Chapter 2. Defining Logical Data Units

In XML Schema, data units that are a collection of data fields are defined using complexType elements. Specifying a complex type requires three pieces of information:

For example, the structure shown in Example 2.3, “Simple Structure” is defined in XML Schema as a complex type with two elements.

struct personalInfo
{
  string name;
  int age;
};

Example 2.4, “A complex type” shows one possible XML Schema mapping for the structure shown in Example 2.3, “Simple Structure” The structure defined in Example 2.4, “A complex type” generates a message containing two elements: name and age.

.

<complexType name="personalInfo">
  <sequence>
    <element name="name" type="xsd:string" />
    <element name="age" type="xsd:int" />
  </sequence>
</complexType>

XML Schema has three ways of describing how the fields of a complex type are organized when represented as an XML document and passed on the wire. The first child element of the complexType element determines which variety of complex type is being used. Table 2.1, “Complex type descriptor elements” shows the elements used to define complex type behavior.

If the structure is defined using a choice element, as shown in Example 2.5, “Simple complex choice type”, it generates a message with either a name element or an age element.

<complexType name="personalInfo">
  <choice>
    <element name="name" type="xsd:string"/>
    <element name="age" type="xsd:int"/>
  </choice>
</complexType>

You define the data fields that make up a structure using element elements. Every complexType element should contain at least one element element. Each element element in the complexType element represents a field in the defined data structure.

To fully describe a field in a data structure, element elements have two required attributes:

In addition to name and type, element elements have two other commonly used optional attributes: minOcurrs and maxOccurs. These attributes place bounds on the number of times the field occurs in the structure. By default, each field occurs only once in a complex type. Using these attributes, you can change how many times a field must or can appear in a structure. For example, you can define a field, previousJobs, that must occur at least three times, and no more than seven times, as shown in Example 2.6, “Simple complex type with occurrence constraints”.

<complexType name="personalInfo">
  <all>
    <element name="name" type="xsd:string"/>
    <element name="age" type="xsd:int"/>
    <element name="previousJobs" type="xsd:string:
             minOccurs="3" maxOccurs="7"/>
  </all>
</complexType>

You can also use the minOccurs to make the age field optional by setting the minOccurs to zero as shown in Example 2.7, “Simple complex type with minOccurs set to zero”. In this case age can be omitted and the data will still be valid.

<complexType name="personalInfo">
  <choice>
    <element name="name" type="xsd:string"/>
    <element name="age" type="xsd:int" minOccurs="0"/>
  </choice>
</complexType>

In XML documents, attributes are contained in the element’s tag. For example, in the complexType element in the code below, name is an attribute. To specify an attribute for a complex type, you define an attribute element in the complexType element definition. An attribute element can appear only after the all, sequence, or choice element. Specify one attribute element for each of the complex type’s attributes. Any attribute elements must be direct children of the complexType element.

<complexType name="personalInfo">
  <all>
    <element name="name" type="xsd:string"/>
    <element name="previousJobs" type="xsd:string"
             minOccurs="3" maxOccurs="7"/>
  </all>
  <attribute name="age" type="xsd:int" use="required" />
</complexType>

In the previous code, the attribute element specifies that the personalInfo complex type has an age attribute. The attribute element has these attributes:

In an attribute element, you can specify the optional default attribute, which lets you specify a default value for the attribute.

Apache CXF supports two methods for defining arrays in a contract. The first is define a complex type with a single element whose maxOccurs attribute has a value greater than one. The second is to use SOAP arrays. SOAP arrays provide added functionality such as the ability to easily define multi-dimensional arrays and to transmit sparsely populated arrays.

Complex type arrays are a special case of a sequence complex type. You simply define a complex type with a single element and specify a value for the maxOccurs attribute. For example, to define an array of twenty floating point numbers you use a complex type similar to the one shown in Example 2.9, “Complex type array”.

<complexType name="personalInfo">
  <element name="averages" type="xsd:float" maxOccurs="20"/>
</complexType>

You can also specify a value for the minOccurs attribute.

SOAP arrays are defined by deriving from the SOAP-ENC:Array base type using the wsdl:arrayType element. The syntax for this is shown in Example 2.10, “Syntax for a SOAP array derived using wsdl:arrayType”. Ensure that the definitions element declares xmlns:SOAP-ENC="http://schemas.xmlsoap.org/soap/encoding/".

