UserIdMdcHelper { public static void set(final Subject subject) { checkNotNull(subject); String userId = UserIdHelper.get(subject); log.trace("Set: {}", userId); MDC.put(KEY, userId); } private  UserIdMdcHelper(); static boolean isSet(); static void setIfNeeded(); static void set(final Subject subject); static void set(); static void unknown(); static void system(); static void unset(); static final String KEY; }
@Test(expected = NullPointerException.class) public void set_withNull() { UserIdMdcHelper.set(null); }