8.1 Building a DOM from XML

If you’re like me, the first thing you may be trying to do is build a Document Object Model (DOM) tree from some kind of XML input. Assuming you’ve got the XML in a String the following code will build an XML Document:

XML.SAXParser defaultParserClass processDocumentString: theXMLString 
	beforeScanDo: [ :p | p validate: false].

Though the code above appears as though it should be easy to use, there’s some hidden features you should know about. First, theXMLString can not contain any null bytes. Depending on where your XML comes from it may have a NULL byte at the end (like mine did). Many languages implement strings as an array of bytes (usually printable ones) ending with a null (a character with integer value 0). In my case, the XML was coming from a remote client written in C using middleware to send the message to my server. Since the middleware doesn’t assume to know anything about the message it received, it’s received into a String, null-byte and all. To remove it I used:

XML.SAXParser defaultParserClass
    processDocumentString: (aString copyWithout: 0 asCharacter)
    beforeScanDo: [ :p | p validate: false].

Starting out, I didn’t know much about the value of DTDs either (Document Type Definitions), so I wasn’t using them (more on why you should later). What you need to know is XML comes in two flavors, (three if you include broken as a flavor) well-formed and valid.

Well-formed XML is simply XML following the basic rules, like only one top-level (the document’s root), no overlapping tags, and a few other contraints. Valid XML means not only is the XML well-formed, but it’s also compliant with some kind of rule base about which elements are allowed to follow which other ones, whether or not attributes are permitted and what their values and defaults should be, etc.

There’s no way to get around well-formedness. Most XML tools complain vociferously about missing or open tags. What you may not have lying around, though, is a DTD describing how the XML should be assembled. If you need to skip validation for any reason you must include the selector:

beforeScanDo: [ :p | p validate: false].

Now that you have your XML document, you probably want to access its contents (why else would you want one, right?). Let’s take the following (brief) XML as an example:

<porder porder_num="10351">
  <porder_head>
    <order_date>01/04/2000</order_date>
  </porder_head>
  <porder_line>
    <part>widget</part>
    <quantity>1.0000</quantity>
  </porder_line>
  <porder_line>
    <part>doodad</part>
    <quantity>2.0000</quantity>
  </porder_line>
</porder>

The first thing you probably want to know is how to access the different tags, and more specifically, how to access the contents of those tags. First, by way of providing a roadmap to the elements I’ll show you the Smalltalk code for getting different pieces of the document, assuming the variable you’ve assigned the document to is named doc. I’ll also create instance variables for the various elements as I go along:

I’ve deliberately left-out accessing porder’s attribute because accessing them is different from accessing other nodes. You can get an OrderedCollection of attributes using:

attributes := doc root attributes.

but the ordered collection isn’t really useful. To access any single attribute you’d need to look for it in the collection:

porderNum := (attributes detect: [ :each | each key type = 'porder_num' ]) value.

But that’s not a whole lot of fun, especially if there’s a lot you need to get, and if there’s any possibility the attribute may not exist. Then you have to do the whole detect:ifNone: thing, and boy, does that make the code readable! What I did instead was create a method in my objects’ abstract:

dictionaryForAttributes: aCollection
    ^Dictionary withAll: (aCollection
	collect: [ :each | each key type -> each value ])

Now what you have is an incrementally more useful method for getting attributes:

attributes := self dictionaryForAttributes: doc root attributes.
porderNum := attributes at: 'porder_num'.

At first this appears like more code, and for a single attribute it probably is. But if an element includes more than one attribute the payoff is fairly decent. Of course, you still need to handle the absence of an attribute in the dictionary but I think it reads a little better using a Dictionary than an OrderedCollection:

porderNum := attributes at: 'porder_num' ifAbsent: [].