XPath Expressions

Last modified 26 Oct 2021 10:24 +02:00

OBSOLETE

This functionality is obsolete. It is no longer supported or maintained.

XPath version 2 was a supported expression language in midPoint. It is not supported any longer. This page is kept solely for historical reasons. It is not updated any longer.

Its language URL is:

http://www.w3.org/TR/xpath/

Variables and functions

XPath variables are prefixed by dollar sign ($) and must be specified in a correct namespace (using a valid namespace prefix). For example, following expression will evaluate to the content of the fullName property of a user (using c as a namespace prefix):

$c:user/c:fullName

Following table provides summary of commonly used variables in midPoint expressions. These variable may or may not be set depending on a specific situation in which the expression is used. Please refer to the in-line XSD schema documentation for list of specific variables for each situation.

Variable Name Type Comment

Variable Name	Type	Comment
`$c:user`	`UserType`	The User object, available for most expressions
`$c:account`	`AccountShadowType`	TODO
`$c:configuration`	TODO	TODO
`$c:environment`	TODO	TODO

$c:user

UserType

The User object, available for most expressions

$c:account

AccountShadowType

TODO

$c:configuration

TODO

$c:environment

TODO

The XPath is used not only as a pointer language, but also as a generic purpose expression language. For example, the following code can be used to concatenate first and last name of a user:

concat($c:user/c:givenName,' ',$c:user/c:familyName)

Path Expressions and Generic Expression

The XPath language is used both as a pointer language and as a generic purpose language. Sometimes it may be difficult to distinguish the two different uses.

The path expressions are used in case the schema asks for XPathType. The path expressions should not use XPath functions. It should be a simple relative path expression, usually starting with an variable. An example of a path expression is provided below.

XPath Path Expression

$c:user/c:givenName

Path expressions are frequently used as l-values. The path expressions point to a piece of XML where a value can be assigned or modified.

Path expressions are fixed to XPath version 1 language and are not supposed to be extensible. It is not expected that the pointer language will be anything else than XPath.

On the other hand, generic expressions can use full power of XPath. Generic expressions can be used when the schema asks for ExpressionType. An example of a generic expression is provided below.

XPath Generic Expression

concat('uid=',$c:user/c:name,',ou=people,dc=example,dc=com')

Generic expressions are used as r-values, producing a value in any reasonable way. Generic expression are now supposed to be in XPath version 2. Generic expressions are extensible and there may be more languages than just XPath in the future.

XPath and Namespaces

Similarly as in BPEL, the XPath expressions are using the context of the XML document that contains them. This is used especially to declare namespace prefixes used in the XPath expressions. Following example is a XML snippet that declared the namespace prefix that is later used in the expression.

<whatever xmlns:c="http://midpoint.evolveum.com/xml/ns/public/common/core-1.xsd">
    <c:valueExpression xmlns:foo="http://schemas.foo.com/">
        $c:user/c:extension/foo:bar
    </c:valueExpression>
</whatever>

However, the XML normalization process is not aware about the prefixes used in the expression. As XML prefixes are considered semantically insignificant, XML normalization may rename the prefixes or remove "unused" prefixes. We are not aware of any mechanism that can solve this problem, as this may a systemic problem of XML normalization itself (e.g. there seems to be a hardcoded exception for QName, but only in schema-aware normalization, no similar mechanism was found for XPath). Therefore it is recommended not to use XML normalization, if possible.

In case the normalization cannot be turned off, there is an alternative mechanism how to explicitly declare XML namespaces inside XPath expression. An example is provided below.

<whatever xmlns:c="http://midpoint.evolveum.com/xml/ns/public/common/core-1.xsd">
    <c:valueExpression>
        declare namespace c="http://midpoint.evolveum.com/xml/ns/public/common/core-1.xsd";
        declare namespace foo="http://schemas.foo.com/"
        $c:user/c:extension/foo:bar
    </c:valueExpression>
</whatever>

Such approach is making the code difficult to read and maintain. The namespace prefix declarations are duplicated and have to be provided in each and every expression. However, it may be the only feasible approach when XML normalization is used.

XPath Expressions

Variables and functions

Path Expressions and Generic Expression

XPath and Namespaces

See Also