SCDJWS Study Guide: SOAP


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SOAP Encoding

The SOAP encoding style is based on a simple type system that is a generalization of the common features found in type systems in programming languages, databases and semi-structured data. A type either is a simple (scalar) type or is a compound type constructed as a composite of several parts, each with a type. This is described in more detail below.

This section defines rules for serialization of a graph of typed objects. It operates on two levels. First, given a schema in any notation consistent with the type system described, a schema for an XML grammar may be constructed. Second, given a type-system schema and a particular graph of values conforming to that schema, an XML instance may be constructed. In reverse, given an XML instance produced in accordance with these rules, and given also the original schema, a copy of the original value graph may be constructed.

There are following SOAP encoding types:

Simple Types:

  • Strings
  • Enumerations
  • Array of Bytes

Compound Types:

  • Arrays
  • Structures

SOAP defines a set of serialization rules for encoding datatypes in XML. All data is serialized as elements rather than attributes. Attributes are only used for structural metadata; for example, when references are needed. For simple types such as strings, numbers, dates, and so forth, the datatypes defined in XML Schema Part II—Datatypes are used. For types such as classes or structures, each field in the type is serialized using an element with the same name as the field. For array types, each array element is typically serialized using an element with the same name as the type, although other element names may be used. In both cases, if the field being serialized is itself a structure or an array, then nested elements are used. The top-level element in both the structure case and the array case is namespace qualified. Descendant elements should be unqualified.

The serialization rules apply to children of the Header element as well as children of the Body element. Such children are serialized types just like any other type. A request and any associated response are also treated as types, and are serialized according to the same rules.

Examples Serialization of a structured Java or VB type

package example.org.People;
// Java class definition
class Person
{
  String name;
  float age;
  short height;
}

// VB Type definition
Public Type Person
  name As String
  age As Single
  height As Integer
End Type

<p:Person xmlns:p='urn:example-org:people'>
  <name>Martin</name>
  <age>33</age>
  <height>64</height>
</p:Person>

Serialization of a Java or VB array

package example.org.Num;
// Java class definition
class Numbers
{
   long[5] data;
}
 
// VB Type definition
Public Type Numbers
  data(5) As Long
End Type
 
<p:Numbers xmlns:p='urn:example-org:num'>
  <data enc:arrayType='xsd:long[5]'
     xmlns:enc='http://schemas.xmlsoap.org/soap/encoding/'>
    <enc:long>2</enc:long>
    <enc:long>3</enc:long>
    <enc:long>5</enc:long>
    <enc:long>7</enc:long>
    <enc:long>9</enc:long>
  </data>
</p:Numbers>



Serialization of simple structured data



Serializing data structures, when each field is referred to exactly once, is straightforward. Each field is serialized as an embedded element, a descendant element of the Body element, not as an immediate child. Such an element is called a single-reference accessor, and it provides access to the data in the field at a single location in the message. The element name used to contain the data is the same as the field name used in the programmatic type.

Example Serializing structured data

package example.org.People;
// Java class definitions
class PersonName
{
  String givenName;
  String familyName;
}
 
class Person
{
  PersonName name;
  float age;
  short height;
 
  public static void AddPerson ( Person person );
}
 
// VB Type definitions
Public Type PersonName
  givenName As String
  familyName As String
End Type
 
Public Type Person
  name As PersonName
  age As Single
  height As Integer
End Type
 
Public Sub AddPerson ( ByRef person As Person )
End Sub


<soap:Envelope
   xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/"
   soap:encodingStyle='http://schemas.xmlsoap.org/soap/encoding/'>
  <soap:Body>
    <p:AddPerson
       xmlns:p='urn:example-org:people'>
      <person>
        <name>
          <givenName>Martin</givenName>
          <familyName>Gudgin</familyName>
        </name>
        <age>33</age>
        <height>64</height>
      </person>
    </p:AddPerson>
  </soap:Body>
</soap:Envelope>

Java and VB definitions for a method call taking a structured type representing a Person as a single parameter, followed by the SOAP message representing a request to execute such a method.

