Add fresh parameter : 
public <K> T ajax(String url, Class<K> type, long expire, long fresh,
AjaxCallback<K> callback)
Unlike expire parameter, if the remote connection is available, the data
do not come from the cache. 
Examples: 
Expire: 1 month 
Fresh: 1 day 
With less than a day, the cached data is always returned. 
After two days, when the call fail, the cached data is returned. If the
call work, the remote data is cached and returned. 
After a month, the cache is no longer valid and if the call fails, no
data is returned.