XMLNSC empty elements and null values

Empty elements and null values occur frequently in XML documents.

A robust message flow must be able to recognize and handle empty elements and null values. Similarly, elements in a message tree might have a NULL value, an empty value, or no value at all. This topic explains the parsing and writing of these values by the XMLNSC domain. For details on processing null values in Graphical Data Maps and ESQL, see Handling null values.

Parsing

Description XML input parsed by XMLNSC Value of 'element' in message tree
Empty element value <element/> Empty string
Empty element value <element></element> Empty string
Folder with child elements <element><childElement/></element> No value
Nil element value <element xsi:nil="true"/> No value and a child xsi:nil attribute with the value 'true'.

Both forms of an empty element result in the same value in the message tree.

Writing

Description Value of 'element' in message tree XML output from XMLNSC parser
Empty element value Empty string <element/>
Null element value NULL <element/>
Folder with child elements No value <element><childElement/></element>
Nil element value Empty string, NULL, or Folder <element xsi:nil="true"/>

Note that the XMLNSC parser produces only xsi:nil attributes that are already in the message tree. It does not automatically produce xsi:nil attributes for all message tree elements that have a NULL value and are 'nillable'.

Empty elements

An empty element can take two forms in an XML document: 
- <element/>
- <element></element>

The XMLNSC parser treats both forms in the same way. The element is added to the message tree with a value of "" (the empty string).

When a message tree is produced by the XMLNSC parser, it always uses the first form for elements that have a value of "" (the empty string) in the input XML message.

Elements with an xsi:nil attribute

If validation is enabled for the flow, the XMLNSC parser performs the following validations:

  • The 'nillable' property of an element definition in the XML schema is set to 'true'.

    If the element in the document has an xsi:nil attribute with the value 'true', the element must not have a value, or contain any child elements.

  • The 'nillable' property of an element definition in the XML schema is set to 'false'.

    The element in the input document must not have an xsi:nil attribute.

If an element in the input document has an xsi:nil attribute, the XMLNSC parser creates an element in the message tree with no value, and a child xsi:nil attribute with the value 'true' or 'false'.

When a message tree is written to a bit stream by the XMLNSC parser, if the value of the element is empty, NULL, or no value, and the element has no child elements, the element is written as <element/>. If the element has an xsi:nil attribute, it is written exactly like any other attribute.

Note that the XMLNSC parser produces only xsi:nil attributes that are already in the message tree. It does not automatically produce xsi:nil attributes for all message tree elements that have a NULL value and are 'nillable'.