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# Tuesday, 04 November 2008

Why we've turned our attention to the Saxon implementation?

A considerable part (~75%) of project we're working on at present is creating xslt(s). That's not stylesheets to create page presentations, but rather project's business logic. To fulfill the project we were in need of xslt 2.0 processor. In the current state of affairs I doubt someone can point to a good alternative to the Saxon implementation.

The open source nature of the SaxonB project and intrinsic curiosity act like a hook for such species like ourselves.

We want to say that we're rather sceptical observers of a code: the code should prove it have merits. Saxon looks consistent. It takes not too much time to grasp implementation concepts taking into account that the code routinely follows xpath/xslt/xquery specifications. These code observation and practice with live xslt tasks helped us to form an opinion on the Saxon itself. That's why we dare to critique it.

1. Compilation is fused with execution.

An xslt before being executed passes several stages including xpath data model, and a graph of expressions - objects implementing parts of runtime logic.

Expression graph is optimized to achieve better runtime performace. The optimization logic is distributed throughout the code, and in particular lives in expression objects. This means that expression completes two roles: runtime execution and optimization.

I would prefer to see a smaller and cleaner run time objects (expressions), and optimization logic separately. On the other hand I can guess why Michael Kay fused these roles: to ease lazy optimizations (at runtime).

2. Optimizations are xslt 1.0 by origin

This is like a heritage. There are two main techniques: cached sequences, and global indices of rooted nodes.

This might be enough in xslt 1.0, but in 2.0 where there are diverse set of types, where sequences extend node sets to other types, where sequences may logically be grouped by pairs, tripples, and so on, this is not enough.

XPath data model operates with sequences only (in math sense). On the other hand it defines many set based functions (operators) like: $a intersect $b, $a except $b, $a = $b, $a != $b. In these examples XPath sequences are better to consider as sets, or maps of items.

Other example: for $i in index-of($names, $name) return $values[$i], where $names as xs:string*, $values as element()* shows that a closure of ($names, $values) is in fact a map, and $names might be implemented as a composition of a sequence and a map of strings to indices.

There are other use case examples, which lead me to think that Saxon lacks set based operators. Global indices are poor substitution, which work for rooted trees only.

Again, I guess why Michael Kay is not implementing these operators: not everyone loads xslt with stressful tasks requiring these features. I think xslt is mostly used to render pages, and one rarely deviates from rooted trees.

In spite of the objections we think that Saxon is a good xslt 2.0 implementation, which unfortunately lacks competitors.

Tuesday, 04 November 2008 11:30:36 UTC  #    Comments [0] -
xslt
# Friday, 17 October 2008

We strongly object against persistence frameworks in their contemporary meaning. This includes a long row of names like Hibernate, Java Persistence API, LINQ, and others.

Consider how one of them describes itself:

...high performance object/relational persistence and query service... lets you develop persistent classes following object-oriented idiom - including association, inheritance, polymorphism, composition, and collections... allows you to express queries in its own portable SQL extension...

Sounds good, right?

We think not! Words "own" and "portable" regarding SQL are heard almost like antonyms. When one creates a unified language (a noble rush, opposed to a proprietary one (?)) she will inevitably adds a peer, increasing plurality in the family of languages.

Attempts to create similar layers between data and business logic are not new. This happens throughout the computer history. IDMS, NATURAL, COOL:GEN these are 20-30 years old examples.

Our reasoning (nothing new).

One need to approach to a design (development and maintainance) from different perspectives, thus she will understand the question under the design better, and will estimate skills to accomplish the problem. This will lead to a modularization e.g: business layer, data layer, appearance; and to development (maintainance) roles: program developer, database specialist, appearance speciaist. On a small scale several roles are often fulfilled with one person; this should not mean, however, that these roles are redundant, one just need to try on different roles.

Why does one separate business layer and data layer?

