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# Saturday, 16 February 2008

Hello again!

To see first part about jxom please read.

I'm back with jxom (Java xml object model). I've finally managed to create an xslt that generates java code from jxom document.

Will you ask why it took as long as a week to produce it?

There are two answers:
1. My poor talents.
2. I've virtually created two implementations.

My first approach was to directly generate java text from xml. I was a truly believer that this is the way. I've screwed things up on that way, as when you're starting to deal with indentations, formatting and reformatting of text you're generating you will see things are not that simple. Well, it was a naive approach.

I could finish it, however at some point I've realized that its complexity is not composable from complexity of its  parts, but increases more and more. This is not permissible for a such simple task. Approach is bad. Point.

An alternative I've devised is simple and in fact more natural than naive approach. This is a two stage generation:
  a) generate sequence of tokens - serializer;
  b) generate and then print a sequence of lines - streamer.

Tokens (item()*) are either control words (xs:QName), or literals (xs:string).

I've defined following control tokens:

Token Description
t:indent indents following content.
t:unindent unindents following content.
t:line-indent resets indentation for one line.
t:new-line new line token.
t:terminator separates token sequences.
t:code marks line as code (default line type).
t:doc marks line as documentation comment.
t:begin-doc marks line as begin of documentation comment.
t:end-doc marks line as end of documentation comment.
t:comment marks line as comment.

Thus an input for the streamer looks like:

<xsl:sequence select="'public'"/>
<xsl:sequence select="' '"/>
<xsl:sequence select="'class'"/>
<xsl:sequence select="' '"/>
<xsl:sequence select="'A'"/>
<xsl:sequence select="$t:new-line"/>
<xsl:sequence select="'{'"/>
<xsl:sequence select="$t:new-line"/>
<xsl:sequence select="$t:indent"/>
<xsl:sequence select="'public'"/>
<xsl:sequence select="' '"/>
<xsl:sequence select="'int'"/>
<xsl:sequence select="' '"/>
<xsl:sequence select="'a'"/>
<xsl:sequence select="';'"/>
<xsl:sequence select="$t:unindent"/>
<xsl:sequence select="$t:new-line"/>
<xsl:sequence select="'}'"/>
<xsl:sequence select="$t:new-line"/>

Streamer receives a sequence of tokens and transforms it in a sequence of lines.

One beautiful thing about tokens is that streamer can easily perform line breaks in order to keep page width, and another convenient thing is that code generating tokens should not track indentation level, as it just uses t:indent, t:unindent control tokens to increase and decrease current indentation.

The way the code is built allows mimic any code style. I've followed my favorite one. In future I'll probably add options controlling code style. In my todo list there still are several features I want to implement, such as line breaker to preserve page width, and type qualification optimizer (optional feature) to reduce unnecessary type qualifications.

Current implementation can be found at jxom.zip. It contains:

File Description
java.xsd jxom xml schema.
java-serializer-main.xslt transformation entry point.
java-serializer.xslt generates tokens for top level constructs.
java-serializer-statements.xslt generates tokens for statements.
java-serializer-expressions.xslt generates tokens for expressions.
java-streamer.xslt converts tokens into lines.
DataAdapter.xml sample jxom document.

This was my first experience with xslt 2.0. I feel very pleased with what it can do. The only missed feature is indirect function call (which I do not want to model with dull template matching approach).

Note that in spite that xslt I've built is platform independed I want to point out that I was experimenting with saxon 9. Several times I've relied on efficient tail call implementation (see t:cumulative-integer-sum), which otherwise will lead to xslt stack overflow.

I shall be pleased to see your feedback on the subject.

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