<complexType name="TypeName">
  <complexContent>
    <restriction base="SOAP-ENC:Array">
      <attribute ref="SOAP-ENC:arrayType"
                 wsdl:arrayType="ElementType<ArrayBounds>"/>
    </restriction>
  </complexContent>
</complexType>

Using this syntax, TypeName specifies the name of the newly-defined array type. ElementType specifies the type of the elements in the array. ArrayBounds specifies the number of dimensions in the array. To specify a single dimension array use []; to specify a two-dimensional array use either [][] or [,].

For example, the SOAP Array, SOAPStrings, shown in Example 2.11, “Definition of a SOAP array”, defines a one-dimensional array of strings. The wsdl:arrayType attribute specifies the type of the array elements, xsd:string, and the number of dimensions, with [] implying one dimension.

<complexType name="SOAPStrings">
  <complexContent>
    <restriction base="SOAP-ENC:Array">
      <attribute ref="SOAP-ENC:arrayType"
                 wsdl:arrayType="xsd:string[]"/>
    </restriction>
  </complexContent>
</complexType>

You can also describe a SOAP Array using a simple element as described in the SOAP 1.1 specification. The syntax for this is shown in Example 2.12, “Syntax for a SOAP array derived using an element”.

<complexType name="TypeName">
  <complexContent>
    <restriction base="SOAP-ENC:Array">
      <sequence>
        <element name="ElementName" type="ElementType"
                 maxOccurs="unbounded"/>
      </sequence>
    </restriction>
  </complexContent>
</complexType>

When using this syntax, the element’s maxOccurs attribute must always be set to unbounded.

XML Schema allows you to create new types by restricting the possible values of an XML Schema simple type. For example, you can define a simple type, SSN, which is a string of exactly nine characters. New types defined by restricting simple types are defined using a simpleType element.

The definition of a type by restriction requires three things:

The base type is the type that is being restricted to define the new type. It is specified using a restriction element. The restriction element is the only child of a simpleType element and has one attribute, base, that specifies the base type. The base type can be any of the XML Schema simple types.

For example, to define a new type by restricting the values of an xsd:int you use a definition like the one shown in Example 2.14, “Using int as the base type”.

<simpleType name="restrictedInt">
  <restriction base="xsd:int">
    ...
  </restriction>
</simpleType>

The rules defining the restrictions placed on the base type are called facets. Facets are elements with one attribute, value, that defines how the facet is enforced. The available facets and their valid value settings depend on the base type. For example, xsd:string supports six facets, including:

Each facet element is a child of the restriction element.

Example 2.15, “SSN simple type description” shows an example of a simple type, SSN, which represents a social security number. The resulting type is a string of the form xxx-xx-xxxx. <SSN>032-43-9876<SSN> is a valid value for an element of this type, but <SSN>032439876</SSN> is not.

<simpleType name="SSN">
  <restriction base="xsd:string">
    <pattern value="\d{3}-\d{2}-\d{4}"/>
  </restriction>
</simpleType>

Enumerated types in XML Schema are a special case of definition by restriction. They are described by using the enumeration facet which is supported by all XML Schema primitive types. As with enumerated types in most modern programming languages, a variable of this type can only have one of the specified values.

The syntax for defining an enumeration is shown in Example 2.16, “Syntax for an enumeration”.

<simpleType name="EnumName">
  <restriction base="EnumType">
    <enumeration value="Case1Value"/>
    <enumeration value="Case2Value"/>
    ...
    <enumeration value="CaseNValue"/>
  </restriction>
</simpleType>

EnumName specifies the name of the enumeration type. EnumType specifies the type of the case values. CaseNValue, where N is any number one or greater, specifies the value for each specific case of the enumeration. An enumerated type can have any number of case values, but because it is derived from a simple type, only one of the case values is valid at a time.

For example, an XML document with an element defined by the enumeration widgetSize, shown in Example 2.17, “widgetSize enumeration”, would be valid if it contained <widgetSize>big</widgetSize>, but it would not be valid if it contained <widgetSize>big,mungo</widgetSize>.

<simpleType name="widgetSize">
  <restriction base="xsd:string">
    <enumeration value="big"/>
    <enumeration value="large"/>
    <enumeration value="mungo"/>
  </restriction>
</simpleType>