Serialization of Structured Data with Multiple References

In cases when a field in a data structure is referred to in several places in that data structure (for example, in a doubly linked list), then the field is serialized as an independent element, an immediate child element of Body, and must have an id attribute of type ID. Such elements are called multireference accessors. They provide access to the data in the field from multiple locations in the message. Each reference to the field in the data structure is serialized as an empty element with an href attribute of type IDREF, where the value of the attribute contains the identifier specified in the id attribute on the multireference accessor preceded by a fragment identifier, #.

Example Multi-reference accessors

package example.org.People;
// Java class definition
class PersonName
{
String givenName;
String familyName;
}
class Person
{
PersonName name;
float age;
short height;
 
public static boolean Compare ( Person p1, Person p2 );
}
 
 
<soap:Envelope
   xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/"
   soap:encodingStyle='http://schemas.xmlsoap.org/soap/encoding/'>
<soap:Body xmlns:p='urn:example-org:people'>
  <p:Compare>
   <p1 href='#pid1' />
   <p2 href='#pid1' />
  </p:Compare>
  <p:Person id='pid1' >
  <name>
    <givenName>Martin</givenName>
    <familyName>Gudgin</familyName>
   </name>
   <age>33</age>
   <height>64</height>
  </p:Person>
</soap:Body>
</soap:Envelope>

Java definition for a method call taking two parameters both of type Person, followed by the SOAP message representing a request to execute such a method where both parameters refer to the same instance of Person.

Dealing with null References in Complex Data Structures

In certain cases when reference types exist in a programmatic data structure there is a need to represent a null reference. Such references are modeled in SOAP messages using the nil attribute in the http://www.w3.org/2001/XMLSchema-instance namespace. Setting the value of the attribute to 1 indicates that the accessor on which it appears represents a null reference.

Example Null references

package example.org.Nodes;
 
// Java class definition
class Node
{
 String val;
 Node next;
 
 public static long ListLength ( Node node );
}
 
 
<soap:Envelope
   xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/"
   soap:encodingStyle='http://schemas.xmlsoap.org/soap/encoding/'>
   <next xmlns:xsi='http://www.w3.org/2001/XMLSchema'
      -instance xsi:nil='1' />
  <soap:Body >
  <n:ListLength xmlns:n='urn:example-org:nodes'>
   <node>
    <val>New York</val>
     <next>
      <val>Paris</val>
       <next>
        <val>London</val>
        </next>
     </next>
   </node>
  </n:ListLength>
 </soap:Body>
</soap:Envelope>

Java class definition for a simple linked list. The end of the list is indicated by a null reference in the next field. A list of three items is passed in the request message.

Serializing Dynamically Typed Data

SOAP provides for serialization of dynamically typed data; that is, data typed at run-time, through a polymorphic accessor. Such accessors look like normal accessors apart from the presence of a type in the http://www.w3.org/2001/XMLSchema-instance namespace. This attribute indicates the type the accessor actually holds. The value of this attribute may well vary from message to message.

Example dynamically typed data

package example.org.Poly;
 
// Java definitions
class Poly
{
public static void Execute ( Object param );
}
 
' Visual Basic Definition
Public Sub Execute ( param As Variant )
End Sub
 
 
<soap:Envelope
   xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/"
   soap:encodingStyle='http://schemas.xmlsoap.org/soap/encoding/'>
<soap:Body>
  <p:Execute
  xmlns:p='urn:example-org:poly'
  xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance'
  xmlns:xsd='http://www.w3.org/2001/XMLSchema' >
   <param xsi:type='xsd:long' >2000</param>
  </p:Execute>
</soap:Body>
</soap:Envelope>
 
<soap:Envelope
   xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/"
   soap:encodingStyle='http://schemas.xmlsoap.org/soap/encoding/'>
<soap:Body>
  <p:Execute
  xmlns:p='urn:example-org:poly'
  xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance'
  xmlns:pre='urn:example-org:people'>
   <param xsi:type='pre:Person' >
    <name>
     <givenName>Martin</givenName>
     <familyName>Gudgin</familyName>
    </name>
    <age>33</age>
    <height>64</height>
   </param>
  </p:Execute>
</soap:Body>
</soap:Envelope>

Java and VB definitions for a method call taking a dynamically typed parameter followed by several SOAP messages representing a request to execute such a method. The first SOAP message passes a parameter of type long whereas the second passes a parameter of type Person.

