Spicing Up Your Web Services with XSLT
February 10, 2009
In the first thirteen parts of this series, Ive introduced you to some of the many features available within the IBM Data Studio integrated development environment (IDE) thats available for use with the IBM data servers. Specifically, Ive shown you how to set up and use database connection objects, how to generate an overview diagram of your database architecture, how to build OLE DB functions that can be used to easily integrate data from external data sources that have an OLE DB provider, how to create an SQL statement using either the SQL Builder or the SQL Editor in IBM Data Studio, and how to take an SQL statement and quickly turn it into a stored procedure. Ive also shown you how to wrap both an SQL statement and a stored procedure as a Web service. Most recently, I showed you how to test your Web service using the Web Services Explorer thats integrated into IBM Data Studio or through a Web browser using the Web Services Explorer.
In this article, Im going to show you how to apply Extensible Stylesheet Language Transformations (XSLT) to your Web services. The output of the Web service you created earlier in this series was XML. For example, when executing the FEMALEPERSONNEL Web service from a representational state transfer (REST) interface in a Web browser, the output looked like this:
As you can see, business logic was executed through a URL-based invocation of a Web service, but the output is just plain XML. XML isnt a presentation language; rather its a data semantic language. Using XSLT, you can transform XML into all sorts of different output formats. For example, perhaps you want the output format of this data to be a Really Simple Syndication (RSS) feed, hypertext markup language (HTML)Braille text, or practically anything. In this article, were going to create a new Web service and transform the output such that it displays as a nicely formatted HTML Web page using XSLT.
Getting ready for this article
I assume in this article that you completed all the steps in Part 12: Testing your Web Service using the Web Services Explorer. From there, all you need to do in order to follow the steps in this article is ensure that the application server you defined and deployed your Web services to in Part 11 is started and running such that the Servers tab looks like this:
I also assume that your Data Project Explorer view looks similar to this:
Finally, I assume you have access to the Mozilla Firefox Internet browser.
What is Extensible Stylesheet Language Transformation?
Extensible Stylesheet Language Transformation (XSLT) is an XML-based technology used to transform the structure of XML documents. In fact, the term stylesheet is part of the XSLT name because of its ability to change the content of the XML document think cascading stylesheets (CSS) with HTML and you get the point.
XSLT is designed for use as part of the Extensible Stylesheet Language (XSL). XML Path Language (XPath) is an XSL sub-language that is designed to be used with XSLT. It is used for identifying or addressing parts of a source XML document. Every node within an XML document can be uniquely identified and located using the syntax defined for XPath.
One of the coolest things about XSLT is the fact that you can shape your XML into almost any output you want. For example, perhaps an XML response envelope from your Web service returns some XML data. Using XSLT, you could transform this data for output on a Web page in HTML. Another example of the use of XSLT is to screen-scrape an XHTML page and distribute that content to an RSS or Atom Syndication Format (ASF) feed.
When you configure XSL transformations for your Web services using a REST GET invocation, the message flow for a Web service operation follows these steps:
1. A client application sends an HTTP GET (TEXT/XML) message that accesses an operation in a Web service. The message is tagged according to a custom XML schema.
2. The message is transformed so that it is tagged according to the default XML schema.
3. The Web service receives the message and passes to the database the SQL statement or stored procedure call that is in the operation.
4. The Web service receives the response from the database and packages the response in an XML message that is tagged according to the default XML schema for the operation.
5. The message is transformed so that it is either in XML (and is tagged according to the custom XML schema), or in a non-XML format, such as HTML or plain text.
6. The Web service sends the response to the client application.
This process is shown in the following figure:
As you can see in the previous figure, IBM Data Web Services gives you options to apply XSLT to incoming XML messages and outgoing XML messages. (In this article, the example shows you how to apply an XSLT transformation to the outbound XML message.)
In this series, Im illustrating how to build Web services using a bottom-up approach where the business logic (in this case, an SQL statement or a stored procedure) has been built and the intent is to expose this logic as a Web service. In a top-down approach, you start with a service specification (for example, a WSDL file) and then implement the underlying code to match that specification. By applying XSLT to input service messages, in many cases, you can map the service format of the bottom-up style messages to the top-down design. This alleviates some of the top-down service format requirements while allowing you to develop bottom-up Web services; of course, this is all done with XSLT, only in this case on an input message.
Depending on how you build your Web service (for example, a SOAP service), this process varies somewhat; refer to the IBM Data Studio Information Center or integrated help for more information.
Here are some excerpts of the XSLT document that you will be using in this example:
<?xml version="1.0" encoding="UTF-8"?> <xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0" xmlns:xalan="http://xml.apache.org/xslt" xmlns:tns="http://www.w3.org/1999/xhtml"> <xsl:output method="html" version="1" encoding="UTF-8" omit-xml-declaration="yes" standalone="yes" indent="yes" />
As you can see, the excerpts of this XSLT document show you that XSLT documents are indeed written in XML. You can also see that this XSLT document will transform the XML into HTML, as indicated by the xsl:output element where the method attribute is set to "html". Perhaps you are transforming an XML document into an RSS feed for which you would define an output clause similar to this: <xsl:output method="xml" encoding="UTF-8" media-type="application/rss+xml"/>
<xsl:template match="/"> <html><body> <h2>Employee Record</h2> <xsl:element name="form"> <xsl:attribute name="action">http://localhost:8080/DemoMyWebService/rest/Demo/updateEmp</xsl:attribute> <xsl:attribute name="method">POST</xsl:attribute> <table border="1"> <tr> <th rowspan="12"> <img> <xsl:attribute name="src">data:image/gif;base64,<xsl:value-of select="//row/PICTURE/text()"/></xsl:attribute> </img> </th>
As you further inspect the sample XSLT document used in this article, you can see how the previous code shapes the input XML data from our Web service into an HTML output. The following code tells an XSLT processor how to take the <EMPNO> XML element and transform it into HTML.
<td>EMPNO:</td> <td> <xsl:value-of select="//row/EMPNO/text()"/> <xsl:element name="input"> <xsl:attribute name="value"> <xsl:value-of select="//row/EMPNO/text()"/></xsl:attribute> <xsl:attribute name="type">hidden</xsl:attribute> <xsl:attribute name="name">empno</xsl:attribute> </xsl:element> </td>
You can see how XML technologies are used with XSLT by noticing the XPath notation of the textual value of the <EMPNO>: //row/EMPNO/text().
Of course, all of the transformed data will be displayed in table format because all of the transformation logic resides within the following tags:
Its outside the scope of this article to delve into the depths of transformations with XML data; however, the W3 Schools Web site has some terrific resources on this (and many other) XML-related technologies. Check out http://www.w3schools.com/xsl/ for more information.