System.LimitException: Maximum stack depth reached: 1001

global class testFBInt {


String username = 'username';
String password = 'password';
String clientId = 'consumer_id';
String clientSecret = 'consumer_secret';
String tokenEndpoint = 'https://test.salesforce.com/services/oauth2/token';

public testFBInt () {
    this.username = username;
    this.password = password;
    this.clientId = clientId;
    this.clientSecret = clientSecret;
}


testFBInt upf = new testFBInt ();

global void Execute(){

String sessionId = upf.requestAccessToken();

System.Debug('@sesionid' + upf.requestAccessToken());
exInt(sessionId);
}

public String requestAccessToken() {
    HttpRequest req = new HttpRequest();
    req.setEndpoint(tokenEndpoint);
    req.setMethod('POST');
    req.setBody(buildHttpQuery(new Map<String, String> {
        'grant_type' => 'password',
        'username' => username,
        'password' => password,
        'client_id' => clientId,
        'client_secret' => clientSecret
    }));

    Http http = new Http();
    HttpResponse resp = http.send(req);

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

    return (String) m.get('access_token');
}

static String buildHttpQuery(Map<String, String> queryParams) {
    if (queryParams.isEmpty()) {
        return '';
    }

    String[] params = new String[] {};
    for (String k : queryParams.keySet()) {
        String v = EncodingUtil.urlEncode(queryParams.get(k), 'UTF-8');

        params.add(String.format('{0}={1}', new String[] { k, v }));
    }

    return String.join(params, '&');
}


 @future(callout=true)
 public static void exInt(string sessionId) {




String dashboardImageURL = 'https://ap4.salesforce.com/servlet/servlet.ChartServer?rsid=.....';

String base64Data;
httprequest req = new httprequest();
req.setEndpoint(dashboardImageURL);
req.setmethod('GET');
req.setHeader('Authorization', 'OAuth ' + sessionId);
http http = new http();
httpresponse res = http.send(req);
Blob image = res.getBodyAsBlob();
base64Data =  EncodingUtil.base64Encode(image);


Document docatt = new Document();
docatt.Name = 'Chart 1.jpg';
docatt.Body = EncodingUtil.base64Decode(base64Data);
docatt.FOLDERID = '00l6F005101pqGi'; //Your folder id
docatt.IsPublic = true;

insert docatt;

System.Debug('@@ doc ' + docatt);
}
}