Arrays

SOAP provides comprehensive array support. Single and multidimensional arrays are supported, along with sparse and jagged arrays and partial transmission. Arrays in SOAP are always of type Array in the http://schemas.xmlsoap.org/soap/encoding/ namespace, or a type derived by restriction from that type. If they are of the Array type, they are encoded using an Array element also in the http://schemas.xmlsoap.org/soap/encoding/ namespace. If they are of a derived type, then any element name may be used. In either case, an arrayType attribute in the http://schemas.xmlsoap.org/soap/encoding/ namespace is mandatory. The type of this attribute is string, but it in fact indicates the type of the array along with dimension information. Each dimension appears in square brackets after the QName for the type, separated by commas. Each array item is serialized as an element. The name of this element can be the type name or some arbitrary name.

Example Simple array example

<soap:Envelope
   xmlns:soap='http://schemas.xmlsoap.org/soap/envelope/'
   soap:encodingStyle='http://schemas.xmlsoap.org/soap/encoding/'>
<soap:Body>
  <m:MethodResponse
    xmlns:m='urn:example-org:someuri' >
   <enc:Array
   xmlns:enc='http://schemas.xmlsoap.org/soap/encoding/'
    xmlns:xsd='http://www.w3.org/2001/XMLSchema'
      enc:arrayType='xsd:long[5]' >
      <enc:long>2</enc:long>
      <enc:long>3</enc:long>
      <enc:long>5</enc:long>
      <enc:long>7</enc:long>
      <enc:long>9</enc:long>
   </enc:Array>
  </m:MethodResponse>
</soap:Body>
</soap:Envelope>

A response message containing an array of five long values. Note the value of the arrayType attribute indicating the size of the array.

Multidimensional Arrays

Multidimensional arrays can be encoded by specifying multiple dimensions separated by commas inside the square brackets in the arrayType attribute. Any number of dimensions may be specified.

Example Multidimensional array example

<soap:Envelope
  xmlns:soap='http://schemas.xmlsoap.org/soap/envelope/'
  soap:encodingStyle='http://schemas.xmlsoap.org/soap/encoding/'>
<soap:Body>
  <m:Method
    xmlns:m='urn:example-org:some-uri' >
   <enc:Array
   xmlns:enc='http://schemas.xmlsoap.org/soap/encoding/'
      xmlns:xsd='http://www.w3.org/2001/XMLSchema'
      enc:arrayType='xsd:string[2,3]' >
    <item>row 1 column 1</item>
    <item>row 1 column 2</item>
    <item>row 1 column 3</item>
    <item>row 2 column 1</item>
    <item>row 2 column 2</item>
    <item>row 2 column 3</item>
   </enc:Array>
  </m:Method>
</soap:Body>
</soap:Envelope>

A request message containing a two-dimensional array of strings. Note the value of the arrayType attribute indicating the type and dimensions of the array.

Partial Transmission of Arrays

In certain scenarios an array of a certain size may need to be transmitted, but only a subset of the items needs to be sent. For such arrays the array element is annotated with an offset attribute in the http://schemas.xmlsoap.org/soap/encoding/ namespace. The value of the offset attribute indicates the zero-based offset of the first element. The value appears in square brackets. Listed items are assumed to appear at contiguous locations in the array. Items may be omitted from the end of the array.

