@future (callout=true)
 global static void Getfields(String JIRA_Key) {

    //Construct HTTP request and response
    HttpRequest req = new HttpRequest();
    HttpResponse res = new HttpResponse();
    Http http = new Http();

    //Modify these variables:
    String username = 'admin';
    String password = 'xxx';
    String jiraURL = 'https://mysfjira.atlassian.net/';

    //Construct Authorization and Content header
    Blob headerValue = Blob.valueOf(username+':'+password);
    String authorizationHeader = 'Basic ' + EncodingUtil.base64Encode(headerValue);
    req.setHeader('Authorization', authorizationHeader);
    req.setHeader('Content-Type','application/json');

    String endpoint = jiraURL+'/rest/api/2/issue/'+JIRA_Key;

    req.setMethod('GET');
    req.setEndpoint(endpoint);
    res = http.send(req);

    System.debug('Serialization :::' +res.getBody());

    Map<String, Object> m = (Map<String, Object>) JSON.deserializeUntyped(res.getBody());

    System.debug('Deserialization :::' +m);

    //mapping the fields with case object fields

    List<Case> casestoUPsert = new List<Case>();


    for(Map<String, Case> c : m){


           Case cas = new Case();
           cas.Id = c.id;
           cas.JIRA_Key__c = c.key;
           cas.Status = c.Status;
           cas.Origin = c.Priority;

           casestoUPsert.add(cas);

    }
    if(casesToUpsert.size()>0){
        Database.upsert(casestoUpsert,false) ;
}

}

}