Salesforce 獲取Salesforce Token

//獲取Salesforce Token
    public String getSalesforceToken() 
    {
       String getTokenUrl = 'https://login.salesforce.com/services/oauth2/token';
        String grant_type = 'password';
        String username = 'XXXXXX';
        String password = 'XXXXXX';
        String client_id = 'XXXXXXX';
        String client_secret = 'XXXXXX';

        //獲取token的鏈接和請求參數
        String endpoint = getTokenUrl + '?' + 'grant_type=' + grant_type + '&client_id=' + client_id + '&client_secret=' + client_secret + '&username=' + username + '&password=' + password;
        HttpRequest req = new HttpRequest();
        system.debug('Endpoint is : ' + endpoint);
        req.setEndpoint(endpoint);
        req.setMethod('POST');
        //發送http請求
        Http http = new Http();
        HTTPResponse res = http.send(req);
        String result = res.getBody();
        system.debug('result is : ' + result);

        if(String.isNotEmpty(result)) {
            return result;
        }
        return null;
    }