How to send a person to a 404 page if f:viewParam / converter returns null?

Question

Lets say you had a page with a view param, like /widgets?widgetId=1

Answer 1

Use ExternalContext#responseSendError() in the Converter when the condition is met.

context.getExternalContext().responseSendError(404, message);
context.responseComplete();
return null;

Don't forget to call FacesContext#responseComplete() afterwards, this isn't implicitly been done for some reason, in contrary to ExternalContext#redirect(). Otherwise JSF will append the current page to end of response, or throw an IllegalStateException when it's already committed.

Instead of the magic number 404 you can also use HttpServletResponse.SC_NOT_FOUND.

context.getExternalContext().responseSendError(HttpServletResponse.SC_NOT_FOUND, message);
context.responseComplete();
return null;