How to handle JSON input with Saxon XSLT

Question

Recent Saxon releases contain a command line argument "-json:myfile.json" to input a JSON file.

But how do I implement the XSLT to parse this JSON? I did not find any doc which handles this directly (without making use of "json-to-xml" or similar).

I only found this: how to convert json to xml with saxonjs?

But this does not help me, because my json starts with an array:

[
  {
    "eid": "2122.5",
    "ecat": "show",
    "day": "1629410400",
    "spcat": "Bühne",
    "time": "19:30",
    "text": "Welle",
    "remarks": "",
    "location": ""
  },
  {
    "eid": "2122.6",
    "ecat": "show",
    "day": "1629496800",
    "spcat": "Bühne",
    "time": "19:30",
    "text": "Welle",
    "remarks": "",
    "location": ""
  }
]

By writing an intermediate XSLT using the function "json-to-xml", I can convert this JSON to xml that looks like this:

<?xml version="1.0" encoding="UTF-8"?>
<array xmlns="http://www.w3.org/2005/xpath-functions">
   <map>
      <string key="eid">2122.5</string>
      <string key="ecat">show</string>
      <string key="day">1629410400</string>
      <string key="spcat">Bühne</string>
      <string key="time">19:30</string>
      <string key="text">Welle</string>
      <string key="remarks"/>
      <string key="location"/>
   </map>
   <map>
      <string key="eid">2122.6</string>
      <string key="ecat">show</string>
      <string key="day">1629496800</string>
      <string key="spcat">Bühne</string>
      <string key="time">19:30</string>
      <string key="text">Welle</string>
      <string key="remarks"/>
      <string key="location"/>
   </map>
</array>

How can I create a template that matches the root item, and how can I call "apply-templates" to trigger another template that handles the items?

Martin Honnen · Accepted Answer · 2022-09-21 10:15:25Z

1

The JSON you have shown is an array so it will be mapped to the XPath/XSLT XDM type array(*), or, in your case, array(map(xs:string, xs:string)). To match on that use e.g. <xsl:template match=".[. instance of array(*)]">..</xsl:template> or e.g. <xsl:template match=".[. instance of array(map(xs:string, xs:string))]">..</xsl:template>.

More complete example would be online at the fiddle, doing:

<?xml version="1.0" encoding="utf-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  version="3.0"
  xmlns:xs="http://www.w3.org/2001/XMLSchema"
  xmlns:map="http://www.w3.org/2005/xpath-functions/map"
  exclude-result-prefixes="#all"
  expand-text="yes">
  
  <xsl:output method="xml" indent="yes"/>

  <xsl:template match=".[. instance of array(map(xs:string, xs:string))]">
    <items>
      <xsl:apply-templates select="?*"/>
    </items>
  </xsl:template>
  
  <xsl:template match=".[. instance of map(xs:string, xs:string)]">
    <xsl:variable name="map" select="."/>
    <item>
      <xsl:iterate select="map:keys(.)">
        <value key="{.}">{$map(.)}</value>
      </xsl:iterate>      
    </item>
  </xsl:template>
  
</xsl:stylesheet>

As for grouping:

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  version="3.0"
  xmlns:xs="http://www.w3.org/2001/XMLSchema"
  xmlns:map="http://www.w3.org/2005/xpath-functions/map"
  exclude-result-prefixes="#all"
  expand-text="yes">
  
  <xsl:output method="xml" indent="yes"/>

  <xsl:template match=".[. instance of array(map(xs:string, xs:string))]">
    <items>
      <xsl:for-each-group select="?*" group-by="?text">
        <group name="{current-grouping-key()}">
          <xsl:apply-templates select="current-group()"/>
        </group>
      </xsl:for-each-group>
    </items>
  </xsl:template>
  
  <xsl:template match=".[. instance of map(xs:string, xs:string)]">
    <xsl:variable name="map" select="."/>
    <item>
      <xsl:iterate select="map:keys(.)[not(. = current-grouping-key())]">
        <value key="{.}">{$map(.)}</value>
      </xsl:iterate>      
    </item>
  </xsl:template>
  
</xsl:stylesheet>

edited Sep 21, 2022 at 10:15

answered Sep 20, 2022 at 15:28

Martin Honnen

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4 Comments

Michael Kay Over a year ago

Note also, for future reference, that there is a proposal to allow the match pattern to be written as match="array(map(s:string, xs:string))".

wknauf Over a year ago

Great, this works. Is there any doc that could point me to the correct syntax (or a tutorial on how to parse JSON)? I have to admit that I don't understand all those curly brackets ;-). In my real sample, I would have to use for-each-group in the next step to group over the field "text". How could this be done? But in the end, I think I will switch to a two-step process: first convert json to xml, then use a "classic" XSLT to process the xml. Reason: the "-json" command line argument is only supported in the Java parser HE, but not in the .NET version...

Martin Honnen Over a year ago

@wknauf, the .NET Saxon HE 10 release, like the Java HE 10 release, doesn't have the the -json command line option, as it was newly introduced in Saxon 11, I think. For .NET that means currently, SaxonCS from Saxonica and .NET 5 or 6, or, as I have managed to cross-compile Saxon HE 11.4 Java to .NET 6, or you could, if you have the .NET 6 SDK installed, try my recently release dotnet tool nuget.org/packages/SaxonHE11NetXslt/11.4.0-alpha1 which runs Saxon 11.4 HE within .NET 6 as a command line tool/app (with the -json option). As for the grouping, raise a new question on that.

Martin Honnen Over a year ago

The mapping of JSON to XDM is of course explained in the XPath 3.1 spec or in tutorials on XPath 3.1, like altova.com/training/xpath3.

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