This page is part of archived documentation for openHAB 3.3. Go to the current stable version

# XPath Transformation Service

Transforms an XML (opens new window) input using an XPath (opens new window) expression.

# Examples

# Basic Example

Given a retrieved XML

Input XML

<?xml version="1.0" encoding="UTF-8"?>
<PTZStatus version="2.0" >
    <AbsoluteHigh>
        <elevation>0</elevation>
        <azimuth>450</azimuth>
        <absoluteZoom>10</absoluteZoom>
    </AbsoluteHigh>
</PTZStatus>

The XPath /PTZStatus/AbsoluteHigh/azimuth/text() returns the document

# Advanced Example

Given a retrieved XML (e.g. from an HIK Vision device with the namespace xmlns="http://www.hikvision.com/ver20/XMLSchema"):

Input XML

<?xml version="1.0" encoding="UTF-8"?>
<PTZStatus version="2.0" xmlns="http://www.hikvision.com/ver20/XMLSchema">
    <AbsoluteHigh>
        <elevation>0</elevation>
        <azimuth>450</azimuth>
        <absoluteZoom>10</absoluteZoom>
    </AbsoluteHigh>
</PTZStatus>

A simple xpath query to fetch the Azimut value does not work as it does not address the namespace.

There are two ways to address the namespace.

Simple path which may not work in complex XML.
With full qualified path.

The XPath

[name()='PTZStatus']/*[name()='AbsoluteHigh']/*[name()='azimuth']/
/*[local-name()='PTZStatus' and namespace-uri()='http://www.hikvision.com/ver20/XMLSchema']/*[local-name()='AbsoluteHigh' and namespace-uri()='http://www.hikvision.com/ver20/XMLSchema']/*[local-name()='azimuth' and namespace-uri()='http://www.hikvision.com/ver20/XMLSchema']

returns

<azimuth>450</azimuth>

# In Setup

.items

String  Temperature_xml "Temperature [XPATH(/*[name()='PTZStatus']/*[name()='AbsoluteHigh']/*[name()='azimuth']/):%s °C]" {...}
Number  Temperature "Temperature [%.1f °C]"

.rules

rule "Convert XML to Item Type Number"
  when
    Item Temperature_xml changed
  then
    // use the transformation service to retrieve the value
    // Simple
    val mytest = transform("XPATH", "/*[name()='PTZStatus']
                                     /*[name()='AbsoluteHigh']
                                     /*[name()='azimuth']
                                     /text()", 
                                    Temperature_xml.state.toString )  
    // Fully qualified
    val mytest = transform("XPATH", "/*[local-name()='PTZStatus'    and namespace-uri()='http://www.hikvision.com/ver20/XMLSchema']
                                     /*[local-name()='AbsoluteHigh' and namespace-uri()='http://www.hikvision.com/ver20/XMLSchema']
                                     /*[local-name()='azimuth'      and namespace-uri()='http://www.hikvision.com/ver20/XMLSchema']
                                     /text()",
                                    Temperature_xml.state.toString )

    // post the new value to the Number Item
    Temperature.postUpdate( newValue )
end

Now the resulting Number can also be used in the label to change the color (opens new window) or in a rule as value for comparison.

# Usage as a Profile

The functionality of this TransformationService can be used in a Profile on an ItemChannelLink too. To do so, it can be configured in the .items file as follows:

String <itemName> { channel="<channelUID>"[profile="transform:XPATH", function="<xpath>", sourceFormat="<valueFormat>"]}

The XPath expression to be executed has to be set in the function parameter. The parameter sourceFormat is optional and can be used to format the input value before the transformation, i.e. %.3f. If omitted the default is %s, so the input value will be put into the transformation without any format changes.

Please note: This profile is a one-way transformation, i.e. only values from a device towards the item are changed, the other direction is left untouched.

# Further Reading

An introduction (opens new window) to XPath at W3School
An informative explanation of common mistakes (opens new window).
Online validation tools like this (opens new window) to check the syntax.

Caught a mistake or want to contribute to the documentation? Edit this page on GitHub (opens new window)

← Scale XSLT →