Optimal JAXB XJC code generation and optimal XML s

There have been a lot of similar questions which focus on one aspect to optimize but each solution had an ugly downside.

Suppose I want to develop an XML schema (XSD) which allows the following documents and want to generate classes using XJC:

<Catalogue>
    <Book>...</Book>
    <Journal>...</Journal>
    <Book>...</Book>
    ...
</Catalogue>

The schema should model the type hierarchy (Book and Journal are subclasses of Publication). Naturally, this should also be the case for the generated Java classes.

I tried the following approches which all have a major issue:

1.) modeling Catalogue to contain a xsd:choice of all possible subtypes.

<xsd:complexType name="Catalogue">
    <xsd:choice maxOccurs="unbounded">
        <xsd:element ref="Book" />
        <xsd:element ref="Magazine" />
    </xsd:choice>
</xsd:complexType>

<xsd:element name="Publication" abstract="true" type="Publication" />
<xsd:element name="Book" type="Book"/>
<xsd:element name="Magazine" type="Magazine"/>

<xsd:complexType name="Publication">
    <xsd:sequence></xsd:sequence>
</xsd:complexType>

<xsd:complexType name="Book">
    <xsd:complexContent>
        <xsd:extension base="Publication">
        </xsd:extension>
    </xsd:complexContent>
</xsd:complexType>
...

The problem here is that I have to mention all possible subtypes in the choice element which could be a lot in a real application. A minor issue is that, although the Catalogue attribute has the correct type List<Publication> it has an ugly name bookAndMagazine. Because of the redundant schema definition, not an option!

2.) modeling Catalogue to contain a xsd:sequence of the parent class

<xsd:complexType name="Catalogue">
    <xsd:choice maxOccurs="unbounded">
        <xsd:element ref="Publication" maxOccurs="unbounded"/>
    </xsd:choice>
</xsd:complexType>

This does only work if the XML documents are formulated like <Publication xsi:type="Book"...> . Thus, not an option!

3.) Use substitutionGroup like mentioned here http://www.xfront.com/ElementHierarchy.html

<xsd:complexType name="Catalogue">
    <xsd:choice maxOccurs="unbounded">
        <xsd:element ref="Publication" maxOccurs="unbounded"/>
    </xsd:choice>
</xsd:complexType>

<xsd:element name="Publication" abstract="true" type="Publication" />
<xsd:element name="Book" type="Book" substitutionGroup="Publication"/>
<xsd:element name="Magazine" type="Magazine" substitutionGroup="Publication"/>

<xsd:complexType name="Publication">
    <xsd:sequence></xsd:sequence>
</xsd:complexType>

<xsd:complexType name="Book">
    <xsd:complexContent>
        <xsd:extension base="Publication">
        </xsd:extension>
    </xsd:complexContent>
</xsd:complexType>
...

Here, the code generation is the issue because the inner element of Catalogue is mapped to List<JaxbElement<? extends Publication>> rather than List<Publication> . Hence, also this isn't an option.

How to bring all my objectives together?:

canonical, non-redundant schema which models inheritance (such as in 2.) or 3.))
simple and clean Java classes generated from this schema and which models inheritance (like in 2.) and partly in 1.) )
clean XML documents (not like in 2.) )
usage of standard JAXB and preferable not much binding metadata

And if there is no solution which matches all these objectives, which one would you prefer?

回答1:

I would suggest using something similar to your first option as I have never seen a cleaner solution to this problem.

XSD

<xs:element name="Publications" minOccurs="0">
  <xs:complexType>
    <xs:sequence>
      <xs:choice minOccurs="0" maxOccurs="unbounded">
        <xs:element name="Magazine" type="magazine"/>
        <xs:element name="Book" type="book"/>
      </xs:choice>
    </xs:sequence>
  </xs:complexType>
</xs:element>

 <xs:complexType name="magazine">
  <xs:complexContent>
   <xs:extension base="publication">
    <xs:sequence>
      <xs:element name="issueName" type="xs:string" minOccurs="0"/>
    </xs:sequence>
  </xs:extension>
 </xs:complexContent>
</xs:complexType>

<xs:complexType name="publication">
  <xs:sequence>
    <xs:element name="name" type="xs:string" minOccurs="0"/>
  </xs:sequence>
</xs:complexType>

<xs:complexType name="book">
  <xs:complexContent>
    <xs:extension base="publication">
      <xs:sequence>
        <xs:element name="title" type="xs:string" minOccurs="0"/>
      </xs:sequence>
    </xs:extension>
  </xs:complexContent>
</xs:complexType>

JAVA This is the Java code I used to generate the above XSD.

@XmlElements({
  @XmlElement(name="Magazine", type=Magazine.class),
  @XmlElement(name="Book", type=Book.class)
})
@XmlElementWrapper(name="Publications")
public List<Publication> publications;

回答2:

If you can start from the code (instead of the XSD) and are OK with using MOXY check out this blog post. It uses method 3 without the JAXBElement wrappers. I'd love a way to do this starting with the XSD, but I haven't found it yet.