final String hostName = store.getDefaultString(PreferenceConstants.P_HOST_ENDPOINT_STRING);
should be replaced with
String hostName = store.getString(PreferenceConstants.P_HOST_ENDPOINT_STRING);
Please also rename the label "Host name:" to "URL to HAC:" or something that better express that it must be a URL.
The default hostname string "http://localhost:9001/hac" is always used for validation action and not the configured url in preference page. In ValidateImpexAction
final String hostName = store.getDefaultString(PreferenceConstants.P_HOST_ENDPOINT_STRING);
should be replaced withString hostName = store.getString(PreferenceConstants.P_HOST_ENDPOINT_STRING);
Please also rename the label "Host name:" to "URL to HAC:" or something that better express that it must be a URL.