Pragmatic perspective. There are databases, which may accomplish most of data storage tasks in a more efficient way than one may achieve without database. There are two worlds of database specialists and program developers. These two layers and roles are facts of reality.

A desiner's goal is to keep these roles separate:

  • do not force a database specialist to know the business logic details;
  • do not force a program developer to know details on how to organize a storage in more efficient way, or on how to optimize a particular query;

Modularity helps here. Databases are well equipped to solve these tasks: the data layer should expose a database API through stored procedures, functions, and views, while the business layer should use this API to access the database.

With persistence frameworks there are two alterantives:

  1. still use data layer API;
  2. rely on a persistence framework.

When the first case is selected then a framework provides almost no aditional value comparing to traditional database access (jdbc, ado.net, an so on).

When one relies on a framework then a data layer interface virtually disappears (in fact a framework substitutes this interface). Database specialist has very little control over tuning the data structure, and optimizing queries, unless she starts digging in the business code but even then she always cannot control queries to the database. Moreover database specialist must learn a proprietary query language.

Result is that a persistence framework erodes a division of responsibilities, complicating development and maintainance.

We often hear a following explanation on why one should use Persistence Frameworks: "It eases database vendor switch". This is the most stupid reason to use Persistence Frameworks! It looks as if they plan to switch vendors once a day.

A design needs to focus on a modularity. This will make code more robust, faster and maintainable. This also eases potential migration process, as the data layer should be migrated only, with minimal (mostly configurational) changes in the business layer.

Friday, 17 October 2008 19:57:28 UTC  #    Comments [2] -
Tips and tricks
# Saturday, 27 September 2008

We are certain xslt/xquery are the best for web application frameworks from the design perspective; or, in other words, pipeline frameworks allowing use of xslt/xquery are preferable way to create web applications.

Advantages are obvious:

  • clear separation of business logic, data, and presentation;

  • richness of languages, allowing to implement simple presentation, complex components, and sophisticated data binding;

  • built-in extensibility, allowing comunication with business logic, written in other languages and/or located at different site.

It seems the agitation for a such technologies is like to force an open door. There are such frameworks out there: Orbeon Forms, Cocoon, and others. We're not qualified to judge of their virtues, however...

Look at the current state of affairs. The main players in this area (well, I have a rather limited vision) push other technologies: JSP/JSF/Faceletes and alike in the Java world, and ASP.NET in the .NET world. The closest thing they are providing is xslt servlet/component allowing to generate an output.

Their variants of syntaxis, their data binding techniques allude to similar paradigms in xslt/xquery:

<select>
  <c:forEach var="option" items="#{bean.options}">
    <option value="#{option.key}">#{parameter.value}</option>
  </c:forEach>
</select>

On the surface, however, we see much more limited (in design and in the application) frameworks.

And here is a contradiction: how can it be that at present such a good design is not as popular, as its competitors, at least?

Someone can say, there is no such a problem. You can use whatever you want. You have a choice! Well, he's lucky. From our perspective it's not that simple.

We're creating rather complex web applications. Their nature isn't important in this context, but what is important is that there are customers. They are not thoroughly enlightened in the question, and exactly because of this they prefer technologies proposed by leaders. It seems, everything convince them: main stream, good support, many developers who know technology.

There is no single chance to promote anything else.

We believe that the future may change this state, but we're creating at present, and cannot wait...

Saturday, 27 September 2008 10:36:06 UTC  #    Comments [3] -
Tips and tricks | xslt
# Tuesday, 16 September 2008

I've uploaded jxom.zip

Now, it contains a state machine generator. See "What you can do with jxom".

The code is in the java-state-machine-generator.xslt. The test is in the java-state-machine-test.xslt.

Tuesday, 16 September 2008 11:02:09 UTC  #    Comments [0] -
xslt
# Monday, 08 September 2008

Java has no value types: objects allocated inplace, in contrast to objects referred by a pointer in the heap. This, in my opinion, has a negative impact on a program design and on a performance.

