Trouble with Schemas

I had to spend some time this week working on XML schemas. The trouble with schemas is that you can go all out and implement an almost object-oriented inheritance hierarchy. Which is exactly what I did.

I unwittingly set foot into the rarefied area of substitutionGroups and the like. And trying to find information when you don't know what to look for is a huge problem. So to save you the trouble and for my own future reference, here's what I learned. I could be wrong in several places, but it should be a good starting point for you.

This is what I was I had in mind. I needed a generic Object and a simple Collection-like container that could hold 1 or more Objects. And if you are not using XMLSpy...well good luck!

This is what the Object looks like:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns="http://www.javaforu.com/schemas/lesson1" xmlns:xs="http://www.w3.org/2001/XMLSchema" targetNamespace="http://www.javaforu.com/schemas/lesson1" elementFormDefault="qualified" attributeFormDefault="unqualified">

<xs:complexType name="GenericObjectType"></xs:complexType>

<xs:element name="GenericObject" type="GenericObjectType"></xs:element>

</xs:schema>

And this is what the Object Collection looks like:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns="http://www.javaforu.com/schemas/lesson1" xmlns:xs="http://www.w3.org/2001/XMLSchema" targetNamespace="http://www.javaforu.com/schemas/lesson1" elementFormDefault="qualified" attributeFormDefault="unqualified">


<xs:complexType name="GenericObjectCollectionType">
 <xs:sequence>
  <xs:element ref="GenericObject" maxOccurs="unbounded"></xs:element>
 </xs:sequence>
</xs:complexType>

<xs:element name="GenericObjectCollection" type="GenericObjectCollectionType"></xs:element>

</xs:schema>

That was easy enough. What I need now is a Person object that inherits from Object.

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns="http://www.javaforu.com/schemas/lesson1" xmlns:xs="http://www.w3.org/2001/XMLSchema" targetNamespace="http://www.javaforu.com/schemas/lesson1" elementFormDefault="qualified" attributeFormDefault="unqualified">


<xs:complexType name="PersonType">
 <xs:complexContent>
  <xs:extension base="GenericObjectType">
   <xs:sequence>
    <xs:element name="firstName" type="xs:string"></xs:element>
    <xs:element name="lastName" type="xs:string"></xs:element>
    <xs:element name="age" type="xs:positiveInteger"></xs:element>
    <xs:element name="gender" type="xs:string"></xs:element>
   </xs:sequence>
  </xs:extension>
 </xs:complexContent>
</xs:complexType>

<xs:element name="Person" type="PersonType" substitutionGroup="GenericObject"></xs:element>

</xs:schema>

Note the use of substitutionGroup="GenericObject" in the Person element. This means that I can create Person tags/elements where ever GenericObject tags are allowed. That's very cool!

Here comes the clincher..If I need a special PersonCollection that only allows Person objects but still inherits from the GenericCollection, this is what I'd have to do:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns="http://www.javaforu.com/schemas/lesson1" xmlns:xs="http://www.w3.org/2001/XMLSchema" targetNamespace="http://www.javaforu.com/schemas/lesson1" elementFormDefault="qualified" attributeFormDefault="unqualified">


<xs:complexType name="PersonCollectionType">
 <xs:complexContent>
  <xs:restriction base="GenericObjectCollectionType">
   <xs:sequence>
    <xs:choice maxOccurs="unbounded">
     <xs:element ref="Person"></xs:element>
    </xs:choice>
   </xs:sequence>
  </xs:restriction>
 </xs:complexContent>
</xs:complexType>

</xs:schema>

Now, pay attention: For some reason, I'm yet to discover why, you cannot just replace the ref to the GenericObject with a Person ref when you have an unbounded occurrence rule. Try it. It has to be wrapped in a choice rule.

And of course the substitutionGroup="GenericObjectCollection" will you use the PersonCollection where ever the GenericCollection is allowed.

<xs:element name="PersonCollection" substitutionGroup="GenericObjectCollection"></xs:element>

This is what a sample looks XML like:

<?xml version="1.0" encoding="UTF-8"?>
<DocRoot xmlns="http://www.javaforu.com/schemas/lesson1" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.javaforu.com/schemas/lesson1 Lesson1.xsd">

 <GenericObject></GenericObject>
 
 <Person>
  <firstName>sdsfxcv</firstName>
  <lastName>xsdf</lastName>
  <age>12</age>
  <gender>m</gender>
 </Person>

 <GenericObjectCollection>
  <GenericObject></GenericObject>

  <Person>
   <firstName>asd</firstName>
   <lastName>fgdfgd</lastName>
   <age>77</age>
   <gender>f</gender>
  </Person>
 </GenericObjectCollection>

 <PersonCollection>
  <Person>
   <firstName>abcd</firstName>
   <lastName>def</lastName>
   <age>20</age>
   <gender>m</gender>
  </Person>
 </PersonCollection>

</DocRoot>

And the full schema:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns="http://www.javaforu.com/schemas/lesson1" xmlns:xs="http://www.w3.org/2001/XMLSchema" targetNamespace="http://www.javaforu.com/schemas/lesson1" elementFormDefault="qualified" attributeFormDefault="unqualified">

 <xs:complexType name="GenericObjectType"/>

 <xs:complexType name="GenericObjectCollectionType">
  <xs:sequence>
   <xs:element ref="GenericObject" maxOccurs="unbounded"></xs:element>
  </xs:sequence>
 </xs:complexType>

 <xs:complexType name="DocRootType">
  <xs:sequence>
   <xs:element ref="GenericObject" minOccurs="0" maxOccurs="unbounded"></xs:element>
   <xs:element ref="GenericObjectCollection" minOccurs="0" maxOccurs="unbounded"></xs:element>
  </xs:sequence>
 </xs:complexType>

 <xs:element name="GenericObject" type="GenericObjectType"></xs:element>
 <xs:element name="GenericObjectCollection" type="GenericObjectCollectionType"></xs:element>
 <xs:element name="DocRoot" type="DocRootType"></xs:element>

 <!---->

 <xs:complexType name="PersonType">
  <xs:complexContent>
   <xs:extension base="GenericObjectType">
    <xs:sequence>
     <xs:element name="firstName" type="xs:string"></xs:element>
     <xs:element name="lastName" type="xs:string"></xs:element>
     <xs:element name="age" type="xs:positiveInteger"></xs:element>
     <xs:element name="gender" type="xs:string"></xs:element>
    </xs:sequence>
   </xs:extension>
  </xs:complexContent>
 </xs:complexType>

 <xs:complexType name="PersonCollectionType">
  <xs:complexContent>
   <xs:restriction base="GenericObjectCollectionType">
    <xs:sequence>
     <xs:choice maxOccurs="unbounded">
      <xs:element ref="Person"></xs:element>
     </xs:choice>
    </xs:sequence>
   </xs:restriction>
  </xs:complexContent>
 </xs:complexType>

 <xs:element name="Person" type="PersonType" substitutionGroup="GenericObject"></xs:element>
 <xs:element name="PersonCollection" substitutionGroup="GenericObjectCollection"></xs:element>

</xs:schema>

If you find any mistakes or have any other such interesting bits of information to share, I'd love to hear it.