JSF exception handling, get request param - jsf-2

Question

i followed instructions from for implementing exception handling in jsf web app.
my problem is to show attribute value, that i set in ExceptionHandler
here is ExceptionHandler.java
#Override
public void handle() throws FacesException {
final Iterator<ExceptionQueuedEvent> i = getUnhandledExceptionQueuedEvents().iterator();
while (i.hasNext()) {
ExceptionQueuedEvent event = i.next();
ExceptionQueuedEventContext context = (ExceptionQueuedEventContext) event.getSource();
Throwable t = context.getException();
FacesContext fc = FacesContext.getCurrentInstance();
Map<String, Object> requestMap = fc.getExternalContext().getRequestMap();
NavigationHandler nav = fc.getApplication().getNavigationHandler();
try {
// Push some useful stuff to the request scope for
// use in the page
System.err.println("DefaultExceptionHandler.handle()...exceptionMessage = " + t.getMessage());
requestMap.put("exceptionMessage", t.getMessage());
nav.handleNavigation(fc, null, "error/500");
fc.renderResponse();
} finally {
i.remove();
}
}
getWrapped().handle();
}
and 500.xhtml
<ui:composition
xmlns="http://www.w3.org/1999/xhtml"
xmlns:h="http://java.sun.com/jsf/html"
xmlns:ui="http://java.sun.com/jsf/facelets"
xmlns:f="http://java.sun.com/jsf/core">
The error message is:
<br />
<b>
3. : #{exceptionMessage}
</b>
<br />
<b>
1. : #{requestScope['exceptionMessage']}
</b>
<br />
<b>
2. : #{param['exceptionMessage']}
</b>
and page in browser looks like:
The error message is:
3. :
1. :
2. :
thanks in advance!!

Answer 1

First of all, that's not a request parameter at all. Request parameters are the data which the enduser has sent to the server along with the HTTP request. In GET requests they are visible in query string part of the URL. In POST requests, they are hidden in request body (but visible with a HTTP traffic monitor such as browser's builtin one accessible by F12 key). What you were setting is a request attribute. So, from the attempts, only #{exceptionMessage} and #{requestScope['exceptionMessage']} should have worked, but #{param['exceptionMessage']} definitely not.
Coming back to your concrete problem, this can happen when you're sending a redirect on navigation by either <redirect> in navigation case, or by appending ?faces-redirect=true to the outcome somewhere in a custom navigation handler. A redirect basically instructs the client to create a brand new HTTP request, hereby trashing the initial HTTP request including all of its attributes.
If that's also not the problem, then the (customized) FacesContext or ExternalContext implementation being used is likely broken. Hard to tell as you didn't tell anything about the JSF impl/version used nor about the server used nor any "3rd party" libraries. E.g. Spring Web Flow's ExternalContext implementation is known to be broken like that in some versions.