Incidentally, I've thought of a use case, which can be understood as a value type by the jvm implementations. Consider an example:

class A
{
  private final B b = new B();
}

Implementation may layout class A, in a way that field b will be a content of an instance of class B itself rather than a pointer to an instance of a class B. This way we save a pointer and a heap allocation of instance B. Another example:

class C
{
  C(int size)
  {
    values = new D[size];

    for(int i = 0; i < values.length; i++)
    {
      values[i] = new D();
    }
  }

  private final D[] values;
}

Here field values is never a null and each item of array contains a non null value. Assuming these conditions are kept for a whole life cycle, and values are not passed by reference, we can consider values as an array of value types.

A use case conditions are following:

  • a field contains a non null value;
  • the field value is an instance of the field type and not descendant type;
  • if the field is an array, then all elements of the array are initialized with instances of element type, and not descendant type.
  • the field or an element of the array can be assigned through the operator new only (field = new T(), array[i] = new T());
  • the array field is not passed by reference (Arrays.sort(array) never happens).

JIT's allowed to interpret a field as a value type provided it proves these conditions.

Later...

There is another use case to detect value types:

  • a method variable contains no null value, and
  • that variable is never stored in any field, and
  • no synchronization is used on the instance of value in variable, and
  • a value to the variable is assigned through the operator new only.

A variable can be layed out directly onto the stack, provided a preceding conditions are satisfied.

P.S. In spite that .NET has built in value types, it may use the very same technique to optimize reference types.

Monday, 08 September 2008 08:01:51 UTC  #    Comments [0] -
Tips and tricks
# Friday, 05 September 2008

We're facing a task of conversion of a java method into a state machine. This is like to convert a SAX Parser, pushing data, into an Xml Reader, which pulls data.

The task is formalized as:

  • for a given method containing split markers create a class perimitting iteration;
  • each iteration performs part of a logic of a method.

We have defined rules converting all statements into a state machine except of the statement synchronized. In fact the logic is rather linear, however the most untrivial conversion is for try statement. Consider an example:

public class Test
{
  void method()
    throws Exception
  {
    try
    {
      A();
      B();
    }
    catch(Exception e)
    {
      C(e);
    }
    finally
    {
      D();
    }

    E();
  }

  private void A()
    throws Exception
  {
    // logic A
  }

  private void B()
    throws Exception
  {
    // logic B
  }

  private void C(Exception e)
    throws Exception
  {
    // logic C
  }

  private void D()
    throws Exception
  {
    // logic D
  }

  private void E()
    throws Exception
  {
    // logic E
  }
}

Suppose we want to see method() as a state machine in a way that split markers are after calls to methods A(), B(), C(), D(), E(). This is how it looks as a state machine:

Callable<Boolean> methodAsStateMachine()
  throws Exception
{
  return new Callable<Boolean>()
  {
    public Boolean call()
      throws Exception
    {
      do
      {
        try
        {
          switch(state)
          {
            case 0:
            {
              A();
              state = 1;

              return true;
            }
            case 1:
            {
              B();
              state = 3;

              return true;
            }
            case 2:
            {
              C(ex);
              state = 3;

              return true;
            }
            case 3:
            {
              D();

              if (currentException != null)
              {
                throw currentException;
              }

              state = 4;

              return true;
            }
            case 4:
            {
              E();
              state = -1;

              return false;
            }
          }

          if (currentException == null)
          {
            currentException = new IllegalStateException();
          }
        }
        catch(Throwable e)
        {
          currentException = null;

          switch(state)
          {
            case 0:
            case 1:
            {
              if (e instanceof Exception)
              {
                ex = (Exception)e;
                state = 2;
              }
              else
              {
                currentException = e;
                state = 3;
              }

              continue;
            }
            case 2:
            {
              currentException = e;
              state = 3;

              continue;
            }
          }

          currentException = e;
          state = -1;
        }
      }
      while(false);

      return this.<Exception>error();
    }

    @SuppressWarnings("unchecked")
    private <T extends Throwable> boolean error()
      throws T
    {
      throw (T)currentException;
    }

    private int state = 0;
    private Throwable currentException = null;
    private Exception ex = null;
  };
}

Believe it, or not but this transformation can be done purely in xslt 2.0 with the help of the jxom (Java xml object model). We shall update jxom.zip whenever this module will be implemented and tested.

