I'm using JBossAS 6.0.0.M2 and replaced the RESTEasy JARs according to 2.0-beta-2. The following error didn't occur when using JBossAS 5.1 and RESTEasy 1.2 within a WAR.

I have an application class:
public class JaxrsApplication extends Application {
private Set<Class<?>> classes = new HashSet<Class<?>>();
private Set<Object> singletons = new HashSet<Object>();

public JaxrsApplication()

{ singletons.add(new KundenverwaltungResource()); // resource class: works fine singletons.add(new KundeValidationExceptionMapper()); // WILL BE A PROBLEM ... }

@Override
public Set<Class<?>> getClasses()

{ return classes; }

@Override
public Set<Object> getSingletons()

{ return singletons; }

}

The exception mapper:
@Provider
public class KundeValidationExceptionMapper implements ExceptionMapper<KundeValidationException> {
private static final Logger LOGGER = LoggerFactory.getLogger(KundeValidationExceptionMapper.class);
private static final boolean DEBUG = LOGGER.isDebugEnabled();

public Response toResponse(KundeValidationException e) {
if (DEBUG) LOGGER.debug("BEGINN toResponse: " + e.getViolations());

final Collection<ConstraintViolation<Kunde>> violations = e.getViolations();
final StringBuilder sb = new StringBuilder();
for (ConstraintViolation<Kunde> v: violations)

{ sb.append(v.getMessage()); sb.append(" "); }

final String responseStr = sb.toString();
final Response response = Response.status(NOT_FOUND)
.type(TEXT_PLAIN)
.entity(responseStr)
.build();

if (DEBUG) LOGGER.debug("ENDE toResponse: " + responseStr);
return response;
}
}

In the JBoss console I only see the resources being registered as singletons, but not the exception mappers:
16:32:15,868 INFO [org.jboss.web.tomcat.service.deployers.TomcatDeployment] deploy, ctxPath=/hskaREST
16:32:17,065 INFO [org.jboss.resteasy.spi.ResteasyDeployment] Deploying javax.ws.rs.core.Application: class de.hska.util.JaxrsApplication
16:32:17,070 INFO [org.jboss.resteasy.spi.ResteasyDeployment] Adding singleton resource de.hska.kundenverwaltung.rest.KundenverwaltungResource from Application javax.ws.rs.core.Application