Example Partial array tranmission

<soap:Envelope
   xmlns:soap='http://schemas.xmlsoap.org/soap/envelope/'
   soap:encodingStyle='http://schemas.xmlsoap.org/soap/encoding/'>
<soap:Body>
  <m:Method xmlns:m='urn:example-org:someuri' >
   <enc:Array
   xmlns:enc='http://schemas.xmlsoap.org/soap/encoding/'
    xmlns:xsd='http://www.w3.org/2001/XMLSchema'
      enc:arrayType='xsd:string[9]'
      enc:offset='[2]'>
    <item>Earth</item>
    <item>Mars</item>
    <item>Jupiter</item>
   </enc:Array>
  </m:Method>
</soap:Body>
</soap:Envelope>

A request message that transmits the third, fourth, and fifth items in a nine-item array

Sparse Arrays

Sparse arrays, those in which noncontiguous items need to be transmitted, are also supported. Each serialized array item is annotated with a position attribute in the http://schemas.xmlsoap.org/soap/encoding/ namespace. The value of the position attribute is a zero-based offset of the position of the item in the array, enclosed in square brackets.

Example Sparse arrays

<soap:Envelope
   xmlns:soap='http://schemas.xmlsoap.org/soap/envelope/'
   soap:encodingStyle='http://schemas.xmlsoap.org/soap/encoding/'>
<soap:Body>
  <m:Method xmlns:m='urn:example-org:someuri' >
   <enc:Array
   xmlns:enc='http://schemas.xmlsoap.org/soap/encoding/'
   xmlns:xsd='http://www.w3.org/2001/XMLSchema'
      enc:arrayType='xsd:string[9]' >
    <item enc:position='[1]'>Venus</item>
    <item enc:position='[3]'>Mars</item>
    <item enc:position='[7]'>Neptune</item>
   </enc:Array>
  </m:Method>
</soap:Body>
</soap:Envelope>

A request message that transmits the second, fourth, and eighth items in a nine-item array.

Jagged arrays

SOAP supports jagged arrays, also known as arrays of arrays. The arrayType attribute contains a type that includes empty square brackets, as many as necessary to indicate how many dimensions each array has, followed by the dimensions of the array of arrays in square brackets as normal. The inner array elements are also annotated with the appropriate arrayType attribute.

Examples Jagged arrays with single-reference accessors

<soap:Envelope
   xmlns:soap='http://schemas.xmlsoap.org/soap/envelope/'
   soap:encodingStyle='http://schemas.xmlsoap.org/soap/encoding/'>
<soap:Body>
   <enc:Array
   xmlns:enc='http://schemas.xmlsoap.org/soap/encoding/'
     xmlns:xsd='http://www.w3.org/2001/XMLSchema'
      enc:arrayType='xsd:string[][2]' >
    <enc:Array enc:arrayType='xsd:string[2]'>
     <item>Mercury</item>
     <item>Venus</item>
    </enc:Array>
    <enc:Array enc:arrayType='xsd:string[6]'>
     <item>Mars</item>
     <item>Jupiter</item>
     <item>Saturn</item>
     <item>Uranus</item>
     <item>Neptune</item>
     <item>Pluto</item>
    </enc:Array>
   </enc:Array>
  </m:Method>
</soap:Body>
</soap:Envelope>

A request message that transmits an array of arrays of strings. Each array is encoded using a single-reference accessor.

Jagged arrays with multireference accessors

<soap:Envelope
   xmlns:soap='http://schemas.xmlsoap.org/soap/envelope/'
   soap:encodingStyle='http://schemas.xmlsoap.org/soap/encoding/'>
<soap:Body
  xmlns:enc='http://schemas.xmlsoap.org/soap/encoding/'
  xmlns:xsd='http://www.w3.org/2001/XMLSchema' >
  <m:Method xmlns:m='urn:some-uri' >
   <enc:Array enc:arrayType='xsd:string[][2]' >
    <item href='#id1' />
    <item href='#id2' />
   </enc:Array>
  </m:Method>
  <enc:Array id='id1' enc:arrayType='xsd:string[2]'>
   <item>Mercury</item>
   <item>Venus</item>
  </enc:Array>
  <enc:Array id='id2'
        enc:arrayType='xsd:string[6]'>
   <item>Mars</item>
   <item>Jupiter</item>
   <item>Saturn</item>
   <item>Uranus</item>
   <item>Neptune</item>
   <item>Pluto</item>
  </enc:Array>
</soap:Body>
</soap:Envelope>

A request message that transmits an array of arrays of strings. Each array is encoded using a multireference accessor.



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