/*End point Url to web service callout*/
private final static String ENP_POINT_URL ='https://login.salesforce.com/services/oauth2/token';
//For development and production https://login.salesforce.com/services/oauth2/token
//And for sandbox https://test.salesforce.com/services/oauth2/token
private final static String CONSUMER_KEY = 'Your_Org_Consumer_Key';
private final static String CONSUMER_SECRET = 'Your_Org_Consumer_Secret';
  private final static String USERNAME = 'Your_Username';
private final static String PASSWORD = 'Your_Password';
private final static String REQUEST_BODY = 'grant_type=password&client_id={0}&client_secret=
                                            {1}&username={2}&password={3}';




/*To generate Access token Method*/
private static OAuth getAccessToken(){
    try{
        HttpRequest req = new HttpRequest();
        req.setEndpoint(ENP_POINT_URL);
        req.setMethod('POST');
        Blob headerValue = Blob.valueOf(USERNAME + ':' + PASSWORD);
        String authorizationHeader = 'BASIC ' +
        EncodingUtil.base64Encode(headerValue);
        req.setHeader('Authorization', authorizationHeader);
        req.setBody(String.format(REQUEST_BODY ,new string[]{CONSUMER_KEY,CONSUMER_SECRET,
                                                             USERNAME,PASSWORD}));
        req.setTimeout(60000);
        Http http = new Http();
        HttpResponse res = http.send(req);
        OAuth objAuthenticationInfo = (OAuth)JSON.deserialize(res.getbody(), OAuth.class);
        return objAuthenticationInfo;
    }catch(CallOutException ce){
        throw ce;
    }
    return null;
}

/*To get Access token property*/
public static OAuth authenticationDetail{
    get{
        if(authenticationDetail == null){
            authenticationDetail = getAccessToken();
        }
        return authenticationDetail;
    }set;
}

/*To get aouthentication detail Wrapper*/
public class OAuth{
    public String id{get;set;}
    public String issued_at{get;set;}
    public String instance_url{get;set;}
    public String signature{get;set;}
    public String access_token{get;set;}
  }
}