Friday, 05 September 2008 15:39:50 UTC  #    Comments [0] -
xslt
# Wednesday, 03 September 2008

In the xslt one can express logically the same things in different words like:

  exists($x)
and
  every $y in $x satisfies exists($y)

newbie> Really the same?
expert> Ops... You're right, these are different things!

What's the difference?

Wednesday, 03 September 2008 12:34:06 UTC  #    Comments [0] -
xslt
# Saturday, 30 August 2008

I was already writing about tuples and maps in the xslt (see Tuples and maps - Status: CLOSED, WONTFIX, and Tuples and maps in Saxon).

Now, I want to argue on a use case, and on how xslt processor can detect such a use case and implement it as map. This way, for a certain conditions, a sequences could be treated as maps (or as sets).

Use case.

There are two stages:

  • a logic collecting nodes/values satisfying some criteria.
  • process data, and take a special action whenever a node/value is collected on the previous stage.

Whenever we're talking of nodes than result of the first stage is a sequence $set as node()*. The role of this sequence is a set of nodes (order is not important).

The second stage is usually an xsl:for-each, an xsl:apply-templates, or something of this kind, which repeatedly verifies whether a some $node as node()? belongs to the $set, like a following: $node intersect $set, or $node except $set.

In spite of that we're still using regular xpath 2.0, we have managed to express a set based operation. It's a matter of xslt processor's optimizer to detect such a use case and consider a sequence as a set. In fact the detection rule is rather simple.

For expressions $node except $set and $node intersect $set:
  • $set can be considered as a set, as order of elements is not important;
  • chances are good that a $set being implemented as a set outperforms implementation using a list or an array.

Thus what to do? Well, I do not think I'm the smartest child, quite opposite... however it worth to hint this idea to xslt implementers (see Suggest optimization). I still do not know if it was fruitful...

P.S. A very similar use case exists for a function index-of($collection, $item).

Saturday, 30 August 2008 07:44:44 UTC  #    Comments [0] -
xslt
# Tuesday, 12 August 2008

I know we're not the first who create a parser in xslt. However I still want to share our implementation, as I think it's beautiful.

In our project, which is conversion from a some legacy language to java, we're dealing with dynamic expressions. For example in the legacy language one can filter a collection using an expression defined by a string: collection.filter("a > 0 and b = 7");

Whenever expression string is calculated there is nothing to do except to parse such string at runtime and perform filtering dynamically. On the other hand we have found that in the majority of cases literal strings are used. Thus we have decided to optimize this route like this:

  collection.filter(
    new Filter<T>()
    {
      boolean filter(T value)
      {
        return (value.getA() > 0) and (value.getB() = 7);
      }
    });

This means that we're converting that expression string into java code on the generation stage.

In the xslt - our generator engine - this means that we have to convert a string into expression tree like this:

(a > 7 or a= 3) and c * d = 2.2

to

<and>
  <or>
    <gt>
      <identifier>a</identifier>
      <integer>7</integer>
    </gt>
    <eq>
      <identifier>a</identifier>
      <integer>3</integer>
    </eq>
  </or>
  <eq>
    <mul>
      <identifier>c</identifier>
      <identifier>d</identifier>
    </mul>
    <decimal>2.2</decimal>
  </eq>
</and>

Our parser fits naturally to the world of parsers: it uses xsl:analyze-string instruction to tokenize input and parses tokens according to an expression grammar. During implementation I've found some new to me things. I think they worth mentioning:

  • As tokenizer is defined as a big regular expression, we have rather verbose regex attribute over xsl:analyze-string. It was hard to edit such a big line until I've found there is flag="x" option that solves formatting problems:

    The flags attribute may be used to control the interpretation of the regular expression... If it contains the letter x, then whitespace within the regular expression is ignored.

