Android Http从Servlet获取字符串数据

我试图从android中的Javaservlet返回一个简单的url字符串。我可以得到的网址回来，但有一个空的前缀

以下是我用来调用servlet的Android代码：

HttpClient client = new DefaultHttpClient();
HttpGet request = new HttpGet("http://localhost:8888/mcheck_gen_url_fu");
HttpResponse response = client.execute(request);
//Here i try to read the response
BufferedReader rd = new BufferedReader(new InputStreamReader(response.getEntity().getContent()));
while ((line = rd.readLine()) != null) {
    output += line + System.getProperty("line.separator");
}
//I am able to get the response but there is a null pre-pended 

以下是servlet代码：

public class mcheck_gen_url_fu extends HttpServlet {
    public void doGet(HttpServletRequest req, HttpServletResponse resp) throws IOException {

        BlobstoreService blobstoreService = BlobstoreServiceFactory.getBlobstoreService();
        String output = blobstoreService.createUploadUrl("/mcheck_file_upload");

        resp.setContentType("text/plain");
        resp.getWriter().print(output);
    }
}

以下是url的外观：

nullhttp://anyusers-MacBook-Air.local:8888/_ah/upload/ag5tY2hlY2stcGF5bWVudHIbCxIVX19CbG9iVXBsb2FkU2Vzc2lvbl9fGBwM

这样做对吗？我试过Gson，但它也被空值阻塞了

提前谢谢

    output = "";
    while ((line = rd.readLine()) != null) {
        output += line + System.getProperty("line.separator");
    }

顺便说一下，最好的做法是

final StringBuilder sb = new StringBuilder();
final String sep = System.getProperty("line.separator");
while ((line = rd.readLine()) != null) {
        sb.append(line).append(sep);
}

output = sb.toString();