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<o:viewParamValidationFailed> allows the developer to handle a view parameter validation failure with either a redirect or a HTTP error status, optionally with respectively a flash message or HTTP error message. This tag can be placed inside <f:metadata> or <f|o:viewParam>. When placed in <f|o:viewParam>, then it will be applied when the particular view parameter has a validation error as per UIViewParameter#isValid(). When placed in <f:metadata>, and no one view parameter has already handled the validation error via its own <o:viewParamValidationFailed>, then it will be applied when there's a general validation error as per FacesContext#isValidationFailed().

The sendRedirect attribute uses under the covers Faces#redirect(String, String...) to send the redirect, so the same rules as to scheme and leading slash apply here. The sendError attribute uses under the covers Faces#responseSendError(int, String) to send the error, so you can customize HTTP error pages via <error-page> entries in web.xml, otherwise the server-default one will be displayed instead. *

f:viewParam required="true" fail

As a precaution, the <f:viewParam required="true"> has in current Mojarra and MyFaces releases (as of now, Mojarra 2.2.7 and MyFaces 2.2.4) a design error. When the parameter is not specified in the query string, then it is retrieved as null which causes that an internal isRequired() check is performed instead of delegating the check to standard UIInput implementation. This has the consequence that PreValidateEvent and PostValidateEvent listeners are never invoked, which the <o:viewParamValidationFailed> is actually relying on. This is fixed in <o:viewParam. *

Examples

With the example below, when at least one view param is absent, then the client will be returned a HTTP 400 error.

                <f:metadata>
                    <o:viewParam name="foo" required="true" />
                    <o:viewParam name="bar" required="true" />
                    <o:viewParamValidationFailed sendError="400" />
                </f:metadata>
                

With the example below, only when the "foo" parameter is absent, then the client will be redirected to "login.xhtml". When the "bar" parameter is absent, nothing new will happen. The process will proceed "as usual". I.e. the validation error will end up as a faces message in the current view the usual way.

                <f:metadata>
                    <o:viewParam name="foo" required="true">
                        <o:viewParamValidationFailed sendRedirect="login.xhtml" />
                    </o:viewParam>
                    <o:viewParam name="bar" required="true" />
                </f:metadata>
                

With the example below, only when the "foo" parameter is absent, regardless of the "bar" or "baz" parameters, then the client will be returned a HTTP 401 error. When the "foo" parameter is present, but either "bar" or "baz" parameter is absent, then the client will be redirected to "search.xhtml".

                <f:metadata>
                    <o:viewParam name="foo" required="true">
                        <o:viewParamValidationFailed sendError="401" />
                    </o:viewParam>
                    <o:viewParam name="bar" required="true" />
                    <o:viewParam name="baz" required="true" />
                    <o:viewParamValidationFailed sendRedirect="search.xhtml" />
                </f:metadata>
                

In a nutshell: when there are multiple <o:viewParamValidationFailed> tags, then they will be applied in the same order as they are declared in the view. So, with the example above, the one nested in <f|o:viewParam> takes precedence over the one nested in <f:metadata>. *

Messaging

By default, the first occurring faces message on the parent component will be copied, or when there is none, then the first occurring global faces message will be copied. When sendRedirect is used, then it will be set as a global flash error message. When sendError is used, then it will be set as HTTP status message.

You can override this message by explicitly specifying the message attribute. This is applicable on both sendRedirect and sendError.

                <o:viewParamValidationFailed sendRedirect="search.xhtml" message="You need to perform a search." />
                ...
                <o:viewParamValidationFailed sendError="401" message="Authentication failed. You need to login." />
                

*

Design notes

You can technically nest multiple <o:viewParamValidationFailed> inside the same parent, but this is not the documented approach and the behavior is unspecified.

You can not change the HTTP status code of a redirect. This is not a JSF limitation, but a HTTP limitation. The status code of a redirect will always end up the one of the redirected response. If you intend to "redirect" with a different HTTP status code, then you should be using sendError instead and specify the desired page as <error-page> in web.xml.