    This means that I can use spaces to format regular expression and /s to specify space as part of expression.
  • Saxon 9.1.0.1 has inefficiency in implementation of xsl:analyze-string instruction, whenever regex contains literal value however with '{' character (e.g. "\p{{L}}"), as it considers the value to be an AVT and delays pattern compilation until runtime, which it does every time instruction is executed.

Use following link to see the xslt: expression-parser.xslt.
To see how to generate java from an xml follow this link: Xslt for the jxom (Java xml object model), jxom.zip.

Tuesday, 12 August 2008 14:45:54 UTC  #    Comments [2] -
xslt
# Thursday, 31 July 2008

Yesterday, incidentally, I've arrived to a problem of a dynamic error during evaluation of a template's match. This reminded me SFINAE in C++. There the principle is applied at compile time to find a matching template.

I think people underestimate the meaning of this behaviour. The effect of dynamic errors occurring during pattern evaluation is described in the specification:

Any dynamic error or type error that occurs during the evaluation of a pattern against a particular node is treated as a recoverable error even if the error would not be recoverable under other circumstances. The optional recovery action is to treat the pattern as not matching that node.

This has far reaching consequences, like an error recovery. To illustrate what I'm talking about please look at this simple stylesheet that recovers from "Division by zero.":

<xsl:stylesheet version="2.0"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  xmlns:xs="http://www.w3.org/2001/XMLSchema">

<xsl:template match="/">
  <xsl:variable name="operator" as="element()+">
    <div divident="10" divisor="0"/>
    <div divident="10" divisor="2"/>
  </xsl:variable>

  <xsl:apply-templates select="$operator"/>
</xsl:template>

<xsl:param name="NaN" as="xs:double" select="1.0 div 0"/>

<xsl:template
  match="div[(xs:integer(@divident) div xs:integer(@divisor)) ne $NaN]">
  <xsl:message select="xs:integer(@divident) div xs:integer(@divisor)"/>
</xsl:template>

<xsl:template match="div">
  <xsl:message select="'Division by zero.'"/>
</xsl:template>

</xsl:stylesheet>

Here, if there is a division by zero a template is not matched and other template is selected, thus second template serves as an error handler for the first one. Definitely, one may define much more complex construction to be handled this way.

I never was a purist (meaning doing everything in xslt), however this example along with indirect function call, shows that xslt is rather equiped language. One just need to be smart enough to understand how to do a things.

See also: Try/catch block in xslt 2.0 for Saxon 9.

Thursday, 31 July 2008 11:52:21 UTC  #    Comments [0] -
Tips and tricks | xslt
# Monday, 28 July 2008

Among other job activities, we're from time to time asked to check technical skills of job applicants.

Several times we were interviewing people who're far below the acceptable professional skills. It's a torment for both sides, I should say.

To ease things we have designed a small questionnaire (specific to our projects) for job applicants. It's sent to an applicant before the meeting. Even partially answered, this questionnaire constitutes a good filter against profanes:

