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The <o:validator> is a taghandler that extends the standard <f:validator> tag family with support for deferred value expressions in all attributes. In other words, the validator attributes are not evaluated anymore on a per view build time basis, but just on every access like as with UI components and bean properties. This has among others the advantage that they can be evaluated on a per-iteration basis inside an iterating component, and that they can be set on a custom validator without needing to explicitly register it in a tagfile.

Usage

When you specify for example the standard <f:validateLongRange> by validatorId="jakarta.faces.LongRange", then you'll be able to use all its attributes such as minimum and maximum as per its documentation, but then with the possibility to supply deferred value expressions.

 <o:validator validatorId="jakarta.faces.LongRange" minimum="#{item.minimum}" maximum="#{item.maximum}" />
 

The validator ID of all standard Faces validators can be found in their javadocs. First go to the javadoc of the class of interest, then go to VALIDATOR_ID in its field summary and finally click the Constant Field Values link to see the value.

It is also possible to specify the validator message on a per-validator basis using the message attribute. Any "{0}" placeholder in the message will be substituted with the label of the referenced input component. Note that this attribute is ignored when the parent component has already validatorMessage specified.

 <o:validator validatorId="jakarta.faces.LongRange" minimum="#{item.minimum}" maximum="#{item.maximum}"
     message="Please enter between #{item.minimum} and #{item.maximum} characters" />
 

JSF 2.3 compatibility

The <o:validator> is currently not compatible with validators which are managed via the managed=true attribute set on the FacesValidator annotation, at least not when using Mojarra. Internally, the converters are wrapped in another instance which doesn't have the needed setter methods specified. In order to get them to work with <o:validator>, the managed=true attribute needs to be removed, so that OmniFaces ValidatorManager will automatically manage them.