When processing a JSF deployment, if the user has configured a different vendor/version of JSF implementation to use, then one step involves setting up the (JBoss) modules correctly for such a custom implementation. Details of which are here[1].

If for whatever reason, the module setup isn't correctly done, then the deployment can run into a deployment exception which looks like:

Caused by: org.jboss.as.server.deployment.DeploymentUnitProcessingException: WFLYJSF0016: Failed to inject JSF from slot mojarra-2.2.8.21

at org.jboss.as.jsf.deployment.JSFDependencyProcessor.addJSFInjection(JSFDependencyProcessor.java:156)

at org.jboss.as.jsf.deployment.JSFDependencyProcessor.deploy(JSFDependencyProcessor.java:97)

at org.jboss.as.server.deployment.DeploymentUnitPhaseService.start(DeploymentUnitPhaseService.java:176) [wildfly-server-9.0.2.Final.jar:9.0.2.Final]

One such example is reported by an user here[2].

That exception stacktrace can be improved a bit if the original cause is included in it, to make it easier to debug such issues.

[1] https://docs.jboss.org/author/display/WFLY/JSF+Configuration#JSFConfiguration-InstallinganewJSFimplementationmanually
[2] https://developer.jboss.org/thread/280549