<questionnaire>
  <item>
    <question>
      Please estimate your knowledge in XML Schema (xsd) as lacking, bad, good, or perfect.
    </question>
    <answer/>
  </item>
  <item>
    <question>
      Please estimate your knowledge in xslt 2.0/xquery 1.0 as lacking, bad, good, or perfect.
    </question>
    <answer/>
  </item>
  <item>
    <question>
      Please estimate your knowledge in xslt 1.0 as lacking, bad, good, or perfect.
    </question>
    <answer/>
  </item>
  <item>
    <question>
      Please estimate your knowledge in java as lacking, bad, good, or perfect.
    </question>
    <answer/>
  </item>
  <item>
    <question>
      Please estimate your knowledge in c# as lacking, bad, good, or perfect.
    </question>
    <answer/>
  </item>
  <item>
    <question>
      Please estimate your knowledge in sql as lacking, bad, good, or perfect.
    </question>
    <answer/>
  </item>
  <item>
    <question>
      For logical values A, B, please rewrite logical expression "A and B" using operator "or".
    </question>
    <answer/>
  </item>
  <item>
    <question>
      For logical values A, B, please rewrite logical expression "A = B" using operators "and" and "or".
    </question>
    <answer/>
  </item>
  <item>
    <question>
      There are eight balls, with only one heavier than some other.
      What is a minimum number of weighings reveals the heavier ball?
      Please be suspicious about the "trivial" solution.
    </question>
    <answer/>
  </item>
  <item>
    <question>
      If A results in B. What one may say about the reason of B?
    </question>
    <answer/>
  </item>
  <item>
    <question>
      If only A or B result in C. What one may say about the reason of C?
    </question>
    <answer/>
  </item>
  <item>
    <question>
      Please define an xml schema for this questionnaire.
    </question>
    <answer/>
  </item>
  <item>
    <question>
      Please create a simple stylesheet creating an html table based on this questionnaire.
    </question>
    <answer/>
  </item>
  <item>
    <question>
      For a table A with columns B, C, and D, please create an sql query selecting B groupped by C and ordered by D.
    </question>
    <answer/>
  </item>
  <item>
    <question>
      For a sequence of xml elements A with attribute B, please write a stylesheet excerpt creating a sequence of elements D, grouping elements A with the same string value of attribute B, sorted in the order of ascending of B.
    </question>
    <answer/>
  </item>
  <item>
    <question>
      Having a java class A with properties B and C, please sort a collection of A for B in ascending, and C in descending order.
    </question>
    <answer/>
  </item>
  <item>
    <question>
      What does a following line mean in c#?
      int? x;
    </question>
    <answer/>
  </item>
  <item>
    <question>
      What is a parser?
    </question>
    <answer/>
  </item>
  <item>
    <question>
      How to issue an error in the xml stylesheet?
    </question>
    <answer/>
  </item>
  <item>
    <question>
      What is a lazy evaluation?
    </question>
    <answer/>
  </item>
  <item>
    <question>
      How do you understand a following sentence?
      For each line of code there should be a comment.
    </question>
    <answer/>
  </item>
  <item>
    <question>
      Have you used any supplemental information to answer these questions?
    </question>
    <answer/>
  </item>
  <item>
    <question>
      Have you independently answered these questions?
    </question>
    <answer/>
  </item>
</questionnaire>

Monday, 28 July 2008 10:54:54 UTC  #    Comments [0] -
Tips and tricks | xslt
# Thursday, 10 July 2008

I've found that proposition to introduce tuples and maps to xslt/xquery type system has not found a support:

At the joint meeting of the XSL and XQuery Working groups 2008-06-23 it was decided that a change of this nature would be too large for the next "point" release of the Recommendations. The request for new functionality will be considered for a future "main" release.

Boor> *****!

Pessimist> Ah, there won't be tuples and maps in xslt/xquery...

Optimist> Wow, chances are good to see this addition by the year 2018!

Thursday, 10 July 2008 05:54:52 UTC  #    Comments [0] -
xslt
# Thursday, 03 July 2008

Today Michael Kay has announced an update for the Saxon processor. The latest version for now is 9.1.

I've checked our saxon.extensions, and has fixed incompatibilities.

The source for the new version of the Saxon can be found at http://saxon.sourceforge.net/.

New features are discussed at: http://www.saxonica.com/documentation/changes/intro.html

Our extensions can be found at saxon.extensions.9.1.zip.

Thursday, 03 July 2008 13:47:13 UTC  #    Comments [0] -
xslt
# Thursday, 26 June 2008

We are designing a rather complex xslt 2.0 application, dealing with semistructured data. We must tolerate with errors during processing, as there are cases where an input is not perfectly valid (or the program is not designed or ready to get such an input).

