David Pollak was right about XML and JSON

David Pollak was right about XML and JSON, but perhaps in a different way. XML cannot be converted to (clean) JSON.

Suppose we have a shopping cart in XML which we want to convert to JSON:

One representation in JSON would be (cart could be omitted):

We convert a list of nodes with the same name to a JSON array, xml2json-xslt does this for example. What happens if we only have one item in our shopping cart?

Then our converter cannot detect that items is a list and will convert the XML to:

which is semantically something completely different. And very unpleasent for the receiver of our JSON code, because sometimes he gets an array and sometimes an object.

One way to solve the problem is to annotate the XML (looks ugly but works):

and adding an additional condition to the XSLT

or namespacing (doesn’t work yet, we get lots of namespaces and misuse XML namespaces) ?

So David was right, I’m not sure he new why ;-)

Thanks for listening.

David Pollak (from Lift): “There’s no way to convert from XML to JSON because XML contains sequences not expressible in JSON”

Hmm[*]. Not sure if this is true (with CDATA, #Text and @attributes handled in some converters). For me the problem is more that there are too many ways to convert XML to JSON. For exampe the Badgerfish convention. Or the the Google and Yahoo versions. Or the XML.com way. And the Parker convention.

But the ways in Javascript to convert XML to JSON are either slow, very basic, use XSLT, use nasty Regex or cannot create simple JSON which feels JSON like.

* Note to self: Should start using Twitter for this [**].
** Did start Twitter

Update: Any ideas for a good XML to JSON conversion which feels JSON like (no need to be bidirectional)?

Update 2: I currently use XSLT with nice results, Safari doesn’t work yet and neither does Chrome. More to come.

JsonBuilder for Scala, REST and Jersey

How to generate Json for REST? If you’re suspicious of automatic generation like me?* I’ve created a markup builder which can be used with Json and made a look into the future in my post “The best Markup Builder I could build in Java”:

“Closures in Java 7 will make it much easier to write a MarkupBuilder. The Action and Loop inner classes will go away and the code will be more Groovy like.”

* For a discussion on why you might not want to use JAXB and XStream see the comments on “REST: Lean JSON and XML from the same code”

Let’s have a look at this quote again. I didn’t use Groovy but with some interest in Scala – because it might be more maintainable than Java after 5-years project life time as it is more concise but not riddled with the Ruby-problems or unreadable for most as Haskell is – I looked at my JsonBuilder code and tried it with Scala.

I went back to the “Experiments to nicely generation JSON” post and did adapt it to Scala.

The generating code looks the same as in Java, with the possibility to include functions for generating nodes. My Java code did need anonymous inner classes to achieve the same – with more noise and less power. The methods are shorter and the focus lies more on generating the JSON data, not the method boilerplate code. Next will be WebBeans or Guice integration.

Ive struggled with some Scala constructs, the JsonAdapter which implements MessageBodyWriter was a little hard to write, especially:

But everything works now and I can move on to adapt and learn how to do it better in Scala.

The migration wasn’t that hard, some problems exists and my Scala code looks more like Java than Scala, but it’s a beginning. One of my fears is to translate between Seqs, Lists, Arrays and Java Lists and Iterators all the time when interfacing with Java libraries. Not sure yet how to fix that, perhaps with some wrappers. Scalaz might help too. We’ll see in my future adventures into Scala.

Thanks for listening.