Re: [xml-dev] parser models

[Aleksander Slominski <aslom@cs.indiana.edu>]

Arjun Ray wrote:

> Which brings me to my long-standing beef with SAX: state maintenance.  A
> lot of the time, what we call state dependence is more accurately context
> dependence.  In SAX, if you split up the event handling in the app among
> various classes, you have to mess with setHandler() calls explicitly and
> track the stack at the same time to get this right.  This is a pain.

hi,

i agree completely and seen it before but i have reached different conclusion:
use pull parser API :-)

> In search of a more "natural" idiom, I've been experimenting with a pure
> push API which supports stack-based delivery of events.  It's built around
> two mutually dependent interfaces that the consumer will have to implement
> which look like this (many details omitted):

this looks very similar to what all applications are doing that has to convert
SAX event callbacks into objects - for example deserializers in AXIS ...

> Cheesy example:
>
>  public class HtmlTable implements Element, Content {
>      ...
>      Element newChild( ) {
>          return (Element) new HtmlTr( ) ; // sexier constructors possible
>      }

how do you know that every child of table has to be tr?

>
>      Content endChild( Content child ) {
>          if ( child instanceof HtmlTr ) {
>              // etc
>          }
>          return this ;
>      }
>  }
>
>  public class HtmlTr implements Element, Content {
>      void gi( String name ) {
>          if ( ! "tr".equals( name ) )
>              throw ScreamAndDieException ;
>      }
>      Content content( ) {
>          return (Content) this ;
>      }
>      // etc
>  }
>
> A lot of this is boilerplate code that can also be "hoisted".  For deep or
> deeply recursive structures in the XML, this works very well, I've found.

i think it can be done much easier with xml pull parser without any
special support, for example to create HtmlTable from XML input:

XmlPullParser pp = ...
HtmlTable table = HtmlTable.unmarshal(pp);

and here is implementation:

public class HtmlTable {
  Vector rows = new Vector();
  public statoc HtmlTable unmarshal(XmlPullParser pp) {
     if(pp.getEventType() != pp.START_TAG || !"table".equals(pp.getName())
        thow new ValidationExceptiopn("expected start tag for HTML table"+pp.getpositionDesacripton());

     HtmlTable table = new HtmlTable();
     //parse table containig of  <tr>s
     while(pp.nextTag() == pp.START_TAG) {
        if("tr".equals(pp.getName())) {
            HtmlTr tr = HtmlTr.umarshal(pp);
            table.rows.addElement(tr);
         } else {
              thow new ValidationExceptiopn(
                    "expected start tag for HTML tr inside table and not start tag"
                        +pp.getName()pp.getpositionDesacripton());//ScreamAndDieException ;
         }
     }
     if(!"table".equals(pp.getName())
        thow new ValidationExceptiopn("expected end tag for HTML table"+pp.getpositionDesacripton());
   }
    return table;
}

public class HtmlTtr {
  Vector content = new Vector();
  public statoc HtmlTr unmarshal(XmlPullParser pp) {
     if(pp.getEventType() != pp.START_TAG || !"tr".equals(pp.getName()) //checking pre-condition
        thow new ValidationExceptiopn("expected start tag for HTML table"+pp.getpositionDesacripton());

    HtmlTtr tr = new HtmlTr();
     //parse content of  <tr>s
      // usnig nextTag() and nextText() and delegating work to sub-classes ...
      return tr;
   }
}

alek