The most typical error is unsatisfied expectation of tree structure like:
  <xsl:variable name="element" as="element()" select="some-element"/>

Obviously, dynamic error occurs if a specified element is not present. To concentrate on primary logic, and to avoid a burden of illegal (unexpected) case recovery we have created a try/catch API. The goal of such API is:

  • to be able to continue processing in case of error;
  • report as much as possible useful information related to an error.

Alternatives:

Do not think this is our arrogance, which has turned us to create a custom API. No, we were looking for alternatives! Please see [xsl] saxon:try() discussion:

  • saxon:try() function - is a kind of pseudo function, which explicitly relies on lazy evaluation of its arguments, and ... it's not available in SaxonB;
  • ex:error-safe  extension instruction - is far from perfect in its implementation quality, and provides no error location.

We have no other way except to design this feature by ourselves. In our defence one can say that we are using innovatory approach that encapsulates details of the implementation behind template and calls handlers indirectly.

Use:

Try/catch API is designed as a template <xsl:template name="t:try-block"/> calling a "try" handler, and, if required, a "catch" hanler using <xsl:apply-templates mode="t:call"/> instruction. Caller passes any information to these handlers by the means of tunnel parameters.

Handlers must be in a "t:call" mode. The "catch" handler may recieve following error info parameters:

<xsl:param name="error" as="xs:QName"/>
<xsl:param name="error-description" as="xs:string"/>
<xsl:param name="error-location" as="item()*"/>

where $error-location is a sequence of pairs (location as xs:string, context as item())*.

A sample:

<xsl:stylesheet version="2.0"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  xmlns:xs="http://www.w3.org/2001/XMLSchema"
  xmlns:t="http://www.nesterovsky-bros.com/xslt/public/"
  exclude-result-prefixes="xs t">

<xsl:include href="try-block.xslt"/>

<xsl:template match="/">
  <result>
    <xsl:for-each select="1 to 10">
      <xsl:call-template name="t:try-block">
        <xsl:with-param name="value" tunnel="yes" select=". - 5"/>
        <xsl:with-param name="try" as="element()">
          <try/>
        </xsl:with-param>
        <xsl:with-param name="catch" as="element()">
          <t:error-handler/>
        </xsl:with-param>
      </xsl:call-template>
    </xsl:for-each>
  </result>
</xsl:template>

<xsl:template mode="t:call" match="try">
  <xsl:param name="value" tunnel="yes" as="xs:decimal"/>

  <value>
    <xsl:sequence select="1 div $value"/>
  </value>
</xsl:template>

</xsl:stylesheet>

The sample prints values according to the formula "1/(i - 5)", where "i" is a variable varying from 1 to 10. Clearly, division by zero occurs when "i" is equal to 5.

Please notice how to access try/catch API through <xsl:include href="try-block.xslt"/>. The main logic is executed in <xsl:template mode="t:call" match="try"/>, which recieves parameters using tunneling. A default error handler <t:error-handler/> is used to report errors.

Error report:

Error: FOAR0001
Description:
Decimal divide by zero

Location:
1. systemID: "file:///D:/style/try-block-test.xslt", line: 34
2. template mode="t:call" match="element(try, xs:anyType)"
  systemID: "file:///D:/style/try-block-test.xslt", line: 30
  context node:
    /*[1][local-name() = 'try']
3. template mode="t:call"
  match="element({http://www.nesterovsky-bros.com/xslt/private/try-block}try, xs:anyType)"
  systemID: "file:///D:/style/try-block.xslt", line: 53
  context node:
    /*[1][local-name() = 'try']
4. systemID: "file:///D:/style/try-block.xslt", line: 40
5. call-template name="t:try-block"
  systemID: "file:///D:/style/try-block-test.xslt", line: 17
6. for-each
  systemID: "file:///D:/style/try-block-test.xslt", line: 16
  context item: 5
7. template mode="saxon:_defaultMode" match="document-node()"
  systemID: "file:///D:/style/try-block-test.xslt", line: 14
  context node:
    /

