WSS4JHandler cannot be used in an IBM WebSphere web services engine runtime because it attempts to set a MessageContext property to null instead of calling MessageContext.removeProperty(). Doing so raises an exception while using UsernameToken (perhaps more) functionality with the WebSphere error "WSWS3211E: Error: Null property value specified."
I was able to track this down in a debugger session to (WSS4J 1.5.7, 1.5.8) WSS4JHandler, line 314:
As it turns out, the WebSphere implementation of MessageContext is a descendant of java.util.Hashtable, which does not allow null values.
I will attach a patch (passes all unit tests in the maven2 build) with a proposed fix, which is to use MessageContext.removeProperty() instead of setProperty(..., null).