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ViewParameter is a component that extends the standard {@link UIViewParameter} and provides a stateless mode of operation and fixes the issue wherein null model values are converted to empty string parameters in query string (e.g. when includeViewParams=true) and the (bean) validation never being triggered when the parameter is completely absent in query string, causing e.g. @NotNull to fail.

The standard UIViewParameter implementation calls the model setter again after postback. This is not always desired when being bound to a view scoped bean and can lead to performance problems when combined with an expensive converter. To solve this, this component by default stores the submitted value as a component property instead of in the model (and thus in the view state in case the binding is to a view scoped bean).

The standard UIViewParameter implementation calls the converter regardless of whether the evaluated model value is null or not. As converters by specification return an empty string in case of null value, this is being added to the query string as an empty parameter. This is not desired.

The standard UIViewParameter implementation uses an internal "is required" check when the submitted value is null, hereby completely bypassing the standard UIInput validation, including any bean validation annotations and even the PreValidateEvent and PostValidateEvent events. This is not desired.

You can use it the same way as <f:viewParam>, you only need to change f: to o:.