XML Hacks

Create a DTD from an Instance

If you need a DTD in a hurry, create it from an XML instance using Trang, Relaxer, DTDGenerator, or xmlspy.

Several free Java tools are available that can generate a DTD based on an XML instance or instances. James Clark’s Trang (http://www.thaiopensource.com/relaxng/trang.html) can, among other things, convert an XML document to a DTD, as can Relaxer (http://www.relaxer.org). Michael Kay’s DTDGenerator (http://saxon.sourceforge.net/dtdgen.html), once part of the Saxon project, consists of a single Java class that is dedicated to XML-to-DTD conversion. This hack walks you through the steps to generate a DTD from a simple instance using each of these tools.

Trang

You can download the current Trang JAR (trang.jar) from http://www.thaiopensource.com/download/, then place the JAR in the working directory. The Trang archive comes with a manual (trang-manual.html) that provides details on how to use Trang. I will cover only what is needed to create a DTD in this section. (If you need help with Java, refer to [Hack #10] .)

To create a DTD from time.xml, run this command:

java -jar trang.jar -I xml -O dtd time.xml generated.dtd

The -I switch indicates the type of input (XML), and -O indicates the type of output (DTD). time.xml is the input file and generated.dtd is the output file. You could simplify this command by skipping the -I and -O options, which will produce the same result:

java -jar trang.jar time.xml generated.dtd

The file generated.dtd looks like ...