XML Interview Questions and Answers

How do applications process documents that use XML namespaces?
Applications process documents that use XML namespaces in almost exactly the same way they process documents that don't use XML namespaces. For example, if a namespace-unaware application adds a new sales order to a database when it encounters a SalesOrder element, the equivalent namespace-aware application does the same. The only difference is that the namespace-aware application:
* Might need to check for xmlns attributes and parse qualified names. Whether it does this depends on whether such processing is already done by lower-level software, such as a namespace-aware DOM implementation.
* Uses universal (two-part) names instead of local (one-part) names. For example, the namespace-aware application might add a new sales order in response to an {http://www.google.com/ito/sales}SalesOrder element instead of a SalesOrder element.

How do I use XML namespaces with SAX 1.0?
The easiest way to use XML namespaces with SAX 1.0 is to use John Cowan's Namespace SAX Filter (see http://www.ccil.org/~cowan/XML). This is a SAX filter that keeps track of XML namespace declarations, parses qualified names, and returns element type and attribute names as universal names in the form:
URI^local-name
For example:
http://www.google.com/ito/sales^SalesOrder
Your application can then base its processing on these longer names. For example, the code:
public void startElement(String elementName, AttributeList attrs)
throws SAXException
{
...
if (elementName.equals("SalesOrder"))
{
// Add new database record.
}
...
}
might become:
public void startElement(String elementName, AttributeList attrs)
throws SAXException
{
...
if (elementName.equals("http://www.google.com/sales^SalesOrder"))
{
// Add new database record.
}
...
}
or:
public void startElement(String elementName, AttributeList attrs)
throws SAXException
{
...
// getURI() and getLocalName() are utility functions
// to parse universal names.
if (getURI(elementName).equals("http://www.foo.com/ito/sales"))
{
if (getLocalName(elementName).equals("SalesOrder"))
{
// Add new database record.
}
}
...
}
If you do not want to use the Namespace SAX Filter, then you will need to do the following in addition to identifying element types and attributes by their universal names:
* In startElement, scan the attributes for XML namespace declarations before doing any other processing. You will need to maintain a table of current prefix-to-URI mappings (including a null prefix for the default XML namespace).
* In startElement and endElement, check whether the element type name includes a prefix. If so, use your mappings to map this prefix to a URI. Depending on how your software works, you might also check if the local part of the qualified name includes any colons, which are illegal.
* In startElement, check whether attribute names include a prefix. If so, process as in the previous point.

How do I use XML namespaces with SAX 2.0?
SAX 2.0 primarily supports XML namespaces through the following methods: * startElement and endElement in the ContentHandler interface return namespace names (URIs) and local names as well as qualified names. * getValue, getType, and getIndex in the Attributes interface can retrieve attribute information by namespace name (URI) and local name as well as by qualified name.

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