valid substitute for BaseType because it corresponds to an anonymous complex type, since since it corresponds to an anonymous type it doesn't have the necessary value for the xsi:type attribute.

Exception in thread "main" javax.xml.bind.MarshalException
 - with linked exception:
[com.sun.istack.internal.SAXException2: Instance of "com.mypackage.A" is substituting "com.mypackage.BaseType", but "com.mypackage.A" is bound to an anonymous type.]
    at com.sun.xml.internal.bind.v2.runtime.MarshallerImpl.write(MarshallerImpl.java:311)
    at com.sun.xml.internal.bind.v2.runtime.MarshallerImpl.marshal(MarshallerImpl.java:236)
    at javax.xml.bind.helpers.AbstractMarshallerImpl.marshal(AbstractMarshallerImpl.java:95)
    at com.mypackage.Demo.main(Demo.java:16)
Caused by: com.sun.istack.internal.SAXException2: Instance of "com.mypackage.A" is substituting "com.mypackage.BaseType", but "com.mypackage.A" is bound to an anonymous type.
    at com.sun.xml.internal.bind.v2.runtime.XMLSerializer.reportError(XMLSerializer.java:237)
    at com.sun.xml.internal.bind.v2.runtime.XMLSerializer.childAsXsiType(XMLSerializer.java:652)
    at com.sun.xml.internal.bind.v2.runtime.property.SingleElementNodeProperty.serializeBody(SingleElementNodeProperty.java:143)
    at com.sun.xml.internal.bind.v2.runtime.ClassBeanInfoImpl.serializeBody(ClassBeanInfoImpl.java:343)
    at com.sun.xml.internal.bind.v2.runtime.XMLSerializer.childAsSoleContent(XMLSerializer.java:582)
    at com.sun.xml.internal.bind.v2.runtime.ClassBeanInfoImpl.serializeRoot(ClassBeanInfoImpl.java:325)
    at com.sun.xml.internal.bind.v2.runtime.XMLSerializer.childAsRoot(XMLSerializer.java:483)
    at com.sun.xml.internal.bind.v2.runtime.MarshallerImpl.write(MarshallerImpl.java:308)
    ... 3 more

Demo Code - That Works

Demo

If instead of A you specified the other subclass B which corresponds to a named complex type.

package com.mypackage;

import javax.xml.bind.*;

public class Demo {

    public static void main(String[] args) throws Exception {
        JAXBContext jc = JAXBContext.newInstance(Root.class);

        Root root = new Root();
        BaseType baseType = new B(); // Valid Value
        root.setBaseType(baseType);

        Marshaller marshaller = jc.createMarshaller();
        marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
        marshaller.marshal(root, System.out);
    }

}

Output

Then you will see everything works as expected an the type name is included when the baseType property is marshalled.

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<root>
    <baseType xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="b"/>
</root>