RefreshTokenClass.cls

public class RefreshTokenClass {
    private auth_response rt;
    public string str {get; set;}
    public string clientid {get; set;}
    public string clientsecret {get; set;}
    
    public pagereference auth_Step_1(){
        String auth_url = 'https://login.salesforce.com/services/oauth2/authorize';
        String params =  
            '?response_type=code' +
            '&client_id=' + encodingUtil.urlencode(clientid,'UTF-8') +
            '&redirect_uri=https://c.na53.visual.force.com/apex/RefreshToken';
        pageReference pr = New PageReference(auth_url + params);
        return pr.setRedirect(false);
        
    }
    public pagereference auth_Step_2(){
        
        HttpRequest req = new HttpRequest();
        Http http = new Http();
        String auth_url = 'https://login.salesforce.com/services/oauth2/token';
        String params =  
            '?code=' + apexPages.currentPage().getParameters().get('code') +
            '&grant_type=authorization_code' + 
            '&client_id=' + encodingUtil.urlencode(clientid,'UTF-8') +
            '&client_secret='+ clientsecret + 
            '&redirect_uri=https://c.na53.visual.force.com/apex/RefreshToken';
        system.debug('auth_Step_2>>>>>>'+params);
        req.setMethod('POST');
        req.setEndpoint(auth_url + params);
        HTTPResponse resp = http.send(req);
        rt = (auth_response)json.deserialize(resp.getBody(),auth_response.class);
        system.debug('rt>>>>'+ rt  );
        str = rt.refresh_token;
        return null ;  
    }
    private class auth_response{
        public string refresh_token;
        public string access_token;
    }
}