Implementation details:

You were not expecting this API to be pure xslt, weren't you? :-)

Well, you're right, there is an extension function. Its pseudo code is like this:

function tryBlock(tryItems, catchItems)
{
  try
  {
    execute xsl:apply-templates for tryItems.
  }
  catch
  {
    execute xsl:apply-templates for catchItems.
  }
}

 

The last thing. Please get the implementation saxon.extensions.zip. There you will find sources of the try/catch, and tuples/maps API.

Thursday, 26 June 2008 09:18:50 UTC  #    Comments [0] -
Announce | Tips and tricks | xslt
# Tuesday, 17 June 2008

Right now we're inhabiting in the java world, thus all our tasks are (in)directly related to this environment.

We want to store stylesheets as resources of java application, and at the same time to point to these stylesheets without jar qualification. In .NET this idea would not appear at all, as there are well defined boundaries between assemblies, but java uses rather different approach. Whenever you have a resource name, it's up to ClassLoader to find this resource. To exploit this feature we've created an uri resolver for the stylesheet transformation. The protocol we use has a following format: "resource:/resource-path".

For example to store stylesheets in the META-INF/stylesheets folder we use uri "resource:/META-INF/stylesheets/java/main.xslt". Relative path is resolved naturally. A path "../jxom/java-serializer.xslt" in previously mentioned stylesheet is resolved to "resource:/META-INF/stylesheets/jxom/java-serializer.xslt".

We've created a small class ResourceURIResolver. You need to supply an instance of TransformerFactory with this resolver:
  transformerFactory.setURIResolver(new ResourceURIResolver());

The class itself is so small that we qoute it here:

import java.io.InputStream;

import java.net.URI;
import java.net.URISyntaxException;

import javax.xml.transform.Source;
import javax.xml.transform.TransformerException;
import javax.xml.transform.URIResolver;

import javax.xml.transform.stream.StreamSource;

/**
 * This class implements an interface that can be called by the processor
 * to turn a URI used in document(), xsl:import, or xsl:include into a
 * Source object.
 */
public class ResourceURIResolver implements URIResolver
{
  /**
   * Called by the processor when it encounters
   * an xsl:include, xsl:import, or document() function.
   *
   * This resolver supports protocol "resource:".
   * Format of uri is: "resource:/resource-path", where "resource-path" is an
   * argument of a {@link ClassLoader#getResourceAsStream(String)} call.
   * @param href - an href attribute, which may be relative or absolute.
   * @param base - a base URI against which the first argument will be made
   *   absolute if the absolute URI is required.
   * @return a Source object, or null if the href cannot be resolved, and
   *   the processor should try to resolve the URI itself.
   */
  public Source resolve(String href, String base)
    throws TransformerException
  {
    if (href == null)
    {
      return null;
    }

    URI uri;

    try
    {
      if (base == null)
      {
        uri = new URI(href);
      }
      else
      {
        uri = new URI(base).resolve(href);
      }
    }
    catch(URISyntaxException e)
    {
      // Unsupported uri.
      return null;
    }

    if (!"resource".equals(uri.getScheme()))
    {
      return null;
    }

    String resourceName = uri.getPath();

    if ((resourceName == null) || (resourceName.length() == 0))
    {
      return null;
    }

    if (resourceName.charAt(0) == '/')
    {
      resourceName = resourceName.substring(1);
    }

    ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
    InputStream stream = classLoader.getResourceAsStream(resourceName);

    if (stream == null)
    {
      return null;
    }

    return new StreamSource(stream, uri.toString());
  }
}

Tuesday, 17 June 2008 07:57:52 UTC  #    Comments [0] -
Tips and tricks | xslt
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