Why choose an XSL-transformation?

Question

For a current project the decision has to be made whether to use XML and an XSL-transformation to produce HTML or to directly use HTML-templates.

I'd be interested in arguments for or against the XSL-approach. I understand that in cases where you have to support many different layouts, an XSL-solution has a lot of advantages, but why would you choose it in those cases where you only have to support one target layout?

Edit: We're talking about Java here.

Answer 1

Depending on your application, having an XML layer that is then transformed to XHTML via XSLT also meens, that you can write easy WebServices to the XML layer - allowing your customers to consume your sites data...

Having the XML sent to the browser with a transformation link (forgot the exact syntax...) also meens less bandwith needed, as the XSLT file will stay the same and you only need to pass the raw XML it is built from - sort of like using an external CSS style sheet instead of adding the style attributes to your markup ;)

Answer 2

I've used XML & XSLT in a previous project, financial web sites, and it worked well for us, but:

1. We had multiple customers, which varied the number of outputs we had. We could replace the XSLT stylesheet and this made changes to the site easier to manage for the developers
2. We had a specialist web editor on the team. We gave them example XML & they could edit the stylesheets directly
3. If there were ever any wording changes that needed to go onto the website yesterday ( it was a bank, this happened surprisingly often), we could just deploy the new XSLT without redeploying the entire site.
4. Multiple different output formats were needed. We used FOP for transformation to PDF, which is based upon the same sort of technology, so wasn't too hard for us to understand :-)

The main reason I see for using XSLT is if you have multiple sites all based upon the same XML, but requiring different HTML output.

Answer 3

XSLT has the advantage of being able to also produce output in other document types (i.e. pdf) and pdf output is very likely nowadays. XML/XSLT does also separate data from the view.

Answer 4

When we have done XSLT in the past, it was to allow the ability to extend our product. The output remained the same, only the presentation layer needed to change. This allowed us a lot of flexibility when we had clients that wanted to "customize" their UI, since all we needed to do was replace the XSLT file. If you foresee needing to make a lot of those kinds of changes, XSLT might be your answer.

However, as stated above, the XSLT syntax and functional programming mentality can make it difficult to effectively produce templates. We found that we liked to stick to the tricks that we learned and when we had client requests that fell outside of what we already knew, no one wanted to volunteer for the ticket. Usually someone eventually figured out how to do the task and our "bag of tricks" got larger, but it was often very cumbersome to figure out new things.

If you don't foresee change the UI ever, or at least not much, XSLT may not be worth the extra effort.

Answer 5

I created an XML/XSLT-driven UI for an enterprise product about 5 years ago. We're still using it, and I can now look back on my experience and see many pros and cons:

Pros:

• XSL is a powerful declarative language, useful & fun for experienced developers, and transforms can do pretty amazing things in a few lines of code
• XSL is designed for use with XML, so if your data is already XML then it makes a lot of sense
• Separation of concerns (rendering vs. data) is better than many template languages
• XSL-based rendering can be easily "subclassed". By that I mean: let's say you have data class A with associated template A.xslt. For class B derived from A, you can easily create B.xslt with only the small differences, and include A.xslt for inherited behaviors. This makes it less succeptible to breaking due to changes in A.xslt.
• The above point also gives you the power to do overrides. For class A with associated A.xslt, we can easily switch the associated template to A-custom.xslt, which is a few small changes plus inheritance of A.xslt. We can do this on the fly in the field and again, the benefit is that A-custom.xslt is only a few lines, not an entire modified copy of the original A.xslt. The small footprint means it's more likely to work with multiple versions of A.xslt.
• In .NET 2.0, XSLT is compiled and becomes very fast. There may be similar tech for Java. (Most template languages do this now too.)
• In .NET, it's possible to create an "Object XPath Navigator", which lets you transform your data objects without having to convert them to an XML object. Again there may be similar tech in Java
• XSLT is smart about HTML & handles escaping, white space issues, etc. well

Cons:

• XSL is a powerful declarative language, confusing to newer programmers - and fewer people know XSLT well
• XSL is verbose. XML is often verbose too.
• XSL transforms are probably slower than "native" templates. Even when compiled there's still more state overhead to XSL than most template languages
• It's hard to pass parameters to XSLs, you have to either send them in line with your data (forcing you to create extra XML) or via system-specific methods (which may also involve constructing XML data)
• If you don't have an ObjectXPathNavigator or equivalent, you'll incur significant overhead when turning your data objects into XML for transformation
• Depending on the capabilities of your transformer, you may also incur buffering overhead as you transform into a string buffer and then send that string to the output device
• The more advanced your XSLT usage, the less likely it is that your tools will support you (specifically as you start to use includes or faster ways to pass XML data in)

I'll try to update as I think of more issues. I think that looking back now, my verdict would be to stick with a common template language. What were once big issues when I selected XML/XSLT have now been addressed by newer and more mature revisions of the major template engines. We do still benefit greatly from the ability to inherit .xslt files, which is something most template engines don't do well. But in the end the value of having lots of developers providing examples is far greater (compare ASP.NET answers vs XSLT answers on StackOverflow, for instance.)

Hope that helps!

Answer 6

I see how the XSL approach can be handy if your data is already XML.
But usually it isn't. It's somewhere in a database, needs to be generated on the spot or comes from some service.
Creating XML from this source to then be able to create HTML from that XML is useless in my opinion. I would stick with (X)Html templates.